Validate double number

**Elysia** · 01-14-2008

I assume buf is your buffer - the string you read from the user:

Code:

char buf[255];
fgets(buf, sizeof(buf), stdin);

Null as in NULL?
Like if (p == NULL)?

**robwhit** · 01-14-2008

Ah I see. Thanks for the explanation. I was thinking of just strtod.

However, if I may:

Code:

if ( strtod(buf, NULL) == 0 && (buf[0] != '0' && ((buf[0] != '\0') && buf[0] != '-' && buf[1] != '0')) ) /* Not a double! */;

?

**guaro555** · 01-14-2008

Ely

does the same function works for floats also as well?

**Elysia** · 01-14-2008

Originally Posted by robwhit

Ah I see. Thanks for the explanation. I was thinking of just strtod.

However, if I may:

Code:

if ( strtod(buf, NULL) == 0 && (buf[0] != '\0') && (buf[0] != '0' && (buf[0] != '-' && buf[1] != '0')) ) /* Not a double! */;

?

Yes! That might be safer!

Originally Posted by guaro555

Ely

does the same function works for floats also as well?

Yes, it works for floats as well, just change strtod to strtof.

**Dino** · 01-14-2008

Well, just call me a fly in the ointment.

This will fail the test and be called valid: 0DLFKD

It converts to zero since it's garbage, and the first char is zero, so it would be accepted.

I know, I know. I'm just stirring the pot.

Todd

**Elysia** · 01-14-2008

Fine!

How about this?

Code:

if (d == 0 && strcmp(buf, "0") != 0 && strcmp(buf, "-0") != 0) /* Not a double! */;

You really are doing your best to thwart my optimization, aren't you?

**whiteflags** · 01-14-2008

Really, to get the most out of strtod you have to take advantage of all of it's arguments. Knowing where strtod stopped its conversion helps you decide what is acceptable and not acceptable input. Dave Sinkula has illustrated the proper technique in a series of snippets.

My understanding is that strtod works for floats just fine:

float fltpt = (float) strtod( example, &endpt );

http://www.daniweb.com/code/snippet597.html

**Elysia** · 01-14-2008

I suppose... I'm used to atof/atod/atoi, which only takes one. Hmmm.

**robwhit** · 01-14-2008

Ah you caught my post before I saw my error....

the strcmp won't work because there might be more characters afterwards.

lol we are such a waste of bandwidth.

**Elysia** · 01-14-2008

Code:

char* pEnd;
double d = strtod(buf, &pEnd);
if (pEnd - buf == 0) /* "Not a double!" */;

That should work fine, too.

Originally Posted by robwhit

Ah you caught my post before I saw my error....

the strcmp won't work because there might be more characters afterwards.

lol we are such a waste of bandwidth.

Not sure I get what you mean with strcmp won't work since d will ONLY be 0 if the user entered "0" or "-0" or a non-string, so what's the error?

**robwhit** · 01-14-2008

I'm assuming it's part of a stream.

I'm beginning to have brain problems...

**Elysia** · 01-14-2008

Still unsure

How do you expect it to fail? Example, please?
Unless it's part of your "problems"?

**robwhit** · 01-14-2008

for example, you read in a buffer from a file:

char buf[128];
fread(buf, sizeof(buf), 1, file);

then you try to see if it contains a double.

oops, what if there is more data than the double? It will not have a \0, and so it will not compare equal.

**guaro555** · 01-14-2008

Originally Posted by Todd Burch

Well, just call me a fly in the ointment.

This will fail the test and be called valid: 0DLFKD

It converts to zero since it's garbage, and the first char is zero, so it would be accepted.

I know, I know. I'm just stirring the pot.

Todd

T

If I understood the statement correctly you have a few && in there so the first option is true as is garbage like you said and the second is true because is 0, so still not a valid double