How to Map or Transform a Vector in C++ (Template Function)?

How to Map or Transform a Vector in C++ (Template Function)? | ninjasquad

Given a vector, e.g. of integers, we want to apply a function to each element and transform to another vector/array. For example, we might want to return a new array that squares each elements in the original array or list. C++ STL doesn’t provide the map function but we can provide a template function to convert an iterator (not necessary vector) to another using the given function.

$ cat test.cpp && make test && ./test
#include <iostream>
#include <vector>
 
using namespace std;
 
template<typename T, typename F>
T convert(T input, F f) {
    T out;
    for (auto x = input.begin(); x != input.end(); x++) {
        out.push_back(f(*x));
    }
    return out;
}
 
int main()
{
    vector<int> a({1, 2, 3});
    auto b = convert(a, [](const int& n) { return n * n; });
    for (int i = 0; i < static_cast<int>(b.size()); i++) {
        cout << b[i] << endl;
    }
}
make: 'test' is up to date.
1
4
9

$ cat test.cpp && make test && ./test
#include <iostream>
#include <vector>

using namespace std;

template<typename T, typename F>
T convert(T input, F f) {
    T out;
    for (auto x = input.begin(); x != input.end(); x++) {
        out.push_back(f(*x));
    }
    return out;
}

int main()
{
    vector<int> a({1, 2, 3});
    auto b = convert(a, [](const int& n) { return n * n; });
    for (int i = 0; i < static_cast<int>(b.size()); i++) {
        cout << b[i] << endl;
    }
}
make: 'test' is up to date.
1
4
9

If we want to directly modify the input, we can use the transform function from the STL::algorithm. The transform function takes four parameters which are:

begin of source iterator
end of source iterator
begin of destination iterator, could be the same of the begin of the source iterator
transform function

$ cat test.cpp && make && ./test
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
 
int main() {
        vector<int> data({1,2,3,4,5});
 
        std::transform(begin(data), end(data), begin(data), [](auto &n) { return n * n; });
        std::for_each(begin(data), end(data), [](auto &c) { cout << c << ", "; });
        cout << endl;
        return 0;
}
 
g++ -o test test.cpp
1, 4, 9, 16, 25,

$ cat test.cpp && make && ./test
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

int main() {
        vector<int> data({1,2,3,4,5});

        std::transform(begin(data), end(data), begin(data), [](auto &n) { return n * n; });
        std::for_each(begin(data), end(data), [](auto &c) { cout << c << ", "; });
        cout << endl;
        return 0;
}

g++ -o test test.cpp
1, 4, 9, 16, 25,

The Makefile:

CC=g++

# compile to binary test
test: test.cpp
        $(CC) -o test test.cpp

# delete the binary
clean:
        rm -f test

–EOF (The Ultimate Computing & Technology Blog) —

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