Same C program but different execution time - c

Question

I wrote a C program to find out the prime numbers in a given parameter. But for the same input and output, the program has different execution times.
#include <stdio.h>
int main(int argc, char const *argv[])
{
int n,p,k;
scanf("%d", &n);
int prime[n+1];
for (p = 2; p <= n; ++p)
{
if (prime[p]!=-1)
{
for (int i = p*2,k=2; i < n; k++,i=k*p)
{
prime[i]=-1;
}
}
}
for (int i = 1; i < n ; ++i)
{
if (prime[i]!=-1)
{
printf("%d ",i );
}
}
return 0;
}

Answer 1

What you are measuring is the time it takes the user to type the input, i.e. 200.
Calculating the result after the input is given will not take 2 to 5 sec on any modern computer.
It is true that "the computer may be doing other things" and therefore give varying execution time - but that won't give you a 3 sec increase in code like this.
To make the measurement more reliable, you need to "remove" the user input, i.e. remove the scanf.
Instead of scanf you should give the value of n as a command line argument.
Use code like
// scanf("%d", &n); Dont use scanf but use lines like the two below.
if (argc < 2) exit(1); // user error - no number given
n = atoi(argv[1]); // convert command line arg to integer
And start the program like:
test2.exe 200
Now the measured time will be much smaller than 2-5 sec and you won't see execution time vary so much.
Note: While atoi is simple to use, it's in general better to use strtol

Answer 2

The execution of a program on an environment is not dependent only on the code but on some other environment variables such as CPU load.
CLOCKS_PER_SEC is a constant which is declared in <time.h>. To get the CPU time used by a task within a C application, use:
clock_t begin = clock();
/* Do the work. */
clock_t end = clock();
double time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
For your program, You can probably check the
CPU Time took on input.
CPU Time is taken to generate Prime Number
...... and so on
#include <stdio.h>
#include<time.h>
int main(int argc, char const *argv[])
{
int n,p,k;
clock_t t , t1, t2;
t = clock();
scanf("%d", &n);
t = clock() - t;
double time_taken = ((double)t)/CLOCKS_PER_SEC; // in seconds
printf("Took %f seconds to take the input\n", time_taken);
t2 = clock();
int prime[n+1];
for (p = 2; p <= n; ++p)
{
if (prime[p]!=-1)
{
for (int i = p*2,k=2; i < n; k++,i=k*p)
{
prime[i]=-1;
}
}
}
t2 = clock() - t2;
double time_taken2 = ((double)t2)/CLOCKS_PER_SEC; // in seconds
printf("Took %f seconds for generating the prime number \n", time_taken2);
for (int i = 1; i < n ; ++i)
{
if (prime[i]!=-1)
{
printf("%d ",i );
}
}
return 0;
}
Output:
200
Took 0.000075 seconds to take the input
Took 0.000004 seconds for generating the prime number
1 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101 103 107 109 1
13 127 131 137 139 149 151 157 163 167 173 179 181 191 193 197 199
The time is depended on your CPU load. For details please visit here