.begin()

MamtaWardhani's avatar
MamtaWardhani's avatar
MamtaWardhani
Published May 16, 2025
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The .begin() function is a built-in method in the C++ Standard Template Library (STL) that retrieves an iterator pointing to the first element in a std::map container. This method is essential for accessing and traversing the elements stored in a map.

The .begin() method returns an iterator that provides access to the first key-value pair in the map, determined by the map’s sorting criterion. Maps in C++ store elements in a sorted order based on their keys, and .begin() gives access to the element that comes first in this order.

Syntax

map_name.begin();

Parameters:

This function does not take any parameters.

Return value:

The .begin() method returns:

  • An iterator pointing to the first element in the map container.
  • If the map is empty, it returns an iterator equivalent to .end().

Example 1: Basic Usage of Map Begin

This example demonstrates how to use the .begin() method to access the first element in a map:

#include <iostream>
#include <map>
#include <string>


int main() {
  // Create a map with integer keys and string values
  std::map<int, std::string> studentNames = {
    {101, "Alice"},
    {102, "Bob"},
    {103, "Charlie"}
  };


  // Access the first element using begin()
  auto firstElement = studentNames.begin();


  // Print the first key-value pair
  std::cout << "First student ID: " << firstElement->first << std::endl;
  std::cout << "First student name: " << firstElement->second << std::endl;


  return 0;
}

This example results in the following output:

First student ID: 101
First student name: Alice

Example 2: Traversing a Map Using .begin() and .end()

This example shows how to traverse all elements in a map using the .begin() method alongside .end():

#include <iostream>
#include <map>
#include <string>


int main() {
  // Create a map of country codes and their names
  std::map<std::string, std::string> countryCodes = {
    {"US", "United States"},
    {"UK", "United Kingdom"},
    {"FR", "France"},
    {"DE", "Germany"},
    {"JP", "Japan"}
  };


  // Traverse the map using begin() and end()
  std::cout << "Country codes and names:" << std::endl;
  for (auto it = countryCodes.begin(); it != countryCodes.end(); ++it) {
    std::cout << it->first << ": " << it->second << std::endl;
  }
  return 0;
}

This example results in the following output:

Country codes and names:
DE: Germany
FR: France
JP: Japan
UK: United Kingdom
US: United States

Notice how the output is sorted by the keys (country codes), as maps automatically maintain their elements in sorted order.

Codebyte Example: Modifying Map Values Using .begin()

This example demonstrates how to modify values in a map using the iterator returned by .begin():

Code
Output
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Frequently Asked Questions


1. How to initialize a map in C++?


A map in C++ can be initialized in several ways:


    

  • Method 1: Using initializer list (C++11 and later)
    • 



std::map<int, std::string> map1 = {
  {1, "one"},
  {2, "two"},
  {3, "three"}
}




    

  • Method 2: Using insert() method
    • 



std::map<int, std::string> map2;
map2.insert({1, "one"});
map2.insert({2, "two"});
map2.insert({3, "three"});




    

  • Method 3: Using emplace() method (more efficient in C++11 and later)
    • 



std::map<int, std::string> map3;
map3.emplace(1, "one");
map3.emplace(2, "two");
map3.emplace(3, "three");




    

  • Method 4: Using square bracket operator
    • 



std::map<int, std::string> map4;
map4[1] = "one";
map4[2] = "two";
map4[3] = "three";




2. How to get first element in map in C++?


The first element in a map can be accessed using the .begin() method:


std::map<int, std::string> myMap = {{1, "one"}, {2, "two"}, {3, "three"}};


// Get the first element
auto firstElement = myMap.begin();


// Access key and value
int firstKey = firstElement->first;      // 1
std::string firstValue = firstElement->second;  // "one"


// Alternative syntax
int firstKey2 = (*firstElement).first;
std::string firstValue2 = (*firstElement).second;




Note that “first” here means the element with the smallest key according to the map’s sorting criterion, not necessarily the first element inserted.


3. Can I use .begin() to modify the contents of a map?


Yes, you can use the iterator returned by .begin() to modify the values of a map, but not the keys. In C++, map keys are constant (const), so they cannot be changed through the iterator.


Here’s an example:


std::map<std::string, int> scores = {
  {"Alice", 85},
  {"Bob", 90}
};


auto it = scores.begin();
it->second = 95; // This is allowed – modifies value


// it->first = "Charlie"; // Not allowed – keys are read-only




If you need to change a key, the only way is to remove the key-value pair and insert a new one with the updated key.


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