diff --git a/src/analyses/escape_analysis.cpp b/src/analyses/escape_analysis.cpp
index b0129dfd0a6..507df39f770 100644
--- a/src/analyses/escape_analysis.cpp
+++ b/src/analyses/escape_analysis.cpp
@@ -319,7 +319,7 @@ bool escape_domaint::merge(
   {
     const std::set<irep_idt> &b_cleanup=cleanup.second.cleanup_functions;
     std::set<irep_idt> &a_cleanup=cleanup_map[cleanup.first].cleanup_functions;
-    unsigned old_size=a_cleanup.size();
+    auto old_size=a_cleanup.size();
     a_cleanup.insert(b_cleanup.begin(), b_cleanup.end());
     if(a_cleanup.size()!=old_size)
       changed=true;
@@ -379,7 +379,7 @@ void escape_domaint::check_lhs(
     {
       // count the aliases
 
-      unsigned count=0;
+      std::size_t count=0;
 
       for(const auto &alias : aliases)
       {
diff --git a/src/analyses/goto_check.cpp b/src/analyses/goto_check.cpp
index b6ee194dd9f..95345814426 100644
--- a/src/analyses/goto_check.cpp
+++ b/src/analyses/goto_check.cpp
@@ -572,7 +572,7 @@ void goto_checkt::integer_overflow_check(
     // The overflow checks are binary!
     // We break these up.
 
-    for(unsigned i=1; i<expr.operands().size(); i++)
+    for(std::size_t i=1; i<expr.operands().size(); i++)
     {
       exprt tmp;
 
diff --git a/src/analyses/invariant_set.cpp b/src/analyses/invariant_set.cpp
index 718df977b33..4ed9d19cd21 100644
--- a/src/analyses/invariant_set.cpp
+++ b/src/analyses/invariant_set.cpp
@@ -27,7 +27,7 @@ Author: Daniel Kroening, kroening@kroening.com
 
 void inv_object_storet::output(std::ostream &out) const
 {
-  for(unsigned i=0; i<entries.size(); i++)
+  for(std::size_t i=0; i<entries.size(); i++)
     out << "STORE " << i << ": " << to_string(i, "") << '\n';
 }
 
@@ -61,7 +61,7 @@ unsigned inv_object_storet::add(const exprt &expr)
 
   assert(s!="");
 
-  unsigned n=map.number(s);
+  mapt::number_type n=map.number(s);
 
   if(n>=entries.size())
   {
@@ -188,11 +188,11 @@ void invariant_sett::add(
   unsigned f_r=eq_set.find(p.first);
   unsigned s_r=eq_set.find(p.second);
 
-  for(unsigned f=0; f<eq_set.size(); f++)
+  for(std::size_t f=0; f<eq_set.size(); f++)
   {
     if(eq_set.find(f)==f_r)
     {
-      for(unsigned s=0; s<eq_set.size(); s++)
+      for(std::size_t s=0; s<eq_set.size(); s++)
         if(eq_set.find(s)==s_r)
           dest.insert(std::pair<unsigned, unsigned>(f, s));
     }
@@ -209,7 +209,7 @@ void invariant_sett::add_eq(const std::pair<unsigned, unsigned> &p)
   bool constant_seen=false;
   mp_integer c;
 
-  for(unsigned i=0; i<eq_set.size(); i++)
+  for(std::size_t i=0; i<eq_set.size(); i++)
   {
     if(eq_set.find(i)==r)
     {
@@ -319,12 +319,12 @@ void invariant_sett::output(
   INVARIANT_STRUCTURED(
     object_store!=nullptr, nullptr_exceptiont, "Object store is null");
 
-  for(unsigned i=0; i<eq_set.size(); i++)
+  for(std::size_t i=0; i<eq_set.size(); i++)
     if(eq_set.is_root(i) &&
        eq_set.count(i)>=2)
     {
       bool first=true;
-      for(unsigned j=0; j<eq_set.size(); j++)
+      for(std::size_t j=0; j<eq_set.size(); j++)
         if(eq_set.find(j)==i)
         {
           if(first)
@@ -367,7 +367,7 @@ void invariant_sett::add_type_bounds(const exprt &expr, const typet &type)
 
   if(type.id()==ID_unsignedbv)
   {
-    unsigned op_width=to_unsignedbv_type(type).get_width();
+    std::size_t op_width=to_unsignedbv_type(type).get_width();
 
     if(op_width<=8)
     {
@@ -852,7 +852,7 @@ exprt invariant_sett::get_constant(const exprt &expr) const
     unsigned r=eq_set.find(a);
 
     // is it a constant?
-    for(unsigned i=0; i<eq_set.size(); i++)
+    for(std::size_t i=0; i<eq_set.size(); i++)
       if(eq_set.find(i)==r)
       {
         const exprt &e=object_store->get_expr(i);
@@ -938,8 +938,8 @@ bool invariant_sett::make_union(const invariant_sett &other)
   eq_set.intersection(other.eq_set);
 
   // inequalities
-  unsigned old_ne_set=ne_set.size();
-  unsigned old_le_set=le_set.size();
+  std::size_t old_ne_set=ne_set.size();
+  std::size_t old_le_set=le_set.size();
 
   intersection(ne_set, other.ne_set);
   intersection(le_set, other.le_set);
diff --git a/src/analyses/uninitialized_domain.cpp b/src/analyses/uninitialized_domain.cpp
index 48830ed204e..4f92eaebd49 100644
--- a/src/analyses/uninitialized_domain.cpp
+++ b/src/analyses/uninitialized_domain.cpp
@@ -81,7 +81,7 @@ bool uninitialized_domaint::merge(
   locationt from,
   locationt to)
 {
-  unsigned old_uninitialized=uninitialized.size();
+  auto old_uninitialized=uninitialized.size();
 
   uninitialized.insert(
     other.uninitialized.begin(),
diff --git a/src/ansi-c/c_typecheck_code.cpp b/src/ansi-c/c_typecheck_code.cpp
index 9715fc2e4d3..a45420839a4 100644
--- a/src/ansi-c/c_typecheck_code.cpp
+++ b/src/ansi-c/c_typecheck_code.cpp
@@ -151,7 +151,7 @@ void c_typecheck_baset::typecheck_asm(codet &code)
 
     typecheck_expr(code.op0());
 
-    for(unsigned i=1; i<code.operands().size(); i++)
+    for(std::size_t i=1; i<code.operands().size(); i++)
     {
       exprt &list=code.operands()[i];
       Forall_operands(it, list)
diff --git a/src/ansi-c/c_typecheck_expr.cpp b/src/ansi-c/c_typecheck_expr.cpp
index 59c684de00b..c41a2240309 100644
--- a/src/ansi-c/c_typecheck_expr.cpp
+++ b/src/ansi-c/c_typecheck_expr.cpp
@@ -2646,7 +2646,7 @@ void c_typecheck_baset::typecheck_function_call_arguments(
     throw 0;
   }
 
-  for(unsigned i=0; i<arguments.size(); i++)
+  for(std::size_t i=0; i<arguments.size(); i++)
   {
     exprt &op=arguments[i];
 
diff --git a/src/cbmc/show_vcc.cpp b/src/cbmc/show_vcc.cpp
index 067b22461c3..620ace0bb99 100644
--- a/src/cbmc/show_vcc.cpp
+++ b/src/cbmc/show_vcc.cpp
@@ -47,7 +47,7 @@ void bmct::show_vcc_plain(std::ostream &out)
     symex_target_equationt::SSA_stepst::const_iterator
       last_it=has_threads?equation.SSA_steps.end():s_it;
 
-    for(unsigned count=1; p_it!=last_it; p_it++)
+    for(std::size_t count=1; p_it!=last_it; p_it++)
       if(p_it->is_assume() || p_it->is_assignment() || p_it->is_constraint())
       {
         if(!p_it->ignore)
diff --git a/src/cpp/cpp_declarator_converter.cpp b/src/cpp/cpp_declarator_converter.cpp
index e01b7bacaf3..2ea5732f436 100644
--- a/src/cpp/cpp_declarator_converter.cpp
+++ b/src/cpp/cpp_declarator_converter.cpp
@@ -252,7 +252,7 @@ void cpp_declarator_convertert::combine_types(
     if(decl_code_type.return_type()==symbol_code_type.return_type() &&
        decl_code_type.parameters().size()==symbol_code_type.parameters().size())
     {
-      for(unsigned i=0; i<decl_code_type.parameters().size(); i++)
+      for(std::size_t i=0; i<decl_code_type.parameters().size(); i++)
       {
         const code_typet::parametert &decl_parameter=
           decl_code_type.parameters()[i];
diff --git a/src/cpp/cpp_exception_id.cpp b/src/cpp/cpp_exception_id.cpp
index 18649696f53..23d7f49a30c 100644
--- a/src/cpp/cpp_exception_id.cpp
+++ b/src/cpp/cpp_exception_id.cpp
@@ -78,7 +78,7 @@ irept cpp_exception_list(
   cpp_exception_list_rec(src, ns, "", ids);
   result.get_sub().resize(ids.size());
 
-  for(unsigned i=0; i<ids.size(); i++)
+  for(std::size_t i=0; i<ids.size(); i++)
     result.get_sub()[i].id(ids[i]);
 
   return result;
diff --git a/src/cpp/cpp_instantiate_template.cpp b/src/cpp/cpp_instantiate_template.cpp
index fe0b3e28ac2..c80c757298e 100644
--- a/src/cpp/cpp_instantiate_template.cpp
+++ b/src/cpp/cpp_instantiate_template.cpp
@@ -415,13 +415,13 @@ const symbolt &cpp_typecheckt::instantiate_template(
       static_cast<const exprt &>(
         template_symbol.value.find("template_methods"));
 
-    for(unsigned i=0; i<template_methods.operands().size(); i++)
+    for(auto &tm : template_methods.operands())
     {
       cpp_saved_scope.restore();
 
       cpp_declarationt method_decl=
         static_cast<const cpp_declarationt &>(
-          static_cast<const irept &>(template_methods.operands()[i]));
+          static_cast<const irept &>(tm));
 
       // copy the type of the template method
       template_typet method_type=
diff --git a/src/cpp/cpp_internal_additions.cpp b/src/cpp/cpp_internal_additions.cpp
index 78384905a83..a193742643e 100644
--- a/src/cpp/cpp_internal_additions.cpp
+++ b/src/cpp/cpp_internal_additions.cpp
@@ -20,7 +20,7 @@ std::string c2cpp(const std::string &s)
 
   result.reserve(s.size());
 
-  for(unsigned i=0; i<s.size(); i++)
+  for(std::size_t i=0; i<s.size(); i++)
   {
     char ch=s[i];
 
diff --git a/src/cpp/cpp_name.cpp b/src/cpp/cpp_name.cpp
index 1206ade0a71..fe456c0ada9 100644
--- a/src/cpp/cpp_name.cpp
+++ b/src/cpp/cpp_name.cpp
@@ -19,9 +19,9 @@ irep_idt cpp_namet::get_base_name() const
   const subt &sub=get_sub();
 
   // find last "::"
-  unsigned base=0;
+  std::size_t base=0;
 
-  for(unsigned i=0; i<sub.size(); i++)
+  for(std::size_t i=0; i<sub.size(); i++)
   {
     if(sub[i].id()=="::")
       base=i+1;
diff --git a/src/cpp/cpp_scope.cpp b/src/cpp/cpp_scope.cpp
index 1521472ccd3..32d3f5a395f 100644
--- a/src/cpp/cpp_scope.cpp
+++ b/src/cpp/cpp_scope.cpp
@@ -187,7 +187,7 @@ void cpp_scopet::lookup_identifier(
     id_set.insert(this);
 
   #if 0
-  for(unsigned i=0; i<parents_size(); i++)
+  for(std::size_t i=0; i<parents_size(); i++)
   {
     cpp_idt &parent= get_parent(i);
     if(parent.identifier == identifier
diff --git a/src/cpp/cpp_typecheck_compound_type.cpp b/src/cpp/cpp_typecheck_compound_type.cpp
index c0715fd03f4..6b5c7c1625b 100644
--- a/src/cpp/cpp_typecheck_compound_type.cpp
+++ b/src/cpp/cpp_typecheck_compound_type.cpp
@@ -599,7 +599,7 @@ void cpp_typecheckt::typecheck_compound_declarator(
 
         // create symbols for the parameters
         code_typet::parameterst &args=code_type.parameters();
-        unsigned i=0;
+        std::size_t i=0;
         for(auto &arg : args)
         {
           irep_idt base_name=arg.get_base_name();
@@ -640,7 +640,7 @@ void cpp_typecheckt::typecheck_compound_declarator(
           expr_call.arguments().reserve(args.size());
           expr_call.arguments().push_back(late_cast);
 
-          for(unsigned i=1; i < args.size(); i++)
+          for(std::size_t i=1; i < args.size(); i++)
           {
             expr_call.arguments().push_back(
               namespacet(symbol_table).lookup(
@@ -657,7 +657,7 @@ void cpp_typecheckt::typecheck_compound_declarator(
           code_func.arguments().reserve(args.size());
           code_func.arguments().push_back(late_cast);
 
-          for(unsigned i=1; i < args.size(); i++)
+          for(std::size_t i=1; i < args.size(); i++)
           {
             code_func.arguments().push_back(
               namespacet(symbol_table).lookup(
diff --git a/src/cpp/cpp_typecheck_virtual_table.cpp b/src/cpp/cpp_typecheck_virtual_table.cpp
index f882db1f1af..ade9c5c1568 100644
--- a/src/cpp/cpp_typecheck_virtual_table.cpp
+++ b/src/cpp/cpp_typecheck_virtual_table.cpp
@@ -23,7 +23,7 @@ void cpp_typecheckt::do_virtual_table(const symbolt &symbol)
 
   const struct_typet &struct_type=to_struct_type(symbol.type);
 
-  for(unsigned i=0; i < struct_type.components().size(); i++)
+  for(std::size_t i=0; i < struct_type.components().size(); i++)
   {
     const struct_typet::componentt &compo=struct_type.components()[i];
     if(!compo.get_bool("is_virtual"))
@@ -83,7 +83,7 @@ void cpp_typecheckt::do_virtual_table(const symbolt &symbol)
 
     exprt values(ID_struct, symbol_typet(vt_symb_type.name));
 
-    for(unsigned i=0; i < vt_type.components().size(); i++)
+    for(std::size_t i=0; i < vt_type.components().size(); i++)
     {
       const struct_typet::componentt &compo=vt_type.components()[i];
       std::map<irep_idt, exprt>::const_iterator cit2 =
diff --git a/src/cpp/parse.cpp b/src/cpp/parse.cpp
index 1cb4bfbe651..4de000d02eb 100644
--- a/src/cpp/parse.cpp
+++ b/src/cpp/parse.cpp
@@ -139,7 +139,7 @@ class new_scopet
   typedef std::map<irep_idt, new_scopet> id_mapt;
   id_mapt id_map;
 
-  unsigned anon_count;
+  std::size_t anon_count;
 
   new_scopet *parent;
 
@@ -382,7 +382,7 @@ class Parser // NOLINT(readability/identifiers)
   bool rMSCuuidof(exprt &);
   bool rMSC_if_existsExpr(exprt &);
 
-  unsigned number_of_errors;
+  std::size_t number_of_errors;
   irep_idt current_function;
 
   void merge_types(const typet &src, typet &dest);
@@ -488,7 +488,7 @@ bool Parser::SyntaxError()
 
   cpp_tokent t[ERROR_TOKENS];
 
-  for(unsigned i=0; i<ERROR_TOKENS; i++)
+  for(std::size_t i=0; i<ERROR_TOKENS; i++)
     lex.LookAhead(i, t[i]);
 
   if(t[0].kind!='\0')
@@ -499,7 +499,7 @@ bool Parser::SyntaxError()
 
     std::string message="parse error before `";
 
-    for(unsigned i=0; i<ERROR_TOKENS; i++)
+    for(std::size_t i=0; i<ERROR_TOKENS; i++)
       if(t[i].kind!='\0')
       {
         if(i!=0)
@@ -1695,7 +1695,7 @@ bool Parser::rOtherDeclaration(
 
     assert(!type_name.get_sub().empty());
 
-    for(unsigned i=0; i < type_name.get_sub().size(); i++)
+    for(std::size_t i=0; i < type_name.get_sub().size(); i++)
     {
       if(type_name.get_sub()[i].id() == ID_operator)
       {
diff --git a/src/goto-instrument/document_properties.cpp b/src/goto-instrument/document_properties.cpp
index 5140459c025..d0cae86735e 100644
--- a/src/goto-instrument/document_properties.cpp
+++ b/src/goto-instrument/document_properties.cpp
@@ -78,7 +78,7 @@ void document_propertiest::strip_space(std::list<linet> &lines)
   for(std::list<linet>::const_iterator it=lines.begin();
       it!=lines.end(); it++)
   {
-    for(unsigned j=0; j<strip && j<it->text.size(); j++)
+    for(std::size_t j=0; j<strip && j<it->text.size(); j++)
       if(it->text[j]!=' ')
       {
         strip=j;
@@ -104,7 +104,7 @@ std::string escape_latex(const std::string &s, bool alltt)
 {
   std::string dest;
 
-  for(unsigned i=0; i<s.size(); i++)
+  for(std::size_t i=0; i<s.size(); i++)
   {
     if(s[i]=='\\' || s[i]=='{' || s[i]=='}')
       dest+="\\";
@@ -125,7 +125,7 @@ std::string escape_html(const std::string &s)
 {
   std::string dest;
 
-  for(unsigned i=0; i<s.size(); i++)
+  for(std::size_t i=0; i<s.size(); i++)
   {
     switch(s[i])
     {
@@ -141,7 +141,7 @@ std::string escape_html(const std::string &s)
 
 bool is_empty(const std::string &s)
 {
-  for(unsigned i=0; i<s.size(); i++)
+  for(std::size_t i=0; i<s.size(); i++)
     if(isgraph(s[i]))
       return false;
 
diff --git a/src/goto-programs/builtin_functions.cpp b/src/goto-programs/builtin_functions.cpp
index fbf77309efe..5d7e950653d 100644
--- a/src/goto-programs/builtin_functions.cpp
+++ b/src/goto-programs/builtin_functions.cpp
@@ -514,7 +514,7 @@ void goto_convertt::do_cpp_new(
     new_call.lhs()=tmp_symbol_expr;
     new_call.add_source_location()=rhs.source_location();
 
-    for(unsigned i=0; i<code_type.parameters().size(); i++)
+    for(std::size_t i=0; i<code_type.parameters().size(); i++)
       if(new_call.arguments()[i].type()!=code_type.parameters()[i].type())
         new_call.arguments()[i].make_typecast(code_type.parameters()[i].type());
 
diff --git a/src/goto-programs/elf_reader.cpp b/src/goto-programs/elf_reader.cpp
index 39bc20de7c6..70d8b404977 100644
--- a/src/goto-programs/elf_reader.cpp
+++ b/src/goto-programs/elf_reader.cpp
@@ -54,7 +54,7 @@ elf_readert::elf_readert(std::istream &_in):in(_in)
     number_of_sections=elf32_header.e_shnum;
 
     // iterate over these
-    for(unsigned i=0; i<elf32_section_header_table.size(); i++)
+    for(std::size_t i=0; i<elf32_section_header_table.size(); i++)
     {
       // go to right place
       in.seekg(elf32_header.e_shoff+i*elf32_header.e_shentsize);
@@ -101,7 +101,7 @@ elf_readert::elf_readert(std::istream &_in):in(_in)
     number_of_sections=elf64_header.e_shnum;
 
     // iterate over these
-    for(unsigned i=0; i<elf64_section_header_table.size(); i++)
+    for(std::size_t i=0; i<elf64_section_header_table.size(); i++)
     {
       // go to right place
       in.seekg(elf64_header.e_shoff+i*elf64_header.e_shentsize);
@@ -141,7 +141,7 @@ std::string elf_readert::get_string(std::streampos index) const
 
 bool elf_readert::has_section(const std::string &name) const
 {
-  for(unsigned i=0; i<number_of_sections; i++)
+  for(std::size_t i=0; i<number_of_sections; i++)
     if(section_name(i)==name)
       return true;
 
diff --git a/src/goto-programs/elf_reader.h b/src/goto-programs/elf_reader.h
index 63aa233894c..f3014fad5e3 100644
--- a/src/goto-programs/elf_reader.h
+++ b/src/goto-programs/elf_reader.h
@@ -122,26 +122,26 @@ class elf_readert
   std::streampos string_table_offset;
   std::string get_string(std::streampos index) const;
 
-  std::string elf32_section_name(unsigned index) const
+  std::string elf32_section_name(std::size_t index) const
   {
     return get_string(elf32_section_header_table[index].sh_name);
   }
 
-  std::string elf64_section_name(unsigned index) const
+  std::string elf64_section_name(std::size_t index) const
   {
     return get_string(elf64_section_header_table[index].sh_name);
   }
 
-  unsigned number_of_sections;
+  std::size_t number_of_sections;
 
-  std::string section_name(unsigned index) const
+  std::string section_name(std::size_t index) const
   {
     return
       elf_class==ELF32?elf32_section_name(index):
                        elf64_section_name(index);
   }
 
-  std::streampos section_offset(unsigned index) const
+  std::streampos section_offset(std::size_t index) const
   {
     return
       elf_class==ELF32?elf32_section_header_table[index].sh_offset:
diff --git a/src/goto-programs/goto_convert.cpp b/src/goto-programs/goto_convert.cpp
index b13702d4d43..430a583e69a 100644
--- a/src/goto-programs/goto_convert.cpp
+++ b/src/goto-programs/goto_convert.cpp
@@ -172,7 +172,7 @@ void goto_convertt::finish_gotos(goto_programt &dest)
       bool stack_is_prefix=true;
       if(label_stack.size()>goto_stack.size())
         stack_is_prefix=false;
-      for(unsigned i=0, ilim=label_stack.size();
+      for(std::size_t i=0, ilim=label_stack.size();
           i!=ilim && stack_is_prefix;
           ++i)
       {
@@ -2189,7 +2189,7 @@ void goto_convert(
   goto_programt &dest,
   message_handlert &message_handler)
 {
-  const unsigned errors_before=
+  const std::size_t errors_before=
     message_handler.get_message_count(messaget::M_ERROR);
 
   goto_convertt goto_convert(symbol_table, message_handler);
diff --git a/src/goto-programs/goto_convert_exceptions.cpp b/src/goto-programs/goto_convert_exceptions.cpp
index f34d4d2f3fa..d51a16c13ad 100644
--- a/src/goto-programs/goto_convert_exceptions.cpp
+++ b/src/goto-programs/goto_convert_exceptions.cpp
@@ -127,7 +127,7 @@ void goto_convertt::convert_try_catch(
   // add a goto to the end of the 'try' block
   dest.add_instruction()->make_goto(end_target);
 
-  for(unsigned i=1; i<code.operands().size(); i++)
+  for(std::size_t i=1; i<code.operands().size(); i++)
   {
     const codet &block=to_code(code.operands()[i]);
 
diff --git a/src/goto-programs/goto_trace.h b/src/goto-programs/goto_trace.h
index 2153cffb040..08dd5234e76 100644
--- a/src/goto-programs/goto_trace.h
+++ b/src/goto-programs/goto_trace.h
@@ -35,7 +35,7 @@ Date: July 2005
 class goto_trace_stept
 {
 public:
-  unsigned step_nr;
+  std::size_t step_nr;
 
   bool is_assignment() const      { return type==typet::ASSIGNMENT; }
   bool is_assume() const          { return type==typet::ASSUME; }
diff --git a/src/goto-programs/remove_function_pointers.cpp b/src/goto-programs/remove_function_pointers.cpp
index bec1ce5ea47..4c35d3b0667 100644
--- a/src/goto-programs/remove_function_pointers.cpp
+++ b/src/goto-programs/remove_function_pointers.cpp
@@ -182,7 +182,7 @@ bool remove_function_pointerst::is_type_compatible(
     if(call_parameters.size()!=function_parameters.size())
       return false;
 
-    for(unsigned i=0; i<call_parameters.size(); i++)
+    for(std::size_t i=0; i<call_parameters.size(); i++)
       if(!arg_is_type_compatible(call_parameters[i].type(),
                                  function_parameters[i].type()))
         return false;
@@ -203,7 +203,7 @@ void remove_function_pointerst::fix_argument_types(
   code_function_callt::argumentst &call_arguments=
     function_call.arguments();
 
-  for(unsigned i=0; i<function_parameters.size(); i++)
+  for(std::size_t i=0; i<function_parameters.size(); i++)
   {
     if(i<call_arguments.size())
     {
diff --git a/src/goto-programs/remove_vector.cpp b/src/goto-programs/remove_vector.cpp
index c0e90033968..a5874999b78 100644
--- a/src/goto-programs/remove_vector.cpp
+++ b/src/goto-programs/remove_vector.cpp
@@ -97,7 +97,7 @@ static void remove_vector(exprt &expr)
       array_exprt array_expr(array_type);
       array_expr.operands().resize(integer2size_t(dimension));
 
-      for(unsigned i=0; i<array_expr.operands().size(); i++)
+      for(std::size_t i=0; i<array_expr.operands().size(); i++)
       {
         exprt index=from_integer(i, array_type.size().type());
 
@@ -123,7 +123,7 @@ static void remove_vector(exprt &expr)
       array_exprt array_expr(array_type);
       array_expr.operands().resize(integer2size_t(dimension));
 
-      for(unsigned i=0; i<array_expr.operands().size(); i++)
+      for(std::size_t i=0; i<array_expr.operands().size(); i++)
       {
         exprt index=from_integer(i, array_type.size().type());
 
diff --git a/src/goto-programs/set_properties.cpp b/src/goto-programs/set_properties.cpp
index 43c9ea9107c..a79802118ba 100644
--- a/src/goto-programs/set_properties.cpp
+++ b/src/goto-programs/set_properties.cpp
@@ -45,7 +45,7 @@ void label_properties(goto_modelt &goto_model)
 
 void label_properties(
   goto_programt &goto_program,
-  std::map<irep_idt, unsigned> &property_counters)
+  std::map<irep_idt, std::size_t> &property_counters)
 {
   for(goto_programt::instructionst::iterator
       it=goto_program.instructions.begin();
@@ -75,7 +75,7 @@ void label_properties(
     if(prefix!="")
       prefix+=".";
 
-    unsigned &count=property_counters[prefix];
+    std::size_t &count=property_counters[prefix];
 
     count++;
 
@@ -87,7 +87,7 @@ void label_properties(
 
 void label_properties(goto_programt &goto_program)
 {
-  std::map<irep_idt, unsigned> property_counters;
+  std::map<irep_idt, std::size_t> property_counters;
   label_properties(goto_program, property_counters);
 }
 
@@ -116,7 +116,7 @@ void set_properties(
 
 void label_properties(goto_functionst &goto_functions)
 {
-  std::map<irep_idt, unsigned> property_counters;
+  std::map<irep_idt, std::size_t> property_counters;
 
   for(goto_functionst::function_mapt::iterator
       it=goto_functions.function_map.begin();
diff --git a/src/goto-programs/string_instrumentation.cpp b/src/goto-programs/string_instrumentation.cpp
index 71cf5959e35..6e696c30042 100644
--- a/src/goto-programs/string_instrumentation.cpp
+++ b/src/goto-programs/string_instrumentation.cpp
@@ -126,16 +126,16 @@ class string_instrumentationt:public messaget
     goto_programt &dest,
     goto_programt::const_targett target,
     const code_function_callt::argumentst &arguments,
-    unsigned format_string_inx,
-    unsigned argument_start_inx,
+    std::size_t format_string_inx,
+    std::size_t argument_start_inx,
     const std::string &function_name);
 
   void do_format_string_write(
     goto_programt &dest,
     goto_programt::const_targett target,
     const code_function_callt::argumentst &arguments,
-    unsigned format_string_inx,
-    unsigned argument_start_inx,
+    std::size_t format_string_inx,
+    std::size_t argument_start_inx,
     const std::string &function_name);
 
   bool is_string_type(const typet &t) const
@@ -392,8 +392,8 @@ void string_instrumentationt::do_format_string_read(
   goto_programt &dest,
   goto_programt::const_targett target,
   const code_function_callt::argumentst &arguments,
-  unsigned format_string_inx,
-  unsigned argument_start_inx,
+  std::size_t format_string_inx,
+  std::size_t argument_start_inx,
   const std::string &function_name)
 {
   const exprt &format_arg=arguments[format_string_inx];
@@ -405,7 +405,7 @@ void string_instrumentationt::do_format_string_read(
     format_token_listt token_list=
       parse_format_string(format_arg.op0().op0().get_string(ID_value));
 
-    unsigned args=0;
+    std::size_t args=0;
 
     for(const auto &token : token_list)
     {
@@ -459,7 +459,7 @@ void string_instrumentationt::do_format_string_read(
     comment += function_name;
     format_ass->source_location.set_comment(comment);
 
-    for(unsigned i=2; i<arguments.size(); i++)
+    for(std::size_t i=2; i<arguments.size(); i++)
     {
       const exprt &arg=arguments[i];
       const typet &arg_type=ns.follow(arguments[i].type());
@@ -495,8 +495,8 @@ void string_instrumentationt::do_format_string_write(
   goto_programt &dest,
   goto_programt::const_targett target,
   const code_function_callt::argumentst &arguments,
-  unsigned format_string_inx,
-  unsigned argument_start_inx,
+  std::size_t format_string_inx,
+  std::size_t argument_start_inx,
   const std::string &function_name)
 {
   const exprt &format_arg=arguments[format_string_inx];
@@ -508,7 +508,7 @@ void string_instrumentationt::do_format_string_write(
     format_token_listt token_list=
       parse_format_string(format_arg.op0().op0().get_string(ID_value));
 
-    unsigned args=0;
+    std::size_t args=0;
 
     for(const auto &token : token_list)
     {
@@ -600,7 +600,7 @@ void string_instrumentationt::do_format_string_write(
   }
   else // non-const format string
   {
-    for(unsigned i=argument_start_inx; i<arguments.size(); i++)
+    for(std::size_t i=argument_start_inx; i<arguments.size(); i++)
     {
       const typet &arg_type=ns.follow(arguments[i].type());
 
diff --git a/src/goto-symex/symex_builtin_functions.cpp b/src/goto-symex/symex_builtin_functions.cpp
index 327c31c5247..aede4347214 100644
--- a/src/goto-symex/symex_builtin_functions.cpp
+++ b/src/goto-symex/symex_builtin_functions.cpp
@@ -299,7 +299,7 @@ void goto_symext::symex_printf(
   const exprt::operandst &operands=tmp_rhs.operands();
   std::list<exprt> args;
 
-  for(unsigned i=1; i<operands.size(); i++)
+  for(std::size_t i=1; i<operands.size(); i++)
     args.push_back(operands[i]);
 
   const irep_idt format_string=
@@ -324,7 +324,7 @@ void goto_symext::symex_input(
 
   std::list<exprt> args;
 
-  for(unsigned i=1; i<code.operands().size(); i++)
+  for(std::size_t i=1; i<code.operands().size(); i++)
   {
     args.push_back(code.operands()[i]);
     state.rename(args.back(), ns);
@@ -349,7 +349,7 @@ void goto_symext::symex_output(
 
   std::list<exprt> args;
 
-  for(unsigned i=1; i<code.operands().size(); i++)
+  for(std::size_t i=1; i<code.operands().size(); i++)
   {
     args.push_back(code.operands()[i]);
     state.rename(args.back(), ns);
@@ -470,7 +470,7 @@ void goto_symext::symex_trace(
 
     irep_idt event=code.arguments()[1].op0().get(ID_value);
 
-    for(unsigned j=2; j<code.arguments().size(); j++)
+    for(std::size_t j=2; j<code.arguments().size(); j++)
     {
       exprt var(code.arguments()[j]);
       state.rename(var, ns);
diff --git a/src/goto-symex/symex_function_call.cpp b/src/goto-symex/symex_function_call.cpp
index 06d519e4941..6dc9a944010 100644
--- a/src/goto-symex/symex_function_call.cpp
+++ b/src/goto-symex/symex_function_call.cpp
@@ -138,7 +138,7 @@ void goto_symext::parameter_assignments(
   if(function_type.has_ellipsis())
   {
     // These are va_arg arguments; their types may differ from call to call
-    unsigned va_count=0;
+    std::size_t va_count=0;
     const symbolt *va_sym=nullptr;
     while(!ns.lookup(
         id2string(function_identifier)+"::va_arg"+std::to_string(va_count),
@@ -282,8 +282,8 @@ void goto_symext::symex_function_call_code(
 
   // read the arguments -- before the locality renaming
   exprt::operandst arguments=call.arguments();
-  for(unsigned i=0; i<arguments.size(); i++)
-    state.rename(arguments[i], ns);
+  for(auto &a : arguments)
+    state.rename(a, ns);
 
   // produce a new frame
   assert(!state.call_stack().empty());
diff --git a/src/goto-symex/symex_target_equation.cpp b/src/goto-symex/symex_target_equation.cpp
index 76a6bae061a..5738f3d0c14 100644
--- a/src/goto-symex/symex_target_equation.cpp
+++ b/src/goto-symex/symex_target_equation.cpp
@@ -492,7 +492,7 @@ void symex_target_equationt::convert_assertions(
   // we find out if there is only _one_ assertion,
   // which allows for a simpler formula
 
-  unsigned number_of_assertions=count_assertions();
+  std::size_t number_of_assertions=count_assertions();
 
   if(number_of_assertions==0)
     return;
@@ -557,7 +557,7 @@ void symex_target_equationt::convert_assertions(
 void symex_target_equationt::convert_io(
   decision_proceduret &dec_proc)
 {
-  unsigned io_count=0;
+  std::size_t io_count=0;
 
   for(auto &step : SSA_steps)
     if(!step.ignore)
diff --git a/src/goto-symex/symex_target_equation.h b/src/goto-symex/symex_target_equation.h
index 90073a84195..06fb2342161 100644
--- a/src/goto-symex/symex_target_equation.h
+++ b/src/goto-symex/symex_target_equation.h
@@ -261,9 +261,9 @@ class symex_target_equationt:public symex_targett
       std::ostream &out) const;
   };
 
-  unsigned count_assertions() const
+  std::size_t count_assertions() const
   {
-    unsigned i=0;
+    std::size_t i=0;
     for(SSA_stepst::const_iterator
         it=SSA_steps.begin();
         it!=SSA_steps.end(); it++)
@@ -272,9 +272,9 @@ class symex_target_equationt:public symex_targett
     return i;
   }
 
-  unsigned count_ignored_SSA_steps() const
+  std::size_t count_ignored_SSA_steps() const
   {
-    unsigned i=0;
+    std::size_t i=0;
     for(SSA_stepst::const_iterator
         it=SSA_steps.begin();
         it!=SSA_steps.end(); it++)
@@ -286,7 +286,7 @@ class symex_target_equationt:public symex_targett
   typedef std::list<SSA_stept> SSA_stepst;
   SSA_stepst SSA_steps;
 
-  SSA_stepst::iterator get_SSA_step(unsigned s)
+  SSA_stepst::iterator get_SSA_step(std::size_t s)
   {
     SSA_stepst::iterator it=SSA_steps.begin();
     for(; s!=0; s--)
diff --git a/src/java_bytecode/java_local_variable_table.cpp b/src/java_bytecode/java_local_variable_table.cpp
index e50e96f41d3..4561d24086f 100644
--- a/src/java_bytecode/java_local_variable_table.cpp
+++ b/src/java_bytecode/java_local_variable_table.cpp
@@ -86,10 +86,12 @@ struct procedure_local_cfg_baset<
   {
     return args.second.begin()->first;
   }
+
   unsigned get_last_node(const method_with_amapt &args) const
   {
     return (--args.second.end())->first;
   }
+
   unsigned nodes_empty(const method_with_amapt &args) const
   {
     return args.second.empty();
@@ -485,7 +487,7 @@ static void populate_live_range_holes(
     merge_into,
     expanded_live_range_start,
     sorted_by_startpc[0]->var.start_pc);
-  for(unsigned idx=0; idx<sorted_by_startpc.size()-1; ++idx)
+  for(std::size_t idx=0; idx<sorted_by_startpc.size()-1; ++idx)
   {
     maybe_add_hole(
       merge_into,
diff --git a/src/java_bytecode/java_types.cpp b/src/java_bytecode/java_types.cpp
index c98616d06c0..b3f908a1ca8 100644
--- a/src/java_bytecode/java_types.cpp
+++ b/src/java_bytecode/java_types.cpp
@@ -241,9 +241,9 @@ typet java_type_from_string(const std::string &src)
         return nil_typet();
       std::string class_name=src.substr(1, src.size()-2);
 
-      for(unsigned i=0; i<class_name.size(); i++)
-        if(class_name[i]=='/')
-          class_name[i]='.';
+      for(auto &letter : class_name)
+        if(letter=='/')
+          letter='.';
 
       std::string identifier="java::"+class_name;
       symbol_typet symbol_type(identifier);
diff --git a/src/langapi/language_ui.cpp b/src/langapi/language_ui.cpp
index 625cc819f42..0d515d81afb 100644
--- a/src/langapi/language_ui.cpp
+++ b/src/langapi/language_ui.cpp
@@ -37,11 +37,9 @@ language_uit::~language_uit()
 
 bool language_uit::parse()
 {
-  for(unsigned i=0; i<_cmdline.args.size(); i++)
-  {
-    if(parse(_cmdline.args[i]))
+  for(const auto &filename : _cmdline.args)
+    if(parse(filename))
       return true;
-  }
 
   return false;
 }
diff --git a/src/memory-models/mm2cpp.cpp b/src/memory-models/mm2cpp.cpp
index 65d74b89cf5..a5be8b35d48 100644
--- a/src/memory-models/mm2cpp.cpp
+++ b/src/memory-models/mm2cpp.cpp
@@ -37,9 +37,8 @@ std::string mm2cppt::text2c(const irep_idt &src)
   std::string result;
   result.reserve(src.size());
 
-  for(unsigned i=0; i<src.size(); i++)
+  for(const auto ch : id2string(src))
   {
-    char ch=src[i];
     if(isalnum(ch))
       result+=ch;
     else
diff --git a/src/pointer-analysis/goto_program_dereference.cpp b/src/pointer-analysis/goto_program_dereference.cpp
index 64f9d5fe071..fbcbcde2ee1 100644
--- a/src/pointer-analysis/goto_program_dereference.cpp
+++ b/src/pointer-analysis/goto_program_dereference.cpp
@@ -108,10 +108,8 @@ void goto_program_dereferencet::dereference_rec(
 
     guardt old_guard=guard;
 
-    for(unsigned i=0; i<expr.operands().size(); i++)
+    for(auto &op : expr.operands())
     {
-      exprt &op=expr.operands()[i];
-
       if(!op.is_boolean())
         throw expr.id_string()+" takes Boolean operands only, but got "+
               op.pretty();
diff --git a/src/pointer-analysis/value_set.cpp b/src/pointer-analysis/value_set.cpp
index 70d2ccb7629..58e0fa1312a 100644
--- a/src/pointer-analysis/value_set.cpp
+++ b/src/pointer-analysis/value_set.cpp
@@ -1435,7 +1435,7 @@ void value_sett::do_function_call(
   // to avoid overwriting actuals that are needed for recursive
   // calls
 
-  for(unsigned i=0; i<arguments.size(); i++)
+  for(std::size_t i=0; i<arguments.size(); i++)
   {
     const std::string identifier="value_set::dummy_arg_"+std::to_string(i);
     exprt dummy_lhs=symbol_exprt(identifier, arguments[i].type());
diff --git a/src/pointer-analysis/value_set_fi.cpp b/src/pointer-analysis/value_set_fi.cpp
index 805c20fec39..c18ae55eb70 100644
--- a/src/pointer-analysis/value_set_fi.cpp
+++ b/src/pointer-analysis/value_set_fi.cpp
@@ -990,7 +990,7 @@ void value_set_fit::assign(
   {
     const struct_union_typet &struct_type=to_struct_union_type(type);
 
-    unsigned no=0;
+    std::size_t no=0;
 
     for(struct_typet::componentst::const_iterator
         c_it=struct_type.components().begin();
@@ -1368,7 +1368,7 @@ void value_set_fit::do_function_call(
   // to avoid overwriting actuals that are needed for recursive
   // calls
 
-  for(unsigned i=0; i<arguments.size(); i++)
+  for(std::size_t i=0; i<arguments.size(); i++)
   {
     const std::string identifier="value_set::" + id2string(function) + "::" +
                                  "argument$"+std::to_string(i);
@@ -1379,7 +1379,7 @@ void value_set_fit::do_function_call(
 
   // now assign to 'actual actuals'
 
-  unsigned i=0;
+  std::size_t i=0;
 
   for(code_typet::parameterst::const_iterator
       it=parameter_types.begin();
diff --git a/src/pointer-analysis/value_set_fivr.cpp b/src/pointer-analysis/value_set_fivr.cpp
index c45c999b507..cbe0a652fb8 100644
--- a/src/pointer-analysis/value_set_fivr.cpp
+++ b/src/pointer-analysis/value_set_fivr.cpp
@@ -232,8 +232,8 @@ void value_set_fivrt::flatten_rec(
   const entryt &e,
   object_mapt &dest,
   flatten_seent &seen,
-        unsigned at_function,
-        unsigned at_index) const
+  unsigned at_function,
+  unsigned at_index) const
 {
   #if 0
   std::cout << "FLATTEN_REC: " << e.identifier << e.suffix << '\n';
@@ -1107,7 +1107,7 @@ void value_set_fivrt::assign(
   {
     const struct_typet &struct_type=to_struct_type(type);
 
-    unsigned no=0;
+    std::size_t no=0;
 
     for(struct_typet::componentst::const_iterator
         c_it=struct_type.components().begin();
@@ -1514,7 +1514,7 @@ void value_set_fivrt::do_function_call(
   to_function=from_function;
   to_target_index=from_target_index;
 
-  for(unsigned i=0; i<arguments.size(); i++)
+  for(std::size_t i=0; i<arguments.size(); i++)
   {
     const std::string identifier="value_set::" + id2string(function) + "::" +
                                  "argument$"+std::to_string(i);
@@ -1536,7 +1536,7 @@ void value_set_fivrt::do_function_call(
 
   // now assign to 'actual actuals'
 
-  unsigned i=0;
+  std::size_t i=0;
 
   for(code_typet::parameterst::const_iterator
       it=parameter_types.begin();
diff --git a/src/pointer-analysis/value_set_fivrns.cpp b/src/pointer-analysis/value_set_fivrns.cpp
index 8e4078ded41..7f05d584fdc 100644
--- a/src/pointer-analysis/value_set_fivrns.cpp
+++ b/src/pointer-analysis/value_set_fivrns.cpp
@@ -794,7 +794,7 @@ void value_set_fivrnst::assign(
   {
     const struct_typet &struct_type=to_struct_type(type);
 
-    unsigned no=0;
+    std::size_t no=0;
 
     for(struct_typet::componentst::const_iterator
         c_it=struct_type.components().begin();
@@ -1165,7 +1165,7 @@ void value_set_fivrnst::do_function_call(
   to_function=from_function;
   to_target_index=from_target_index;
 
-  for(unsigned i=0; i<arguments.size(); i++)
+  for(std::size_t i=0; i<arguments.size(); i++)
   {
     const std::string identifier="value_set::" + id2string(function) + "::" +
                                  "argument$"+std::to_string(i);
@@ -1187,7 +1187,7 @@ void value_set_fivrnst::do_function_call(
 
   // now assign to 'actual actuals'
 
-  unsigned i=0;
+  std::size_t i=0;
 
   for(code_typet::parameterst::const_iterator
       it=parameter_types.begin();
diff --git a/src/util/base_type.cpp b/src/util/base_type.cpp
index f8c29a95e0e..62d40ad32e5 100644
--- a/src/util/base_type.cpp
+++ b/src/util/base_type.cpp
@@ -159,7 +159,7 @@ bool base_type_eqt::base_type_eq_rec(
     if(components1.size()!=components2.size())
       return false;
 
-    for(unsigned i=0; i<components1.size(); i++)
+    for(std::size_t i=0; i<components1.size(); i++)
     {
       const typet &subtype1=components1[i].type();
       const typet &subtype2=components2[i].type();
@@ -190,7 +190,7 @@ bool base_type_eqt::base_type_eq_rec(
     if(parameters1.size()!=parameters2.size())
       return false;
 
-    for(unsigned i=0; i<parameters1.size(); i++)
+    for(std::size_t i=0; i<parameters1.size(); i++)
     {
       const typet &subtype1=parameters1[i].type();
       const typet &subtype2=parameters2[i].type();
diff --git a/src/util/cmdline.cpp b/src/util/cmdline.cpp
index a310a6c28cd..a71ca546879 100644
--- a/src/util/cmdline.cpp
+++ b/src/util/cmdline.cpp
@@ -101,7 +101,7 @@ const std::list<std::string> &cmdlinet::get_values(
 
 int cmdlinet::getoptnr(char option) const
 {
-  for(unsigned i=0; i<options.size(); i++)
+  for(std::size_t i=0; i<options.size(); i++)
     if(options[i].optchar==option)
       return i;
 
@@ -110,7 +110,7 @@ int cmdlinet::getoptnr(char option) const
 
 int cmdlinet::getoptnr(const std::string &option) const
 {
-  for(unsigned i=0; i<options.size(); i++)
+  for(std::size_t i=0; i<options.size(); i++)
     if(options[i].optstring==option)
       return i;
 
diff --git a/src/util/config.h b/src/util/config.h
index 4b1b39c1402..8031b872d3e 100644
--- a/src/util/config.h
+++ b/src/util/config.h
@@ -27,17 +27,17 @@ class configt
   struct ansi_ct
   {
     // for ANSI-C
-    unsigned int_width;
-    unsigned long_int_width;
-    unsigned bool_width;
-    unsigned char_width;
-    unsigned short_int_width;
-    unsigned long_long_int_width;
-    unsigned pointer_width;
-    unsigned single_width;
-    unsigned double_width;
-    unsigned long_double_width;
-    unsigned wchar_t_width;
+    std::size_t int_width;
+    std::size_t long_int_width;
+    std::size_t bool_width;
+    std::size_t char_width;
+    std::size_t short_int_width;
+    std::size_t long_long_int_width;
+    std::size_t pointer_width;
+    std::size_t single_width;
+    std::size_t double_width;
+    std::size_t long_double_width;
+    std::size_t wchar_t_width;
 
     // various language options
     bool char_is_unsigned, wchar_t_is_unsigned;
@@ -65,11 +65,11 @@ class configt
     void set_LP32();  // int=16, long=32, pointer=32
 
     // minimum alignment (in structs) measured in bytes
-    unsigned alignment;
+    std::size_t alignment;
 
     // maximum minimum size of the operands for a machine
     // instruction (in bytes)
-    unsigned memory_operand_size;
+    std::size_t memory_operand_size;
 
     enum class endiannesst { NO_ENDIANNESS, IS_LITTLE_ENDIAN, IS_BIG_ENDIAN };
     endiannesst endianness;
@@ -119,7 +119,7 @@ class configt
 
     bool string_abstraction;
 
-    static const unsigned default_object_bits=8;
+    static const std::size_t default_object_bits=8;
   } ansi_c;
 
   struct cppt
@@ -132,7 +132,7 @@ class configt
     void set_cpp11() { cpp_standard=cpp_standardt::CPP11; }
     void set_cpp14() { cpp_standard=cpp_standardt::CPP14; }
 
-    static const unsigned default_object_bits=8;
+    static const std::size_t default_object_bits=8;
   } cpp;
 
   struct verilogt
@@ -146,16 +146,16 @@ class configt
     classpatht classpath;
     irep_idt main_class;
 
-    static const unsigned default_object_bits=16;
+    static const std::size_t default_object_bits=16;
   } java;
 
   struct bv_encodingt
   {
     // number of bits to encode heap object addresses
-    unsigned object_bits;
+    std::size_t object_bits;
     bool is_object_bits_default;
 
-    static const unsigned default_object_bits=8;
+    static const std::size_t default_object_bits=8;
   } bv_encoding;
 
   // this is the function to start executing
diff --git a/src/util/irep.cpp b/src/util/irep.cpp
index 96c92b6174d..af2f3f18831 100644
--- a/src/util/irep.cpp
+++ b/src/util/irep.cpp
@@ -675,7 +675,7 @@ std::string irept::pretty(unsigned indent, unsigned max_indent) const
     result+=it->second.pretty(indent+2, max_indent);
   }
 
-  unsigned count=0;
+  std::size_t count=0;
 
   forall_irep(it, get_sub())
   {
diff --git a/src/util/json_expr.cpp b/src/util/json_expr.cpp
index 2fbb8155714..b2aa92bb199 100644
--- a/src/util/json_expr.cpp
+++ b/src/util/json_expr.cpp
@@ -353,7 +353,7 @@ json_objectt json(
     result["name"]=json_stringt("array");
     json_arrayt &elements=result["elements"].make_array();
 
-    unsigned index=0;
+    std::size_t index=0;
 
     forall_operands(it, expr)
     {
@@ -377,7 +377,7 @@ json_objectt json(
         "number of struct components should match with its type");
 
       json_arrayt &members=result["members"].make_array();
-      for(unsigned m=0; m<expr.operands().size(); m++)
+      for(std::size_t m=0; m<expr.operands().size(); m++)
       {
         json_objectt &e=members.push_back().make_object();
         e["value"]=json(expr.operands()[m], ns, mode);
diff --git a/src/util/lispexpr.cpp b/src/util/lispexpr.cpp
index 80f62d15c3f..3156fb6374f 100644
--- a/src/util/lispexpr.cpp
+++ b/src/util/lispexpr.cpp
@@ -23,7 +23,7 @@ std::string lispexprt::expr2string() const
 
     case List:
       result="(";
-      for(unsigned j=0; j<size(); j++)
+      for(std::size_t j=0; j<size(); j++)
       {
         if((j+1)==size() && size()!=1)
         {
@@ -151,7 +151,7 @@ std::string escape(const std::string &s)
 {
   std::string result;
 
-  for(unsigned i=0; i<s.size(); i++)
+  for(std::size_t i=0; i<s.size(); i++)
   {
     if(s[i]=='\\' || s[i]=='"')
       result+='\\';
diff --git a/src/util/lispirep.cpp b/src/util/lispirep.cpp
index 22e3e0b99a9..6f223d0c7f6 100644
--- a/src/util/lispirep.cpp
+++ b/src/util/lispirep.cpp
@@ -21,7 +21,7 @@ void lisp2irep(const lispexprt &src, irept &dest)
 
   dest.id(src[0].value);
 
-  for(unsigned i=1; i<src.size(); i++)
+  for(std::size_t i=1; i<src.size(); i++)
     if(!src[i].is_nil())
     {
       const std::string &name=src[i].value;
diff --git a/src/util/mp_arith.cpp b/src/util/mp_arith.cpp
index 6de218f24ed..2ee5fd05f0d 100644
--- a/src/util/mp_arith.cpp
+++ b/src/util/mp_arith.cpp
@@ -56,7 +56,7 @@ std::ostream &operator<<(std::ostream &out, const mp_integer &n)
 /// \return mp_integer
 const mp_integer string2integer(const std::string &n, unsigned base)
 {
-  for(unsigned i=0; i<n.size(); i++)
+  for(std::size_t i=0; i<n.size(); i++)
     if(!(isalnum(n[i]) || (n[i]=='-' && i==0)))
       return 0;