# C how to measure time correctly

algorithmc++measurementperformancetime

This is the "algorithm", but when I want to measure the execution time it gives me zero. Why?

``````#define ARRAY_SIZE 10000
...

clock_t start, end;

start = clock();

for( i = 0; i < ARRAY_SIZE; i++)
{
non_parallel[i] = vec[i] * vec[i];
}
end = clock();
printf( "Number of seconds: %f\n", (end-start)/(double)CLOCKS_PER_SEC );
``````

So What should i do to measure the time?

#### Best Solution

Two things:

1. `10000` is not a lot on a modern computer. Therefore that loop will run in probably less than a millisecond - less than the precision of `clock()`. Therefore it will return zero.

2. If you aren't using the result of `non_parallel` its possible that the entire loop will be optimized out by the compiler.

Most likely, you just need a more expensive loop. Try increasing `ARRAY_SIZE` to something much larger.

Here's a test on my machine with a larger array size:

``````#define ARRAY_SIZE 100000000

int main(){

clock_t start, end;

double *non_parallel = (double*)malloc(ARRAY_SIZE * sizeof(double));
double *vec          = (double*)malloc(ARRAY_SIZE * sizeof(double));

start = clock();

for(int i = 0; i < ARRAY_SIZE; i++)
{
non_parallel[i] = vec[i] * vec[i];
}

end = clock();
printf( "Number of seconds: %f\n", (end-start)/(double)CLOCKS_PER_SEC );

free(non_parallel);
free(vec);
return 0;
}
``````

Output:

``````Number of seconds: 0.446000
``````