String to float in objective c

Question

I have a parser returning some string value I'd like to use as parameter for my class instance initialisation.

I have a method asking two NSString and a float value, but I can't convert the string to float, here is my code:

 NSString *from = [[NSString alloc] initWithString:@"EUR"];
    NSString *to = [[NSString alloc] initWithString:[attributeDict objectForKey:@"currency"]];
    NSNumber *rate = [[NSNumber alloc] initWithFloat:[[attributeDict objectForKey:@"rate"] doubleValue]];


   currency = [[Currency init] alloc];
   [currency addCurrencyWithFrom:from andTo:to andRate:rate];

in the .h

- (id) addCurrencyWithFrom: (NSString *) from_ andTo:(NSString *) to_ andRate:(float *) float_;

Answer 1

No no no no no no no no no.

Use an NSNumberFormatter. This is why it exists. -floatValue and -doubleValue are merely quick-and-easy temporary fixes, but they fail in bad ways. For example:

NSLog(@"%f", [@"123" floatValue]); //logs 123.0000
NSLog(@"%f", [@"abc" floatValue]); //logs 0.0000
NSLog(@"%f", [@"0" floatValue]);   //logs 0.0000
NSLog(@"%f", [@"42" floatValue]);  //logs 42.0000
NSLog(@"%f", [@"42x" floatValue]); //logs 42.0000

-floatValue is unable to distinguish between a proper number (@"0") and an invalid one (@"abc").

NSNumberFormatter, on the other hand, will give you nil if it can't parse it, and an NSNumber if it can. It also takes things like currency symbols, region-specific decimal and thousands separators, and the user's locale into account. -floatValue and friends do not.

See this related question for more info: How to convert an NSString into an NSNumber