Strange output with large vectors - c++

Question

I was solving AMR10G problem on spoj. The problem is just about sorting and is trivial to implement with arrays. I'm a beginner in STL and just to get familiar with STL i was trying to solve it with using some vectors. The code runs fine with small sizes of vector but with large sizes( can be 20,000 in the problem) it prints all 0s. Here is my code.
#include <iostream>
#include <vector>
#include <algorithm>
#include <stdio.h>
using namespace std;
int main() {
int T;
cin>>T;
while(T--){
int N, k, i;
cin>>N >> k;
//***********************************************************
vector<int> ar(N);//problem seems to be here when N ~ 20000
//***********************************************************
for(i = 0; i<N; i++) scanf("%d",&ar[i]);
sort(ar.begin(),ar.end());
//calculate smallest difference
int small = ar[k-1] - ar[0];
for(i = k-1; i<N; i++){
int temp;
if( temp = ar[i] - ar[i-k+1] < small) small = temp;
}
cout<<small <<endl;//print smallest difference
}
return 0;
}
When I changed the type to array it ran perfectly fine. What is the problem with using vectors?

Answer 1

Your code has a lack of error checking:
Check that cin >> N >> k succeeded.
Check that each scanf succeeded.
Check k -1 is within range of the array bounds.
Any of those failures could cause your problem.
There is also a logic error on this line:
if( temp = ar[i] - ar[i-k+1] < small) small = temp;
The control expression is parsed as temp = (ar[i] - ar[i-k+1] < small), so this line will set small = 1 if ar[i] - ar[i-k+1] < small and do nothing otherwise. You probably meant (temp = ar[i] - ar[i-k+1]) < small).