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C parsing double from argument strings

Answer 1

+5 A:

Are you including the relevant header? ie: #include <stdlib.h>

First though (and you should be doing this all the time anyway), try compiling with all warnings on (-Wall on GCC).

If you get a warning about strtod being undefined, that shows where the problem is coming from.

This is a nasty one, because C will implicitly declare any function it doesn't have a prototype for as returning int!

therefromhere 2010-02-08 08:20:18

hmm, why the downvote? I'm pretty sure this is the correct answer. Oh well.

therefromhere 2010-02-08 08:33:22

Not quite: it implicitly takes it as returning an int, not 0. But you're asking the same question I would...

Darius Bacon 2010-02-08 08:46:25

It implicitly *declares* it as returning int. The definition isn't implicit: it's actually calling the library function, the correct definition with the wrong declaration. Hence looking in the wrong place for the return value — an integer register instead of an FP register.

Potatoswatter 2010-02-08 08:59:37

In this case, the returned value is 0.0 because eax is set to zero on entry and the actual value is stored on a floating point register. It could be worse if floating point and integer were stored in the same registers.

jbcreix 2010-02-08 09:00:15

@Darius Ah sorry yeah, I meant to type returns int, not 0 - wrote that in a hurry on the way out of the door. Correcting now.

therefromhere 2010-02-08 09:56:10

Awesome. I thought this was included in the string.h library, but clearly I was wrong. Thanks a lot.

helixed 2010-02-08 15:07:08

Answer 2

+1 A:

You can use sscanf.

double num;
sscanf("123.0", "%lf", &num);

KennyTM 2010-02-08 08:20:54

Unless you're trying to parse multiple items from one string, there's no benefit to using `sscanf` over `strtod`. It's more complicated and has the drawback of making it harder to detect malformed data.

jamesdlin 2010-02-08 10:48:33

Answer 3

A:

You if have to use strtod you can do:

double num = strtod("123.0",NULL);

you can also use sscanf

double num;    
sscanf("123.0","%lf",&num);

codaddict 2010-02-08 08:23:21

-1. You're repeating his code, except not allowing a way to detect errors (that's what the second parameter to strtod is for) and exactly repeating an earlier (though only by 2 minutes) answer.

Roger Pate 2010-02-08 09:01:07

Answer 4

+1 A:

You need to include stdlib.h

dc 2010-02-08 08:25:26

Answer 5

+1 A:

In which language is your operating system? I'm not sure how the C function strtod reacts, but I know that the equivalent delphi function takes the settings of the operating system into account. Some languages (french, german, ...) use a "," instead of a "." as decimal separator.

Name 2010-02-08 08:26:58

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C parsing double from argument strings

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