What loop is faster and why?

Answer 1

#include <stdio.h>
#include <sys/time.h>

long long timeInMilliseconds(void) {
struct timeval tv;

gettimeofday(&tv,NULL);
return (((long long)tv.tv_sec)*1000)+(tv.tv_usec/1000);
}

int main() {

int _cycles = 100000000, i=0;

long long _t1,
_t2,
_t3;

long long d1, d2;

_t1 = timeInMilliseconds();

while ( i++ < _cycles ) ;

_t2 = timeInMilliseconds();

while ( _cycles-- ) ;

_t3 = timeInMilliseconds();

d1 = _t2 - _t1;
d2 = _t3 - _t2;

printf("D1:%ld\n", d1);

printf("D2:%ld\n", d2);

return 0;
}

Answer 2

In my opinion D2 should be less than D1, because of comparing with zero in the second while(){}

Answer 3

I think a "for" loop might be a few nano- seconds faster than a "while" loop, maybe because of the extra increment line in the while (generally speaking).

Answer 4

@blake you're right about the while loop that more can be done with it, making it more accurate. I was basing my "guesstimate" about the for loop being faster on a program I wrote comparing both types. After 2 billion loops, for some reason the while loop took about a second longer. I didn't have any good way to test this except to count to myself 1-1000, 2-1000, etc. I didn't think the testing was precise enough, so I didn't keep the program. Maybe I'll check into it again. This could be a good challenge!

Answer 5

nested for loop is slower for sure.
and for is fastest

Answer 6

for is faster than while loop in c#.for loop is averaged about 2.95 to 3.02ms.while loop is average about 3.05 to 3.37ms.

Answer 7

So, I've tested the code and the result is the same. d1 is equal to d2. d1 = d2. The first while has to increment and compare. The second has to decrement and compare. The time to increment i++ is the same to decrement _cycles--. The time to compare too.
But, I think, the compile could optimize the second while, then, d2 will be less than d1, indeed. It's not the case. It will be possible in assembly language, I think.

Answer 8

while loop is the faster one as it is simple and accurate.

Answer 9

I think while loop produces more clear code. As for the speed: i think it is the same.
So I opt for the while loop

Answer 10

while is faster since it test the conditions first

Answer 11

for loop is more faster than any other loop