Removing cout statement causing value change of a variable - c++

Question

In the following code, when I'm removing cout statement (line after //******)then it is causing a change in the value of "i".
I used TDM-GCC 4.9.2 32 bit release and TDM-GCC 5.1.0 compilers.
I ran this code on codechef and there it runs fine and cout statement is not affecting the value of "i".
#include<iostream>
using namespace std;
int subset(int [], int);
int main()
{
int size,i,ans;
cout<<"size of array : ";
cin>>size;
int arr[size];
for(i = 0 ; i<size;i++)
{
cin>>arr[i];
}
ans = subset(arr,size);
cout<<"ans = "<<ans;
return 0;
}
int subset(int arr[], int size)
{
int i,j, tsum=0, completed=0;
for(i = 0 ;i<size;i++)
tsum = tsum + arr[i];
int carr[tsum+1],temp;
for(i=0;i<size;i++)
{
temp = arr[i];
carr[temp] = 1;
for(j=i+1;j<size;j++)
{
temp = temp + arr[j];
carr[temp] = 1;
}
}
for(i=1;i<=tsum;i++)
{
if(carr[i]!=1)
{
//************************************
cout<<"i : "<<i<<endl;
break;
}
}
return i;
}
Sample input :
size of array : 3
1
2
5
sample output without cout statement :
ans = 6
sample output having cout statement :
i : 4
ans = 4
Actual answere is 4 for the input.

Answer 1

The main problem seems to be that carr is uninitialized.
It is declared as
int carr[tsum+1]
with no initializer.
Later on some elements are set, but always to 1:
carr[temp] = 1;
In the last loop carr is examined:
if(carr[i]!=1)
This condition makes no sense. Either carr[i] has been set, then it is guaranteed to be 1, or it is uninitialized, in which case this comparison has undefined behavior.
Note that variable-length arrays are not standard C++.

Answer 2

To solve the problems as stated by Some Programmer Dude and melpomene, i.e. Variable-length arrays are not standard C++ and carr is uninitialized. Use c++ vectors and initialize them correctly. That would look something like this:
#include <iostream>
#include <vector>
using namespace std;
int subset(const std::vector<int>, const int);
int main()
{
int size, i, ans;
cout << "size of array : ";
cin >> size;
std::vector<int> arr(size);
for (i = 0; i < size; i++)
{
cin >> arr[i];
}
ans = subset(arr, size);
cout << "ans = " << ans;
return 0;
}
int subset(const std::vector<int> arr, const int size)
{
int i, j, tsum = 0, completed = 0;
for (i = 0; i < size; i++)
tsum = tsum + arr[i];
std::vector<int> carr(tsum + 1, 0);
int temp;
for (i = 0; i < size; i++)
{
temp = arr[i];
carr[temp] = 1;
for (j = i + 1; j < size; j++)
{
temp = temp + arr[j];
carr[temp] = 1;
}
}
for (i = 1; i <= tsum; i++)
{
if (carr[i] != 1)
{
//************************************
cout << "i : " << i << endl;
break;
}
}
return i;
}