How to create a map function in c++?

问题:

Say there is a list of integers [1,2,3,4,5] and a map function that multiplies each element with 10 and returns modified list as [10,20,30,40,50] , with out modifying the original list. How this can be done efficiently in c++.

回答1:

Here's an example:

#include <vector>
#include <iostream>
#include <algorithm>

using namespace std;

int multiply(int);

int main() {
    vector<int> source;
    for(int i = 1; i <= 5; i++) {
    source.push_back(i);
    }

    vector<int> result;
    result.resize(source.size());
    transform(source.begin(), source.end(), result.begin(), multiply);

    for(vector<int>::iterator it = result.begin(); it != result.end(); ++it) {
        cout << *it << endl;
    }
}

int multiply(int value) {
    return value * 10;
}

回答2:

Along the lines of @darids answer, but C++03 (current at the time of original post):

#include <vector>
#include <algorithm>
#include <functional>

std::vector<int> src;
std::vector<int> dst;

std::transform(src.begin(), src.end(),
               std::back_inserter(dst),
               std::bind1st(std::multiplies<int>(), 10));

回答3:

I only post this to illustrate using a functor in transform rather than a global function:

#include <cstdlib>
#include <iostream>
#include <algorithm>
#include <functional>
#include <iostream>

using namespace std;

struct MulBy : public std::unary_function<int, int>
{
    MulBy(int v) : v_(v) {}
    int operator()(int lhs) const
    {
        return lhs * v_;
    }
private:
    int v_;
};

int main()
{
    int incoming[5] = {1, 2, 3, 4, 5};
    int result[5] = {0, 0, 0, 0, 0};
    transform(&incoming[0], &incoming[5], &result[0], MulBy(10));
    copy(&result[0], &result[5], ostream_iterator<int>(cout, " "));
    return 0;
}

回答4:

If you can use it, probably the best idea is to use a function in the Standard Template Library.

For example, you might want to check out for_each or transform, which basically do just that.

回答5:

   #include <iostream>
    #include <algorithm>
    #include <vector>
    #include <functional>
    using namespace std;

    struct MulBy : public std::unary_function<int, int>
    {
        MulBy(int v) : v_(v) {}
        int operator()(int lhs) const
        {
            return lhs * v_;
        }
    private:
        int v_;
    };
    int main()
    {
        vector<int> ListOfNumber;
        ListOfNumber.push_back(1);
        ListOfNumber.push_back(2);
        ListOfNumber.push_back(3);
        ListOfNumber.push_back(4);
        ListOfNumber.push_back(5);
        vector<int> ListResult;
        ListResult.resize(ListOfNumber.size());

        //Produces a new list
        transform(ListOfNumber.begin(),ListOfNumber.end(),ListResult.begin(),MulBy(10));
     copy(ListOfNumber.begin(),ListOfNumber.end(),ostream_iterator<int>(cout,"\t"));
        //Modifies the original list
    transform(ListOfNumber.begin(),ListOfNumber.end(),ListOfNumber.begin(),MulBy(10));
    copy(ListResult.begin(),ListResult.end(),ostream_iterator<int>(cout,"\t"));
        cin.get();
    }