Question

Somebody asked me a question:
Which one is the fastest among the below two scenario:

Case 1: assume int count = 0;

for (int i = 0; i < 10; i++)
{
    for (int j = 0; j < 5; j++)
    {
        count++;
    }
}

Case 2: assume int count = 0;

for (int i = 0; i < 5; i++)
{
    for (int j = 0; j < 10; j++)
    {
        count++;
    }
}

In both the scenario’s the final valuse of count will be 50.
But I am not sure which one will be faster? I think CASE II is faster but not sure…

It will be great if anyone can throw some light on it. which is faster and why?

Answer 1

this is the only example i can think of where it matters which variable you iterate where

int array[n][m];
//fast
for (int i=0;i<n;i++)
{ for(int j=0;j<m;j++)
  { count+=array[i][j];
  }
}
//slow
for (int i=0;i<m;i++)
{ for(int j=0;j<n;j++)
  { count+=array[j][i];
  }
}

the second one is slower because you don’t iterate the locations in memory one after the other, but because you jump by m locations at a time. the processor caches the memory locations positioned immediately after an accessed location.