CODEWITHSUNDEEP
c++functionpointersvector
user3348949
user3348949

Reputation: 344

Using functions on vectors in C++

I'm trying to change the data inside a vector's element by using a function. It happens that the elements are changed just inside the function. How can I keep the changings outside the function? Do I must use pointers?

The code:

#include <iostream>
#include <vector>

using namespace std;

void populate( int size_, vector<int> pop)
{
    //Set the vector's size and populate the vector
    pop.resize(size_);
    for(int i = 0; i<3 ; i++)
    {
        pop[i] = i;
    }
}

int main()
{
    vector<int> vec;
    int size = 3;
    populate(size, vec);

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

The output at cout should be : 0 1 2 But it is empty.

Upvotes: 1

Views: 175

Answers (3)

jrok
jrok

Reputation: 55395

What you're trying to do is easily and idiomaticaly done with standard library facilities:

int size = 3;
std::vector<int> vec(size);
std::iota(vec.begin(), vec.end(), 0);  // include algorithm for this

Upvotes: 3

Cory Kramer
Cory Kramer

Reputation: 117866

You need to take the vector by reference

void populate( int size_, vector<int>& pop)

Otherwise you are passing in a copy of the vector, populating it, then returning, but the original vector is unmodified.

Or as @juanchopanza recommended, since the sole purpose of this function is to make this vector for you, it could be

vector<int> populate( int size_ )
{
    vector<int> temp(size_);
    for(int i = 0; i < size_ ; i++)
    {
        pop[i] = i;
    }
    return temp;
}

Then

int main()
{
    int size = 3;
    vector<int> vec = populate(size, vec);
    for(vector<int>::iterator it = vec.begin(); it != vec.end(); ++it)
    {
        cout << *it << endl;
    }   
}

Upvotes: 2

Steven Lu
Steven Lu

Reputation: 43427

You are sending in the vector to populate by value. This creates a copy, so pop is a copy of vec. Changes you make only affect pop.

Upvotes: 0

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