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How to: Iterate Over STL Collection with for each

Visual Studio 2008

In addition to .NET Framework collections, the for each keyword can also be used to iterate over Standard C++ Library (STL) collections. An STL collection is also known as a container. For more information, see STL Containers.

Example

This sample uses for each in a <map>.

Copy

// for_each_stl.cpp
// compile with: /EHsc
#include <map>
#include <iostream>
#include <string>
using namespace std;

int main() {
   int retval  = 0;
   map<string, int> months;

   months["january"] = 31;
   months["february"] = 28;
   months["march"] = 31;
   months["april"] = 30;
   months["may"] = 31;
   months["june"] = 30;
   months["july"] = 31;
   months["august"] = 31;
   months["september"] = 30;
   months["october"] = 31;
   months["november"] = 30;
   months["december"] = 31;

   map<string, int> months_30;

   for each( pair<string, int> c in months )
      if ( c.second == 30 )
         months_30[c.first] = c.second;

   for each( pair<string, int> c in months_30 )
      retval++;

   cout << "Months with 30 days = " << retval << endl;
}

Months with 30 days = 4

This sample uses const& for an iteration variable with STL containers. You can use an & as an iteration variable on any collection of a type that can be declared as a T&.

Copy

// for_each_stl_2.cpp
// compile with: /EHsc
#include <vector>
#include <iostream>
using namespace std;

int main() {
   int retval = 0;
      
   vector<int> col(3);
   col[0] = 10;
   col[1] = 20;
   col[2] = 30;
   
   for each( const int& c in col )
      retval += c;

   cout << "retval: " << retval << endl;
}

retval: 60

Reference

for each, in

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FAQ

A Note on enumerating over a std::map and stdext::hash_map.

When enumerating over a map, both ordered and unordered(hash_map), the type for the loop identifier must be std::pair<const KeyType, ValueType> if you wish to get a const reference to the current element. This is especially true when the value type is another container or class type. When getting a temperary copy, any compatible std::pair will work otherwise.

Example:

for (int i = 0; i != 5; ++i)
	for (int c = 0; c != 5; ++c)
		mv[i].push_back(c);

for each (const std::pair<const int, std::vector<int>> & p in mv)
	for each (int i in p.second)
		std::cout << "mv[" << p.first << "][" << i <<
"] = " << i << std::endl;

History

10/17/2008
6XGate