Objective C Convert int to NSString (iPhone)

Question

I have the following code that is meant to convert milliseconds into hours, mins and seconds:

int hours = floor(rawtime / 3600000);

int mins = floor((rawtime % 3600000) / (1000 * 60));
int secs = floor(((rawtime % 3600000) % (1000 * 60)) / 1000);

NSLog(@"%d:%d:%d", hours, mins, secs);

NSString *hoursStr = [NSString stringWithFormat:@"%d", hours];
NSString *minsStr = [NSString stringWithFormat:@"%d", mins];
NSString *secsStr = [NSString stringWithFormat:@"%d", secs];

NSLog(@"%a:%a:%a", hoursStr, minsStr, secsStr);

Fairly straightforward. Rawtime is an int with value 1200. The output is like this:

0:0:1

0x1.3eaf003d9573p-962:0x1.7bd2003d3ebp-981:-0x1.26197p-698

Why is it that converting the ints to strings gives such wild numbers? I've tried using %i and %u and they made no difference. What is happening?

Answer 1

You have to use %@ as the conversion specifier for an NSString. Change your last line to:

NSLog(@"%@:%@:%@", hoursStr, minsStr, secsStr);

%a means something totally different. From the printf() man page:

aA
The double argument is rounded and converted to hexadecimal notation in the style

[-]0xh.hhhp[+-]d

where the number of digits after the hexadecimal-point character is equal to the precision specification.

Answer 2

Instead of rolling your own string formatting code, you should be using an NSNumberFormatter for numbers or an NSDateFormatter for dates/times. These data formatters take care of localization of format to the user's locale and handle a variety of formats built-in.

For your use, you need to convert your millisecond time into an NSTimeInterval (typedef'd from a double):

NSTimeInterval time = rawtime/1e3;

Now you can use an NSDateFormatter to present the time:

NSDate *timeDate = [NSDate dateWithTimeIntervalSinceReferenceDate:time];
NSString *formattedTime = [NSDateFormatter localizedStringFromDate:timeDate
                                                         dateStyle:NSDateFormatterNoStyle 
                                                         timeStyle:NSDateFormatterMediumStyle];
NSString *rawTime = [[formattedTime componentsSeparatedByString:@" "] objectAtIndex:0];

on OS X where the last line removes the "AM/PM". This will work for any time less than 12 hrs and will give a formatted string in the localized format for HH:MM:SS. On the iPhone, localizedStringFromDate:dateStyle:timeStyle: isn't available (yet). You can achieve the same effect with setTimeStyle:, setDateStyle: and stringFromDate: on a date formatter instance.