parsing NSString to Double

Question

so I want to convert NSString to double. I found the following example:

NSString * s = @"1.5e5";
NSLog(@"%lf", [s doubleValue]);

It works but if doubleValue cannot convert the string to double it simply returns 0.0 which is not what I need. I need some method that tries to convert a string representation of double to double and if indicate somehow if it can't be converted. c# has an excellent method

double d;
boolean Double.TryParse(str, out d)

Is there any method similar to the above one in Objective C? or maybe it's better to use regex? however, i don't really know how to do that.

Another example of people thinking that regex is the ultimate magic solution for everything and that it brings world peace... — , Oct 12 '12 at 19:50
By the way, what you're doing is **undefined behavior.** I won't be surprised if it printed out the correct value if you used `%lf` instead of `%f` (which is for `float`s and not for `double`s...) — , Oct 12 '12 at 19:52
@H2CO3: According to [String Programming Guide/Format Specifiers](https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/Strings/Articles/formatSpecifiers.html), `%f` works with `float` and `double` when formatting. Only for scanning you have to differentiate. — Martin R, Oct 12 '12 at 19:58
@MartinR Strange. Upper on the same page it's written that NSString format specifiers support everything `printf()` does - and for printf, `%f` is for floats and `%lf` is for doubles, and using them interchangeably is undefined behavior. — , Oct 12 '12 at 20:02
@H2CO3: I am not an expert on this, but http://stackoverflow.com/questions/4264127/correct-format-specifier-for-double-in-printf states that `%f` can be used for `double`. That is also what I understand from http://pubs.opengroup.org/onlinepubs/009695399/functions/printf.html. — Martin R, Oct 12 '12 at 20:09
@MartinR Bingo! Asked the poster of the answer you linked and he explained how default type promotion with variadic functions works. You're right. — , Oct 12 '12 at 20:16

score 9 · Accepted Answer · answered Oct 12 '12 at 19:48

You can use the NSScanner class:

NSString *s = @"1.5e5";
NSScanner *scanner = [NSScanner scannerWithString:s];
double d;
BOOL success = [scanner scanDouble:&d];

If you want to ensure that the entire string has been scanned (no extra characters after the number), use

BOOL isAtEnd = [scanner isAtEnd];

parsing NSString to Double

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