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

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.

Solution 1

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);

Solution 2

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.

Solution 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

Solution 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

Solution 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;
}   

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Comments

[ervinbosenbacher]

This syntax is not yet supported by vs2011 for example. At least not my copy.But +1.

[chris]

@xebo, ah, that sucks seeing as how this is one of the best features of C++11 imo.

[Steve Jessop]

This is the answer I was looking for, transform is closer to Python's map than for_each is, since map generates a list of outputs, whereas for_each calls a function but discards the outputs.

[chris]

For any future visitors, ranged-for was added to Visual Studio 2012.

[Alex Bitek]

How does this answer the question? Where in your code do you apply a function to each element of a std::vector ? I only see iteration. Moreover, your first example copies the elements, and the second example passes them by const &, therefore any modifications are done on the copies or can't be done at all.

[chris]

@BadDesign, Technically, outputting it is applying a function, operator<<(std::cout, element). My main goal with the post was to open the OP (and any future visitors) to this syntax. The reason for not wanting to loop through was not stated and could just as well have been not wanting to type it all out. There's a loop present in for_each as well. The second example is just that - an example. It shows that you can use either or, which leaves people free to change it as they wish. Besides, who said you need the element to be modifiable to call a function on it?

[abcd]

would one expect speed improvements from using this (as opposed to the loop)?

[Alexis Wilke]

@dbliss, the implementation of for_each() is likely a loop just the same and since it uses templates, there should be no difference. However, with C++17, they may introduce parallelism which would be totally transparent to you and it could very well end up begin faster.

[sanjivgupta]

std::for_each(v.begin(), v.end(), &foo); OR std::for_each(v.begin(), v.end(), foo);

[KulaGGin]

Why not just use range-based for(const auto& item : myCollection)? Just less code if you have another function in the class or header/source file, or when you use a lambda.

[KulaGGin]

Or not use lambda and use a range-based for loop: for(int& n: nums) n++;. Less code - easier to understand.

[Jerry Coffin]

@KulaGGin: Mostly because the question specifies: "...without using a loop running over all vector elements?" Admittedly, that may not be the most reasonable request, but suggesting a loop when the question is specifically about not using a loop wouldn't really answer the question.

[KulaGGin]

Oops, sorry, my bad, I missed that. I did read the title and skimmed through the text.

[gordon_freeman]

This should be the top answer imo.