problem with array of vectors

[carl.alv]

Hi all!

I have this function which takes as an input two arrays of vectors declared as


vector<int> v1[N], v2[N];

The function is


void fun3(Base** b, vector<int>* v1, vector<int>* v2, int N){
for(int i = 0; i < N; i++){
v1[i].clear();
v2[i].clear();
}
int cont = 0;
for(int i = 0; i < N-1; i++){
int vali = b[i]->get_priv();
for(int j = i+1; j < N; j++){
int valj = b[j]->get_priv();
int dif = vali - valj;
if(dif < 0){
v1[i].push_back(j);
cout << j << " " << v1[i][cont] << " ";
v2[j].push_back(i);
cout << i << " " << v2[j][cont] << endl;
cont++;
}
}
cout<<endl;
}
}

which for N = 4 I expect to print


1 1 0 0
2 2 0 0
3 3 0 0

2 2 1 1
3 3 1 1

3 3 2 2
but instead i'm getting

1 1 0 0
2 2 0 0
3 3 0 0

2 0 1 0
3 0 1 0

3 0 2 0

I suppose that there is a memory allocation problem due to my wrong implementation of the vector array, but I don't know where is my fault. Can someone help me??

[smartbei]

Ok, your problem is your use of the cont variable.

After the first run (which works for v1, not for v2 (it appears to, more on that in a bit)) cont = 3 right?

Now, i = 1. The problem is that the length of v1[1] = 1 (after the first push_back), so you are trying to access v1[3].

Here is how to solve. Firstly, the stl vector library provides a conveniant .at() method, which is just like brackets but imposes range safety - so if you try to access a value beyond the range of the defined vector elements it will raise an error. Use it. :)

In your case though, you can just replace cont with v1[i].size()-1, so it would be (just the interesting part):


if (dif < 0){
v1[i].push_back(j);
cout << j << " " << v1[i][v1[i].size()-1] << " ";
v2[j].push_back(i);
cout << i << " " << v2[j][v2[j].size()-1] << endl;
}

Since you just need to print the values. Note that I have not tested this - there may be syntax errors.

The reason that it appears to work on the first run for v2 is that the values are supposed to be zero anyway.

[carl.alv]

Thank you for your solution smartbei!!!

[hod139]

Since you have a solution, hopefully you don't mind me taking this thread off topic. Why are mixing C-style arrays with vectors? Why not use a vector of vectors? Also, why are you using C-style arrays of your class Base, and not vectors?