NSTimeInterval expirationTime = (secondsSinceUnixEpoch*1000)+120000;
expirationTime = ceil(expirationTime/2);
int expirationInt = (int)expirationTime;
NSLog(#"%d", expirationInt);
The log output is always negative, even though before I convert it to an int it's positive... I tried just multiplying it by -1 to make it positive again and it's just staying negative! I'm totally perplexed.... don't know much about C, am I just doing something silly??
The number (secondsSinceUnixEpoch*1000)+120000 looks to me like it's going to be way too large to fit in an int. Chances are the integer is overflowing and becoming negative.
Converting to long long is one solution. As you stated in a comment, you want to show a whole number for use in a URL. Just do this:
NSTimeInterval expirationTime = (secondsSinceUnixEpoch*1000)+120000;
expirationTime = ceil(expirationTime/2);
NSString *urlString = [NSString stringWithFormat:#"http://example.com?time=%.0f", expirationTime];
This will format the decimal number as a whole number.
Related
I'm currently parsing NSString values to NSNumbers and then adding them into a NSMutableArray called operands in an object called "data" like so:
NSNumberFormatter * f = [[NSNumberFormatter alloc] init];
[f setNumberStyle:NSNumberFormatterDecimalStyle];
NSNumber * myNumber = [f numberFromString:*operandString];
[data.operands addObject:myNumber];
I then retrieve those numbers, perform some math on them, then update the array:
double x = [[data.operands objectAtIndex: i]doubleValue];
double y = [[data.operands objectAtIndex: i + 1]doubleValue];
double answer = x * y;
[data.operands replaceObjectAtIndex:(i) withObject:[NSNumber numberWithDouble:answer]];
When I get the answer, everything looks fine eg: ( 3.33 * 5 = 16.65)
BUT, when I look in the debugger I'm seeing some crazy values for x and answer, such as:
x = 3.3300000000000001
answer = 16.649999999999999
Why is this happening? Am I loosing some precision with parsing these back and fourth? Is it how I've used the NSNumberFormatter to parse the string?
The reason I'm in trouble with this is because I'm trying to ensure there's no double overflow errors so I'm using this simple test to check the integrity:
if (answer / y != x){
//THROW OVERFLOW ERROR
}
With the above crazy numbers this is always inconsistent. When I NSLog the answer it comes out fine:
NSLog (#"%g", [[data.operands objectAtIndex:i]doubleValue]]);
Same for
NSLog (#"%f", [[data.operands objectAtIndex:i]doubleValue]]);
You are not losing any precision that you need to worry about. Those are the correct values. There are only about 2^60 different double numbers, that finite set has to try to approximately cover the infinite 'number of numbers' in the range that doubles cover.
In other words, there are no exact answers in computer land and your
if (answer / y != x){
//THROW OVERFLOW ERROR
}
Will not work. Or it may work much of the time, but fail if you push it. Instead you need to acknowledge the limited precision (which is pretty high precision) of doubles:
//Don't waste time worrying like this...
if (fabs(answer / y - x) > 1e-12*fabs(answer)){
//Not correct or useful thing to check don't use this - i did not check
}
// let the math package handle it:
if (isnan(answer)){
// we gots problems
}
if (!isnormal(answer)){
// we gots some other problems
}
Also don't forget that 10^300 is a very large number, doubles work pretty well. To use 32 bit floats you need to pay much more attention to order of execution, etc.
NSLog is likely outputting with fewer decimals of precision, and rounds to the nearest thing, so the answers look better.
I'm trying to get an NSNumber out of an NSMutableArray that's been previously manipluated as a double and then added to the array to print out in a label (NSString).
It's important that the number stays as an accurate representatoin of a double with no scientific notation to abbreviate the answer.
The other requirement is to have it print to maybe 15 or 16 decimal places, rounding is optional but not required.
I also do not want trailing 0's when displaying the double
I've tried the following but these do not work...
This is ok but ends the number with a . (eg: 1+1=2.)
double test = [[data.argOperands objectAtIndex:0]doubleValue];
label.text = [NSString stringWithFormat:#"%3.2f", test];
label.text = [label.text stringByTrimmingCharactersInSet:[NSCharacterSet characterSetWithCharactersInString:[NSString stringWithFormat:#"0"]]];
I then try something like this, which is wrong because if I do 9^99 it'll print inf or 0.0003/4 it'll give scientific numbers instead of the value
float y = [[calcData.argOperands objectAtIndex:0]doubleValue];;
label.text = [NSString stringWithFormat:#"%g", y];
If I do the following using double it's getting close, 9^99 works, but 3.33/5 returns 0.666000 with trailing 0's
double y = [[data.argOperands objectAtIndex:0]doubleValue];
label.text = [NSString stringWithFormat:#"%f", y];
Any code examples of how to do it this way using either NSNumberFormatter or NSDecimalNumber would be greatly appreciated.
"%.13f" Would give you 13 decimal places, but that would give you trailing zeros.
You may need to create an NSNumberFormatter and use that.
I suspect you're not going to be happy no matter what. Binary floating point is not an exact representation of decimal. The decimal value .1 is not an exact value in binary, and might display as something like .09999999998 if you display it with 13 decimal places.
You also might look at using NSDecimalNumber, which stores values as decimal digits. It's slower than other ways of doing math but you can control the results exactly.
After looking over http://nshipster.com/nsformatter/ and the giant NSNumberFormatter_Class doc I've come up with this code that prints everything to my requirements:
NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
[numberFormatter setUsesSignificantDigits: YES];
numberFormatter.maximumSignificantDigits = 100;
[numberFormatter setGroupingSeparator:#""];
[numberFormatter setNumberStyle:NSNumberFormatterDecimalStyle];
label.text = [numberFormatter stringFromNumber:stringFromNumber:#(1234.567800000555)];
This will actually print 1234.56780000056 (missing the 12th decimal place and rounding it up to the 11th decimal place) though I'm happy enough with this.
I'm still cleaning up the answer, I don't need maximumSignificantDigits = 100 obviously, but generally having a large number there helps to ensure I'm getting all the decimal places I need.
I had to set setGroupingSeparator:#"" because NSNumberFormatterDecimalStyle puts commas in numbers and I don't want them (eg: Instead of getting 1,000 I want 1000).
in iOS Objetive-C I am trying to get the number typed by the user in a text field to set the upper bounder of a random number generation function in C.
- (IBAction)pushTheButton2:(id)sender {
u_int32_t upperBound = (u_int32_t) textField3.text;
textField4.text = [NSString stringWithFormat:#"%d", arc4random_uniform(upperBound)];
}
The output is a giant number that makes no sense. To test if function works, if I hardcode the actual upper bound in the arc4random_uniform function, such as arc4random_uniform(5), then it works!
I figured this could be some kind of literal conversion, so I tried to make this work with u_int32_t but still not outputting the right range.
Can someone help? Thanks
You are currently taking the memory reference pointer of the text and using that as the upper bound.
Try doing something like this instead...
NSInteger upperBound = [textfield.text intValue];
This will convert the string into an int that you can then use in the arc random function.
To parse string to integer you should do:
NSInteger upperBound = [textfield.text integerValue];
I'm adding a bunch of coordinates into quad tree and when I'm asking for the closest coordinate near my location, sometimes I've coordinate with 0 at the end, added automatically perhaps by the quad tree or I don't know how.
The problem is when I'm asking the double value in my core data using predicate it won't match because of the 0 digit addition to the number.
I thought about removing it when I've 0 but I'm sure there is a better way doing it.
For example:
Near location 31.123456, 34.123456, the nearest is 31.123444, 34.123450
when '34.123450' is actually 34.12345 in the database.
//Convert float to String
NSString *str_lat = #"34.123450";
NSString *trimmedString=[str_lat substringFromIndex:MAX((int)[str_lat length]-1, 0)];
if([trimmedString isEqualToString:#"0"])
{
str_lat = [str_lat substringToIndex:[str_lat length] - 1];
}
else
{
}
NSLog(#"%#",str_lat);
First: You should not store numbers as strings. 7.3 and 7.30 are the same values with simply different representations. You should not compare the representations, but the value.
Second: You should not compare floating-point numbers with == but their difference to a delta. In a calculation precision might get lost, rounding is applied and so on. The "mathematical" equal values might be physical different by a more or less small amount.
// remove the zeros from values (if you have them as floats)
NSString *valueFromTheDataBase = [NSString stringWithFormat:#"%g", 34.123450];
NSString *yourValue = [NSString stringWithFormat:#"%g", 34.12345];
if([yourValue isEqualToString:valueFromDataBase]) {
// they are equal
}
OR Make Them floats and compare them
// make them floats and compare them
CGFloat floatFromDB = [valueFromDB floatValue];
CGFloat yourFloat = [yourString floatValue];
if((floatFromDB - yourFloat) == 0) {
// they are equal
}
UPDATED as #Amin Negm says
I'm building an app that allows the user to perform some calculations except the calculations result in numbers with lots of decimal digits. It's fine for me to see that kind of precision but I want to let the users be able to choose how many significant digits they want shown. I'm creating a result string using a double and using the %g format shown here:
NSString *resultString = [[NSString alloc] initWithFormat:#"%.14g", result];
I have created a stepper that the users can interact with and storing the number they have chosen in another double. My question is, how can insert that double where the 14 is to change the number of significant digits? Or is this even possible? Please comment if you need clarification.
Any field width or precision in a format can be replaced by an * to indicate a dynamic value which is supplied by an int argument.
For example:
double d = 1.0/7;
for(int i = 4; i < 12; i++)
NSLog(#"%.*g", i, d);
Outputs:
0.1429
0.14286
0.142857
0.1428571
0.14285714
0.142857143
0.1428571429
0.14285714286