Display a negative time in Obj-C

Question

It may sound simple however I am unsure how to do this. How do I display time in the following format?:

-00:00:00

I have tried using float and int values of the interval difference between two times however neither give a consistent display in the 00:00:00 format. I have also tried converting the time difference into date and then display as String.

This is the code I have used to convert my intervals :

NSDate * now = [NSDate date];
NSTimeInterval totalTime1 = [now timeIntervalSinceDate: timeEntry1];
NSTimeInterval totalTime2 = [now timeIntervalSinceDate: timeEntry2];

//must always be this way
int adjustedTime = totalTime1 - totalTime2;

int hours = adjustedTime / 3600;
int minutes = (adjustedTime / 60) % 60;
int seconds = adjustedTime % 60;

NSString * newTime = [NSString stringWithFormat:@"%02u:%02u:%02u", hours, minutes, seconds];

The above works fine for displaying positive time differences. However presents a variety of 00:423456:978098 and so on when it goes negative in both the NSLog and the Label.

When I convert and save as a type of NSDate I get (null) in NSLog and nothing in my Label.

When I use float it works but does not consistently display in the 00:00:00 format.

NOTE

The code I am using works immaculately for positive time differences. I need negative time differences to also display.

I also need to be able to save the negative time to CoreData. If this is not possible then I will work around, but displaying negative time formatted correctly is the main issue.

EDIT

My new revised code:

NSDate * now = [NSDate date];
NSTimeInterval totalTime1 = [now timeIntervalSinceDate: timeEntry1];
NSTimeInterval totalTime2 = [now timeIntervalSinceDate: timeEntry2];

//must always be this way
int adjustedTime = (int) (totalTime1 - totalTime2);
NSLog (@"What is the adjustedTime? %d", adjustedTime);

int hours = adjustedTime / 3600;
int minutes = (adjustedTime / 60) % 60;
int seconds = adjustedTime % 60;

NSString * newTime = [NSString stringWithFormat:@"%@%02d:%02d:%02d", adjustedTime < 0 ?@"-":@"", hours, minutes, seconds];
NSLog(@"What is the newTime? %@", newTime);

It is closer as now it displays negative numbers however the display is still incorrect when negative.

EDIT 2

A person who answered below suggested I try checking for negative if it is a boolean. Displaying did not change. Below are more log statements to demonstrate. NOTE I stopped using an updated seconds for sake of working out whether it affected the display and stored seconds separate to test, which is why there is no - sign or alteration to the seconds.

2015-01-09 09:30:14.526 App2.0[8398:498707] What is time 2 : 720
2015-01-09 09:30:14.526 App2.0[8398:498707] What is time 1 : 771
2015-01-09 09:30:14.526 App2.0[8398:498707] What is the adjusted time? 51
2015-01-09 09:30:14.527 App2.0[8398:498707] New Time: 00:00:51

2015-01-09 09:30:18.249 App2.0[8398:498707] What is time 2 : 900
2015-01-09 09:30:18.249 App2.0[8398:498707] What is time 1 : 771
2015-01-09 09:30:18.249 App2.0[8398:498707] What is the adjusted time? -129
2015-01-09 09:30:18.249 App2.0[8398:498707] New Time: -00:-2:51

2015-01-09 09:30:20.281 App2.0[8398:498707] What is time 2 : 840
2015-01-09 09:30:20.281 App2.0[8398:498707] What is time 1 : 771
2015-01-09 09:30:20.281 App2.0[8398:498707] What is the adjusted time? -69
2015-01-09 09:30:20.281 App2.0[8398:498707] New Time: -00:-1:51

2015-01-09 09:30:21.725 App2.0[8398:498707] What is time 2 : 780
2015-01-09 09:30:21.726 App2.0[8398:498707] What is time 1 : 771
2015-01-09 09:30:21.726 App2.0[8398:498707] What is the adjusted time? -9
2015-01-09 09:30:21.726 App2.0[8398:498707] New Time: -00:00:51

2015-01-09 09:30:30.161 App2.0[8398:498707] What is time 2 : 1080
2015-01-09 09:30:30.161 App2.0[8398:498707] What is time 1 : 771
2015-01-09 09:30:30.162 App2.0[8398:498707] What is the adjusted time? -309
2015-01-09 09:30:30.162 App2.0[8398:498707] New Time: -00:-5:51

2015-01-09 09:30:33.389 App2.0[8398:498707] What is time 2 : 4680
2015-01-09 09:30:33.389 App2.0[8398:498707] What is time 1 : 771
2015-01-09 09:30:33.390 App2.0[8398:498707] What is the adjusted time? -3909
2015-01-09 09:30:33.390 App2.0[8398:498707] New Time: --1:-5:51

2015-01-09 09:30:36.186 App2.0[8398:498707] What is time 2 : 8280
2015-01-09 09:30:36.187 App2.0[8398:498707] What is time 1 : 771
2015-01-09 09:30:36.187 App2.0[8398:498707] What is the adjusted time? -7509
2015-01-09 09:30:36.187 App2.0[8398:498707] New Time: --2:-5:51

2015-01-09 09:30:43.918 App2.0[8398:498707] What is time 2 : 660
2015-01-09 09:30:43.918 App2.0[8398:498707] What is time 1 : 771
2015-01-09 09:30:43.919 App2.0[8398:498707] What is the adjusted time? 111
2015-01-09 09:30:43.919 App2.0[8398:498707] New Time: 00:01:51

Answer 1

Since you're displaying the time components separately, you'll probably need some conditional logic to adjust the display depending on whether totalTime2 is before or after totalTime1 :

NSString *newTime = nil;

if (adjustedTime < 0) {
    newTime = [NSString stringWithFormat:@"-%02d:%02d:%02d", hours, minutes, seconds];
} 
else {
    newTime = [NSString stringWithFormat:@"%02d:%02d:%02d", hours, minutes, seconds];
}

or if you prefer something more compact:

NSString *newTime = adjustedTime < 0 ? @"-" : @"";
newTime = [newTime stringByAppendingFormat:@"%02d:%02d:%02d", hours, minutes, seconds];

Also, as Wain points out in the comments, you'll need to take the absolute value of each component before using it to display:

int hours = abs(adjustedTime / 3600);
int minutes = abs((adjustedTime / 60) % 60);
int seconds = abs(adjustedTime % 60);

Answer 2

That should do the trick:

  NSDate *now = [NSDate date];
  NSTimeInterval totalTime1 = [now timeIntervalSinceDate: timeEntry1];
  NSTimeInterval totalTime2 = [now timeIntervalSinceDate: timeEntry2];

  NSLog(@"totalTime1: %f", totalTime1); // =>  -60.00;
  NSLog(@"totalTime2: %f", totalTime2); // => 6023.00;

  //must always be this way
  int timeOffset = (int) (totalTime1 - totalTime2);
  NSLog(@"timeOffset: %d", timeOffset); // => -6083;

  BOOL showNegativePrefix = timeOffset < 0;

  int hours   = abs(timeOffset / 3600);       // => 01
  int minutes = abs((timeOffset / 60 ) % 60); // => 41
  int seconds = abs(timeOffset % 60);         // => 23

  NSString * newTime = [NSString stringWithFormat:@"%@%02d:%02d:%02d", showNegativePrefix ? @"-" : @"", hours, minutes, seconds];
  NSLog(@"%@", newTime); // => -01:41:23

Answer 3

You are almost there, all you need extra is a flag if the value is negative and to then format the positive difference preceded by a sign if needed. First set a flag and make the difference always positive:

int adjustedTime = ...;

BOOL isNeg;
if (adjustedTime < 0)
{
   isNeg = YES;
   adjustedTime = -adjustedTime; // make value +ve
}
else
   isNeg = NO;

then do your math as before and change the format line to:

NSString *newTime = [NSString stringWithFormat:@"%@%02d:%02d:%02d", (isNeg ? @"-" : @""), hours, minutes, seconds];

Note: you need to use %d as your values are int not unsigned int.

HTH

Addendum

Here is the code again, including the bit of yours I skipped ("do your math as before"), with added comments:

int adjustedTime = totalTime1 - totalTime2;

// At this point adjustedTime may be negative, use the standard approach
// test if it is -ve, and if so set a flag and make the value +ve

BOOL isNeg;
if (adjustedTime < 0)
{
   // we are here if -ve, set flag
   isNeg = YES;
   // and make the value +ve
   adjustedTime = -adjustedTime;
}
else
   isNeg = NO;

// at this point adjustedValue is ALWAYS +ve, isNeg is set if it was originally -ve

int hours = adjustedTime / 3600;
int minutes = (adjustedTime / 60) % 60;
int seconds = adjustedTime % 60;

// at this point hours, minutes & seconds MUST BE +VE as adjustedTime is +ve

// format the values, an optional sign based on isNeg followed by three POSITIVE numbers
NSString *newTime = [NSString stringWithFormat:@"%@%02d:%02d:%02d", (isNeg ? @"-" : @""), hours, minutes, seconds];

This can only print AT MOST ONE minus sign at the start from the string.

This approach (though with a test for the minimum negative integer as that cannot be negated) is the standard way to handle this issue.

Answer 4

The solutions presented by Sebastian Keller and CRD both suffer a problem when rolling over from negative to positive values, for example when displaying a countdown that starts with a negative value:

-1.896893 -00:00:01
-1.498020 -00:00:01
-1.099442 -00:00:01
-0.996686 00:00:00
-0.896971 00:00:00
-0.195021 00:00:00
-0.095020 00:00:00
0.004988 00:00:00
0.104940 00:00:00
0.504980 00:00:00
0.904981 00:00:00
1.000516 00:00:01
1.104926 00:00:01

As can be seen, both solutions actually display 00:00:00 for 2 seconds. The code below instead produces the correct output:

-1.899441 -00:00:02
-1.499838 -00:00:02
-1.095019 -00:00:02
-0.994941 -00:00:01
-0.899903 -00:00:01
-0.195743 -00:00:01
-0.097564 -00:00:01
0.001758  00:00:00
0.100495  00:00:00
0.503691  00:00:00
0.904986  00:00:00
1.004998  00:00:01
1.103652  00:00:01
2.004936  00:00:02

It's actually a category I wrote on NSNumber but the principle should also be applicable to any floating point number:

- (NSString *)ut_hmsString {
    NSString *sign  = (self.floatValue < 0.0f) ? @"-" : @" ";
    float corr      = (self.floatValue < 0.0f) ? 1.0000001f : 0;
    int seconds     = fmodf(fabsf(floorf(self.floatValue)), 60.0f);
    int fakesec     = fabsf(self.floatValue - corr);
    int minutes     = fakesec / 60 % 60;
    int hours       = fakesec / 3600;
    return [NSString stringWithFormat:@"%@%02i:%02i:%02i", sign, hours, minutes, seconds];
}