How to add commas to number every 3 digits in Objective C?

Don't do your own number formatting. You will almost certainly not get all the edge cases right or correctly handle all possible locales. Use the NSNumberFormatter for formatting numeric data to a localized string representation.

You would use the NSNumberFormatter instance method -setGroupingSeparator: to set the grouping separator to @"," (or better yet [[NSLocale currentLocale] objectForKey:NSLocaleGroupingSeparator]; thanks @ntesler) and -setGroupingSize: to put a grouping separator every 3 digits.

For Swift 4.0

let formatter = NumberFormatter()

formatter.numberStyle = .decimal

formatter.minimumFractionDigits = 2

formatter.maximumFractionDigits = 2

let result = formatter.string(from: NSNumber(value: 123456))

For those who need to do it with strings of numbers and not just integers (I.e. Big Numbers) I made the following macro:

#define addCommas(__string) (\
(^NSString *(void){\
NSString *__numberString = __string;\
NSString *__integerPortion = __numberString;\
NSString *__decimalPortion = @"";\
if ([__string containsString:@"."]) {\
__integerPortion = [__numberString componentsSeparatedByString:@"."][0];\
__decimalPortion = st(@".%@", [__numberString componentsSeparatedByString:@"."][1]);\
}\
int __i = (int)__integerPortion.length-3;\
while (__i > 0) {\
__integerPortion = st(@"%@,%@", substringInRange(__integerPortion, 0, __i), substringInRange(__integerPortion, __i, (int)__integerPortion.length));\
__i -= 3;\
}\
__numberString = st(@"%@%@", __integerPortion, __decimalPortion);\
return __numberString;\
})()\
)