Creating a function: replacing random index values

I am trying to write a function that would be able to look at all the indexes of a vector which have no values stored in them and then replace one of them randomly with the number 2.

How it works(or at least how it should):

Find number of indices with zero ---> used this to find a random number between 1 and number of zero founs ---> finds that index and changes it to 2.

int random_num(std::vector<int>&v){

    int length = std::sqrt(v.size());
    int count = 0;

    for(int i = 0; i < v.size(); i++){
        if(v[i] = 0){
            count++;
        }
    }

    srand(time(0));
    int tmp = (rand()%(count+1));

    for(int i = 0; i < length; i++){
        if(v[i] = 0 && i == (tmp-1)){
            v[i] = 2;
        }
    }
}

The only issue here is that when I compile this, the values of the vector all come out as 0 when I print them out. What is wrong with this?