Initialize a C++ vector with a variable inital value

Question

I was coding up a Union find data structure , and was trying to initialize the parent vector with a value parent[i]=i, Is there a way in c++ to initialize the vector like this , that is declaring a vector of size N , and not assigning fixed values to each element, rather position dependent value to each element. (without using any obvious for loops)

This is what I was looking for:
  std::vector<int> parent(Initializer);

where Initializer is some class or a function.

To try out my hand a bit, I wrote this:

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

class Initializer {
private:
    static int i;
public:
    int operator() ()
    {
        return i++;
    }
};

int main()
{
    vector<int> parent(Initializer);
    cout << parent[0];
    return 0;
}

However I think I have messed up my concepts pretty bad here, and I am not getting what the declaration means, or what it is doing.

Please answer both the questions,

(1) How to initialize a vector with variable initial values.

(2) What exactly is the code I wrote doing?

score 5 · Accepted Answer · edited May 23 '17 at 12:27

This is a function declaration:

vector<int> parent(Initializer);

Becasue Initializer is a type name, you declared a function parent that takes Initializer as a (unnamed) parameter and returns vector<int>. See Most vexing parse.

To do what you want, you can do this:

std::vector<int> parent(N);    // where N is the size you want
std::iota(parent.begin(), parent.end(), 0);  // fill it with consecutive values
                                             // starting  with 0

There's std::generate algorithm that you can use to save result of a function (or function object) in a range:

std::generate(parent.begin(), parent.end(), Initializer());

Live demo.

Thanks! But is it possible to initialize some "functionally dependant values " , like i*i or sqrt(i). — Dref D, Mar 18 '14 at 06:36

score 2 · Answer 2 · answered Mar 18 '14 at 06:40

There are several alternatives. If you want to initialize the vector with increasing values, then you can use std::iota.

std::vector<int> vec(size);
std::iota(std::begin(vec), std::end(vec), 0);

If you want something more general you could use std::generate.

std::vector<int> vec(size);
int n = 0;
std::generate(std::begin(vec), std::end(vec), [&n]() {return n++;});

Initialize a C++ vector with a variable inital value

2 Answers2