The difference between n++ and ++n at the end of a while loop? (ANSI C)

this is probably a dumb question but I just can't figure it out. It has to do with the differences between n++ and ++n (which I thought I understood but apparently not).

#include <stdio.h>
#include <math.h>
long algorithmA(int n);
long algorithmB(int n);
int main(){ long A, B; A = B = 0; int n = 1; while(A >= B){ A = algorithmA(n); B = algorithmB(n); n++; } printf("At n = %d, Algorithm A performs in %ld seconds & " "Algorithm B performs in %ld seconds.", n, A, B);
}
long algorithmA(int n){ return pow(n,4) * 86400 * 4;
}
long algorithmB(int n){ return pow(3,n);
}

Here you can probably tell I'm trying to see at what point Algorithm A outperforms Algorithm B. The functions and units of time were given to me in a homework problem.

Anyways, I always thought that the order of "++" would not matter at the end of a while loop. But if I put ++n instead of n++, I get the wrong answer. Can somebody explain why?

Edit: Well it WAS showing 24 with ++n and 25 with n++, but it must have been for another reason. Because I just checked now and there is no difference. Thanks for your patience and time guys, I just wish I knew what I did!

3 Answers

If you increment without assignment, no difference. However, in the following circumstances, there is:

int n = 1;
int x = n++; // x will be 1 and n will be 2

In this example, the statement gets executed prior to the increment.

int n = 1;
int x = ++n; // both x and n will be 2

However, in this example, increment occurs prior to the execution of the statement.

Operator precedence can help you out.

The only difference between n++ and ++n is that n++ yields the original value of n, and ++n yields the value of n after it's been incremented. Both have the side effect of modifying the value of n by incrementing it.

If the result is discarded, as it is in your code, there is no effective difference.

If your program is behaving differently depending on whether you write

n++;

or

++n;

it must be for some other reason.

In fact, when I compile and execute your program on my system, I get exactly the same output in both cases. Adding newlines to the output format, I get:

At n = 25, Algorithm A performs in 114661785600 seconds &
Algorithm B performs in 282429536481 seconds.

You haven't told us what output you're getting. Please update your question to show the output in both cases.

The prefix versions (++n) alter the variable and then pass along its value. The postfix version (n++) pass along the current value and then alter the variable.