Question

I am a novice to programming and computing.

I am running a C++ based program that is taking approx 6 hours on this machine. I use the timing utility of the framework I work in.

I tried calculating the total number of iterations of my nested loop via a simple program:

{
int k=0;
for (int i = 0; i < 196779; i++)
for (int j= i+1; j< 196779; j++)
{
k++;
if((k+1)%10000 == 0)
cout<< "\n Number of Instructions: " << k;
}
cout<< "\n Total Number of iterations = " << k << endl;
}

Mathematically I would expect it to agree with the value 1.9360889031 × 10^10 , which is the total number of 2-element subsets. I inserted that cout statement to see if something funny was happening, and indeed it does.

1. The outpute exceeds the mathematically expected value. Is my calculation wrong?
2. The output goes into negative values after a while as it exceeds the int range, but it shouldn’t.

Sample output at the end where I manually break

Number of Instructions: -2078590001
 Number of Instructions: -2078580001
 Number of Instructions: -2078570001
 Number of Instructions: -2078560001

I found out the range of Int to be 2147483647, but I had made the calculations and concluded that my k should never exceed the limit. So where is the problem?

Answer 1

Mathematically I would expect it to agree with the value 1.9360889031 × 10^10

1.9360889031 × 10^10 (19,360,889,031) is larger than 2,147,483,647, the maximum value representable by an int (at least on your compiler).

The output goes into negative values after a while as it exceeds the int range, but it shouldn’t.

You can use an unsigned int, which correctly “rolls over” when a computation yields a value too large to be represented (or a negative value). When you overflow an int, you get undefined behavior.