Calling a function on every element of a C++ vector - c++

Question

In C++, is there a way to call a function on each element of a vector, without using a loop running over all vector elements? Something similar to a 'map' in Python.

Answer 1

You've already gotten several answers mentioning std::for_each.
While these respond to the question you've asked, I'd add that at least in my experience, std::for_each is about the least useful of the standard algorithms.
I use (for one example) std::transform, which is basically a[i] = f(b[i]); or result[i] = f(a[i], b[i]); much more frequently than std::for_each. Many people frequently use std::for_each to print elements of a collection; for that purpose, std::copy with an std::ostream_iterator as the destination works much better.

Answer 2

Yes: std::for_each.
#include <algorithm> //std::for_each
void foo(int a) {
std::cout << a << "\n";
}
std::vector<int> v;
...
std::for_each(v.begin(), v.end(), &foo);

Answer 3

On C++ 11: You could use a lambda. For example:
std::vector<int> nums{3, 4, 2, 9, 15, 267};
std::for_each(nums.begin(), nums.end(), [](int &n){ n++; });
ref: http://en.cppreference.com/w/cpp/algorithm/for_each

Answer 4

If you have C++11, there's an even shorter method: ranged-based for. Its purpose is exactly this.
std::vector<int> v {1,2,3,4,5};
for (int element : v)
std::cout << element; //prints 12345
You can also apply references and const to it as well, when appropriate, or use auto when the type is long.
std::vector<std::vector<int>> v {{1,2,3},{4,5,6}};
for (const auto &vec : v)
{
for (int element : vec)
cout << element;
cout << '\n';
}
Output:
123
456

Answer 5

The OP mentions the map function in Python.
This Python function actually applies a function to every element of a list (or iterable) and returns a list (or iterable) that collects all results.
In other words, it does something like this:
def f( x ) :
""" a function that computes something with x"""
# code here
return y
input = [ x1, x2, x3, ... ]
output = map( func, input )
# output is now [ f(x1), f(x2), f(x3), ...]
Hence, the closest C++ standard-library equivalent to Python's map is actually std::transform (from the <algorithm> header).
Example usage is as follows:
#include <vector>
#include <algorithm>
using namespace std;
double f( int x ) {
// a function that computes the square of x divided by 2.0
return x * x / 2.0 ;
}
int main( ) {
vector<int> input{ 1, 5, 10 , 20};
vector<double> output;
output.resize( input.size() ); // unfortunately this is necessary
std::transform( input.begin(), input.end(), output.begin(), f );
// output now contains { f(1), f(5), f(10), f(20) }
// = { 0.5, 12.5, 50.0, 200.0 }
return 0;
}

Answer 6

Use for_each:
// for_each example
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
void myfunction (int i) {
cout << " " << i;
}
struct myclass {
void operator() (int i) {cout << " " << i;}
} myobject;
int main () {
vector<int> myvector;
myvector.push_back(10);
myvector.push_back(20);
myvector.push_back(30);
cout << "myvector contains:";
for_each (myvector.begin(), myvector.end(), myfunction);
// or:
cout << "\nmyvector contains:";
for_each (myvector.begin(), myvector.end(), myobject);
cout << endl;
return 0;
}

Answer 7

You can use std::for_each which takes a pair of iterators and a function or functor.

Answer 8

Thought I would share std::ranges equivalents for for_each and transform, should anyone prefer them:
std::vector<int> v;
std::ranges::for_each(v,[](const auto& n) {});
const auto squared = v | std::views::transform([](const auto& n) { return n*2; });
Running on godbolt: https://godbolt.org/z/zYME6b