problem using mod operator c

Question

I'm trying to perform a mod (%) on 2 ints in c. My first time attempting such a thing.

The code below is in a loop where count is set to 0 outside the loop and increments by 1 every iteration. I'm expecting to see my "read" value change but it stays stuck at the value for "blah".

What am I doing wrong?

int blah=176400;
count+=1;
NSLog(@"the time = %i",count);// prints the correct increments
int read = (int)(blah % count);
NSLog(@"read %i",read); // prints out 1764000 all the time

• Winston Ewert: your code works when I run it. (Aside from modifications to not use NSLog.) Can you try to produce a minimal test case that does reproduce the problem?
• darioo: Prints "176400" or "1764000"? Also, including that for loop would be helpful.
• kriss: The code you show looks fine, the problem may be in the loop around it, or somewhere else. Please post the full code (and avoid things like NSLog, or tag Xcode, that's XCode specific, not C).

Answer 1

Example code:

#include <stdio.h>
int main() {
 int blah = 176400;
 for (int count = 1; count < 20; ++count) {
 printf("%d %% %d = %d\n", blah, count, blah % count);
 }
}

Output:

176400 % 1 = 0
176400 % 2 = 0
176400 % 3 = 0
176400 % 4 = 0
176400 % 5 = 0
176400 % 6 = 0
176400 % 7 = 0
176400 % 8 = 0
176400 % 9 = 0
176400 % 10 = 0
176400 % 11 = 4
176400 % 12 = 0
176400 % 13 = 3
176400 % 14 = 0
176400 % 15 = 0
176400 % 16 = 0
176400 % 17 = 8
176400 % 18 = 0
176400 % 19 = 4

I'd say the problem is elsewhere in your code.

• cHao: Or, count is way bigger than we're expecting it to be.
• dubbeat: @chao. thats raises an alarm. does become very large depending on how long my app has been running.
• darioo: @dubbeat: well, you didn't mention that before. If count is bigger than blah, then you're seeing expected behaviour.
• Steve Jessop: @dubbeat: if you show more code, it might clarify what you mean by "count starts at 0 and increments by 1 every iteration", together with "prints out 176400 all the time". All the time? Including for count == 11?

Answer 2

Your doing "n modulo m" where m is bigger than n. The result is n, and it's correct.

Maybe you wanted count % blah?

• dubbeat: if I switch the values around it just repeatedly prints out the value for count instead of blah.
• cHao: @kriss: Depends on the intent, which i'm not really getting from the limited code in the question. You can't definitively say that one or the other is the "correct order" without knowing what the number was intended to represent. Frankly, though, count % blah makes more sense in most situations i can think of.
• Steve Jessop: 176400 is 49 hours worth of seconds, so if "count" really is a time as indicated by the logging, and you have a 49 hour day on your planet, then "count % blah" would be the time of day. So the numbers are funny, but I think FX might well be right that the order is incorrect.
• kriss: OK, I will remove my downvote if I can, we indeed don't know the value of count and what OP is trying to do, so you may be right. For me to remove my downvote the answer should be edited, maybe putting something like the comment of Steve Jessop about time inside the answer could be useful.
• kriss: Rule of thumb to rember modulo arguments order : it's the same than division.

Answer 3

Understand that blah % count is basically the remainder from dividing blah by count, if both numbers are positive. (Things get a bit hairier when one or both of the numbers are negative.) If count is the larger number, then the result will always be blah (as the division would yield 0, with a remainder of blah). In your case it seems that count is getting very large, leading to this very situation.

It's hard to divine the intent of your using % here, but it seems that either your operands are in the wrong order, or you're misunderstanding what the result should be...or perhaps, that what you actually need is a different operator entirely.