When converting an NSString, which contains standard decimal numbers with two digits (e.g. 8.20) to a NSNumber, I get (from time to time) extra digits and a strange rounding behavior when logging the result via NSLog or saving it in Core Data (as float or double), e.g. 8.20 -> 8.199999999999999.
This is the code I am using to convert the numbers:
NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
[numberFormatter setNumberStyle:NSNumberFormatterDecimalStyle];
[numberFormatter setMaximumFractionDigits:5];
NSNumber *num = [numberFormatter numberFromString:str];
I do not understand why the conversion to NSNumber messes the number up. What is wrong with my code?
This is just how float and double behaves in C/Objective-C (and many other languages). For example, when you type into python 8.0, the result would be 8.000000000001. I recommend using NSScanner to convert them into primitive number types (double, float).
Why would you use NSNumberFormatter to convert string to float, it would be an overkill,
To convert it just use
NSNumber *num = [NSNumber numberWithFloat:[str floatValue]];
I have encounter the problem when I use NSNumber to save the doublevalue
of 8.28 always show the 8.2799999999...,I guess it cause by a computer numerical precision
Try this code.
+ (NSString *)dealWithDouble:(double)doubleValue {
double d2 = doubleValue;
NSString *d2Str = [NSString stringWithFormat:#"%lf", d2];
NSDecimalNumber *num = [NSDecimalNumber decimalNumberWithString:d2Str];
NSString *str2D = [num stringValue];
return str2D;
}
Don't use floatValue. floatValue only gives 24 bit of precision. doubleValue gives 53 bits of precision. If you use numbers over a million dollars for example, floatValue cannot give you any values that are closer than six cent apart. ($1,000,000 followed by $1,000,000.06 etc. )
The rule is: Don't use float unless you know a reason why you should use float and not double.
Related
I'm having problems with converting NSNumber to string and string to NSNumber.
Here's a sample problem:
NSString *stringValue = #"9.2";
NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
NSLog(#"stringvalue:%#",[[formatter numberFromString: stringValue] stringValue]);
Output will be:
stringvalue:9.199999999999999
I need to retrieve the original value, where, in the example should be 9.2.
On the contrary, when the original string is 9.4 the output is still 9.4.
Do you have any idea how to retrieve the original string value without NSNumber doing anything about it?
You are discovering that floating point numbers can't always be represented exactly. There are numerous posts about such issues.
If you need to get back to the original string, then keep the original string as your data and only convert to a number when you need to perform a calculation.
You may want to look into NSDecimalNumber. This may better fit your needs.
NSString *numStr = #"9.2";
NSDecimalNumber *decNum = [NSDecimalNumber decimalNumberWithString:numStr];
NSString *newStr = [decNum stringValue];
NSLog(#"decNum = %#, newStr = %#", decNum, newStr);
This gives 9.2 for both values.
This is my algorithm to find out the speed of my game.
self.speed=.7-self.score/50;
Now how can I make self.speed round to 2 decimal places?
Note: my answer assumes you only care about the number of decimals for the purpose of displaying the value to the user.
When you setup your NSNumberFormatter to format the number into a string for display, setup the formatter with a maximum of two decimal places.
NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
[formatter setNumberStyle:NSNumberFormatterDecimalStyle];
[formatter setMaximumFractionDigits:2];
NSString *formattedNumber = [formatter stringFromNumber:#(self.speed)];
You have the option of using the setRoundingMode: method if you need a specific round method.
BTW - you shouldn't use a string format for this because it doesn't take the user's locale into account to format the number properly.
floats are handled in IEEE754 format, you can't directly decide how many decimal places will be used.You can directly decide how many bits will be used, or indirectly round the las part of the number doing this:
NSString* str=[NSString stringWithFormat: #"%.2f", number];
number= atof([str UTF8String]);
But like maddy pointed, you only need to round/truncate the unwanted decimal digits only when presenting the number to the user, so you could only use the %.2f format specifier when printing it, or use a formatter.
self.speed = (int)(self.speed * 100 + 0.5) / 100.0;
if you want to have that as a string:
NSString *speedString = [NSString stringWithFormat: #"%.2f", self.speed];
If I do the following in Objective-C:
NSString *result = [NSString stringWithFormat:#"%1.1f", -0.01];
It will give result #"-0.0"
Does anybody know how I can force a result #"0.0" (without the "-") in this case?
EDIT:
I tried using NSNumberFormatter, but it has the same issue. The following also produces #"-0.0":
double value = -0.01;
NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
[numberFormatter setNumberStyle:NSNumberFormatterDecimalStyle];
[numberFormatter setMaximumFractionDigits:1];
[numberFormatter setMinimumFractionDigits:1];
NSString *result = [numberFormatter stringFromNumber:[NSNumber numberWithDouble:value]];
I wanted a general solution, independent of the configuration of the number formatter.
I've used a category to add the functionality to NSNumberFormater;
#interface NSNumberFormatter (PreventNegativeZero)
- (NSString *)stringFromNumberWithoutNegativeZero:(NSNumber *)number;
#end
With the implementation:
#implementation NSNumberFormatter (PreventNegativeZero)
- (NSString *)stringFromNumberWithoutNegativeZero:(NSNumber *)number
{
NSString *const string = [self stringFromNumber: number];
NSString *const negZeroString = [self stringFromNumber: [NSNumber numberWithFloat: -0.0f]];
if([string isEqualToString: negZeroString])
{
NSString *const posZeroString = [self stringFromNumber: [NSNumber numberWithFloat: 0.0]];
return posZeroString;
}
return string;
}
#end
How it works
The key feature is to ask the number formatter how it will format -0.0f (i.e., floating point minus zero) as an NSString so that we can detect this and take remedial action.
Why do this? Depending on the formatter configuration, -0.0f could be formatted as: #"-0", #"-0.0", #"-000", #"-0ºC", #"£-0.00", #"----0.0", #"(0.0)", #"😡𝟘.⓪零" really, pretty much anything. So, we ask the formatter how it would format -0.0f using the line: NSString *const negZeroString = [self stringFromNumber: [NSNumber numberWithFloat: -0.0f]];
Armed with the undesired -0.0f string, when an arbitrary input number is formatted, it can be tested to see if it is matches the undesirable -0.0f string.
The second important feature is that the number formatter is also asked to supply the replacement positive zero string. This is necessary so that as before, its formatting is respected. This is done with the line: [self stringFromNumber: [NSNumber numberWithFloat: 0.0]]
An optimisation that doesn't work
It's tempting to perform a numerical test yourself for whether the input number will be formatted as the -0.0f string, but this is extremely non trivial (ie, basically impossible in general). This is because the set of numbers that will format to the -0.0f string depend on the configuration of the formatter. If if happens to be rounding to the nearest million, then -5,000f as an input would be formatted as the -0.0f string.
An implementation error to avoid
When input that formats to the -0.0f string is detected, a positive zero equivalent output string is generated using [self stringFromNumber: [NSNumber numberWithFloat: 0.0]]. Note that, specifically:
The code formats the float literal 0.0f and returns it.
The code does not use the negation of the input.
Negating an input of -0.1f would result in formatting 0.1f. Depending on the formatter behaviour, this could be rounded up and result in #"1,000", which you don't want.
Final Note
For what it's worth, the approach / pattern / algorithm used here will translate to other languages and different string formatting APIs.
Use a NSNumberFormatter. In general, NSString formatting should not be used to present data to the user.
EDIT:
As stated in the question, this is not the correct answer. There is a number of solutions. It's easy to check for negative zero because it is defined to be equal to any zero (0.0f == -0.0f) but the actual problem is that a number of other values can be rounded to the negative zero. Instead of catching such values, I suggest postprocessing - a function that will check if the result contains only zero digits (skipping other characters). If yes, remove leading minus sign.
NSString *result = [NSString stringWithFormat:#"%1.1f", -0.01*-1];
If instead of a value you pass an instance you can check:
float myFloat = -0.01;
NSString *result = [NSString stringWithFormat:#"%1.1f", (myFloat<0? myFloat*-1:myFloat)];
Edit:
If you just want 0.0 as positive value:
NSString *result = [NSString stringWithFormat:#"%1.1f",(int)(myFloat*10)<0?myFloat:myFloat*-1];
Convert the number to NSString by taking the float or double value.
Convert the string back to NSNumber.
NSDecimalNumber *num = [NSDecimalNumber decimalNumberWithString:#"-0.00000000008"];
NSString *st2 = [NSString stringWithFormat:#"%0.2f", [num floatValue]];
NSDecimalNumber *result = [NSDecimalNumber decimalNumberWithString:st2]; //returns 0
The NSNumberFormatter has two methods convert from Number to String, and from String to Number. What if we use method (Number) -> String? twice?
public extension NumberFormatter {
func stringWithoutNegativeZero(from number: NSNumber) -> String? {
string(from: number)
.flatMap { [weak self] string in self?.number(from: string) }
.flatMap { [weak self] number in self?.string(from: number) }
}
}
I have this sample code:
NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
NSString *numberString = #"9.2";
NSNumber *number = [formatter numberFromString:numberString];
NSLog(#"String: %#, Number: %#", numberString, number);
The printed result is:
String: 9.2, Number: 9.199999999999999
I don't understand why this would happen. Am I missing a setting?
According to this reference, an NSNumber can store any ordinary C numeric type. Since this is obtained by parsing a non-integer, the two types to choose from are float and double, and by the displayed value, the chosen representation is double.
Since decimal fractions like 9.2 aren't exactly representable as doubles, you get the closest representable number. That is displayed to default precision, which is about 16 decimal places.
I want to convert a string into a double and after doing some math on it, convert it back to a string.
How do I do this in Objective-C?
Is there a way to round a double to the nearest integer too?
You can convert an NSString into a double with
double myDouble = [myString doubleValue];
Rounding to the nearest int can then be done as
int myInt = (int)(myDouble + (myDouble>0 ? 0.5 : -0.5))
I'm honestly not sure if there's a more streamlined way to convert back into a string than
NSString* myNewString = [NSString stringWithFormat:#"%d", myInt];
To really convert from a string to a number properly, you need to use an instance of NSNumberFormatter configured for the locale from which you're reading the string.
Different locales will format numbers differently. For example, in some parts of the world, COMMA is used as a decimal separator while in others it is PERIOD — and the thousands separator (when used) is reversed. Except when it's a space. Or not present at all.
It really depends on the provenance of the input. The safest thing to do is configure an NSNumberFormatter for the way your input is formatted and use -[NSFormatter numberFromString:] to get an NSNumber from it. If you want to handle conversion errors, you can use -[NSFormatter getObjectValue:forString:range:error:] instead.
Adding to olliej's answer, you can convert from an int back to a string with NSNumber's stringValue:
[[NSNumber numberWithInt:myInt] stringValue]
stringValue on an NSNumber invokes descriptionWithLocale:nil, giving you a localized string representation of value. I'm not sure if [NSString stringWithFormat:#"%d",myInt] will give you a properly localized reprsentation of myInt.
Here's a working sample of NSNumberFormatter reading localized number String (xCode 3.2.4, osX 10.6), to save others the hours I've just spent messing around. Beware: while it can handle trailing blanks such as "8,765.4 ", this cannot handle leading white space and this cannot handle stray text characters. (Bad input strings: " 8" and "8q" and "8 q".)
NSString *tempStr = #"8,765.4";
// localization allows other thousands separators, also.
NSNumberFormatter * myNumFormatter = [[NSNumberFormatter alloc] init];
[myNumFormatter setLocale:[NSLocale currentLocale]]; // happen by default?
[myNumFormatter setFormatterBehavior:NSNumberFormatterBehavior10_4];
// next line is very important!
[myNumFormatter setNumberStyle:NSNumberFormatterDecimalStyle]; // crucial
NSNumber *tempNum = [myNumFormatter numberFromString:tempStr];
NSLog(#"string '%#' gives NSNumber '%#' with intValue '%i'",
tempStr, tempNum, [tempNum intValue]);
[myNumFormatter release]; // good citizen
olliej's rounding method is wrong for negative numbers
2.4 rounded is 2 (olliej's method gets this right)
−2.4 rounded is −2 (olliej's method returns -1)
Here's an alternative
int myInt = (int)(myDouble + (myDouble>0 ? 0.5 : -0.5))
You could of course use a rounding function from math.h
// Converting String in to Double
double doubleValue = [yourString doubleValue];
// Converting Double in to String
NSString *yourString = [NSString stringWithFormat:#"%.20f", doubleValue];
// .20f takes the value up to 20 position after decimal
// Converting double to int
int intValue = (int) doubleValue;
or
int intValue = [yourString intValue];
For conversion from a number to a string, how about using the new literals syntax (XCode >= 4.4), its a little more compact.
int myInt = (int)round( [#"1.6" floatValue] );
NSString* myString = [#(myInt) description];
(Boxes it up as a NSNumber and converts to a string using the NSObjects' description method)
For rounding, you should probably use the C functions defined in math.h.
int roundedX = round(x);
Hold down Option and double click on round in Xcode and it will show you the man page with various functions for rounding different types.
This is the easiest way I know of:
float myFloat = 5.3;
NSInteger myInt = (NSInteger)myFloat;
from this example here, you can see the the conversions both ways:
NSString *str=#"5678901234567890";
long long verylong;
NSRange range;
range.length = 15;
range.location = 0;
[[NSScanner scannerWithString:[str substringWithRange:range]] scanLongLong:&verylong];
NSLog(#"long long value %lld",verylong);
convert text entered in textfield to integer
double mydouble=[_myTextfield.text doubleValue];
rounding to the nearest double
mydouble=(round(mydouble));
rounding to the nearest int(considering only positive values)
int myint=(int)(mydouble);
converting from double to string
myLabel.text=[NSString stringWithFormat:#"%f",mydouble];
or
NSString *mystring=[NSString stringWithFormat:#"%f",mydouble];
converting from int to string
myLabel.text=[NSString stringWithFormat:#"%d",myint];
or
NSString *mystring=[NSString stringWithFormat:#"%f",mydouble];
I ended up using this handy macro:
#define STRING(value) [#(value) stringValue]