# std::map<Key,T,Compare,Allocator>::begin,std::map<Key,T,Compare,Allocator>::cbegin (3) - Linux Manuals

## NAME

std::map<Key,T,Compare,Allocator>::begin,std::map<Key,T,Compare,Allocator>::cbegin - std::map<Key,T,Compare,Allocator>::begin,std::map<Key,T,Compare,Allocator>::cbegin

## Synopsis

iterator begin(); (until C++11)
iterator begin() noexcept; (since C++11)
const_iterator begin() const; (until C++11)
const_iterator begin() const noexcept; (since C++11)
const_iterator cbegin() const noexcept; (since C++11)

Returns an iterator to the first element of the container.
If the container is empty, the returned iterator will be equal to end().
range-begin-end.svg

(none)

## Return value

Iterator to the first element

Constant

## Example

// Run this code

#include <iostream>
#include <map>

int main() {
std::map<int, float> num_map;
num_map[4] = 4.13;
num_map[9] = 9.24;
num_map[1] = 1.09;
// calls a_map.begin() and a_map.end()
for (auto it = num_map.begin(); it != num_map.end(); ++it) {
std::cout << it->first << ", " << it->second << '\n';
}
}

## Output:

1, 1.09
4, 4.13
9, 9.24

Example using a custom comparison function

// Run this code

#include <cmath>
#include <iostream>
#include <map>

struct Point { double x, y; };

typedef Point * PointPtr;
//Compare the x-coordinates of two Point pointers
struct PointCmp {
bool operator()(const PointPtr &lhs, const PointPtr &rhs) const {
return lhs->x < rhs->x;
}
};

int main() {
//Note that although the x-coordinates are out of order, the
// map will be iterated through by increasing x-coordinates
Point points[3] = { {2, 0}, {1, 0}, {3, 0} };

//mag is a map sending the address of node to its magnitude in the x-y plane
//Although the keys are pointers-to-Point, we want to order the map by the
// x-coordinates of the points and NOT by the addresses of the Points. This
// is done by using the PointCmp class's comparison method.
std::map<Point *, double, PointCmp> mag({
{ points, 2 },
{ points + 1, 1 },
{ points + 2, 3 }
});

//Change each y-coordinate from 0 to the magnitude
for(auto iter = mag.begin(); iter != mag.end(); ++iter){
auto cur = iter->first; // pointer to Node
cur->y = mag[cur]; // could also have used cur->y = iter->second;
}

//Update and print the magnitude of each node
for(auto iter = mag.begin(); iter != mag.end(); ++iter){
auto cur = iter->first;
mag[cur] = std::hypot(cur->x, cur->y);
std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
std::cout << iter->second << '\n';
}

//Repeat the above with the range-based for loop
for(auto i : mag) {
auto cur = i.first;
cur->y = i.second;
mag[cur] = std::hypot(cur->x, cur->y);
std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
std::cout << mag[cur] << '\n';
//Note that in contrast to std::cout << iter->second << '\n'; above,
// std::cout << i.second << '\n'; will NOT print the updated magnitude
}
}

## Output:

The magnitude of (1, 1) is 1.41421
The magnitude of (2, 2) is 2.82843
The magnitude of (3, 3) is 4.24264
The magnitude of (1, 1.41421) is 1.73205
The magnitude of (2, 2.82843) is 3.4641
The magnitude of (3, 4.24264) is 5.19615