Question

Suppose one wants to fill a vector with random numbers. Then there is a following obvious solution:

vector<int> result;
result.resize(n);
for (int i = 0; i < n; ++i) {
    result[i] = generateRandomNumber();
}

OK, it obviously works, but I would like to understand what is the simplest STL/Boost way to get rid of the for loop. It is tempting to use std::transform, but it takes a function with one argument. Is there any nice STL way to introduce a dummy argument in a function?

Answer 1

The C++ standard library has std::generate() and std::generate_n();

For example:

#include <iostream>
#include <cstdlib>
#include <algorithm>
#include <vector>
#include <iterator>
int generateRandomNumber()
{
    return std::rand();
}
int main()
{
    int n = 10;
    std::vector<int> result;
    generate_n(back_inserter(result), n, generateRandomNumber);
    copy(result.begin(), result.end(), std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';
}

test: https://ideone.com/5xD6P

As for the second question, if I understand it correctly, is how to create a functor that takes an int argument, ignores it, and calls your int f()?

C++98 way is to actually write the whole functor:

struct IgnoreArgument
{
    typedef int(*fp_t)();
    fp_t fp;
    IgnoreArgument(fp_t f) : fp(f) {}
    int operator()(int) const { return fp(); }
};
...
transform(v.begin(), v.end(), v.begin(), IgnoreArgument(f));

test: https://ideone.com/DTsyl

C++11 way is to use a lambda expression

transform(v.begin(), v.end(), v.begin(), [](int){return f();});

test: https://ideone.com/nAPXI

And the C++98/boost way is to use boost::bind

transform(v.begin(), v.end(), v.begin(), boost::bind(f));

test: https://ideone.com/cvd88