Thread: Validate double number

I think this works pretty good. It uses a regular expression to validate a double. It most likely does NOT illustrate best practices for regular expression use in C.

Code:

#include <stdio.h>
#include <regex.h>

int main (int argc, const char * argv[]) {
	char data[80] ; 
	int rc ; 
	regex_t * myregex ; 
	regmatch_t arrayOfMatches[2];
	
	// Compile the regular expression 
	rc = regcomp( myregex, 
		"^ *[-+]?([0-9]+(\.[0-9]*)?|\.[0-9]+)([eE][-+]?[0-9]+)? *$", 
		REG_EXTENDED | REG_NOSUB ) ; 
	printf("RC from regcomp() = &#37;d\n", rc); 
	
	
	printf("Enter a double value\n");
	scanf("%s",data) ; 
	
	// Compare the entered value to the regex 
	if (!regexec(myregex, data, 1 , arrayOfMatches, 0 ) ) {
		printf("double %s is valid.\n", data) ; 
	}
	else printf("double %s is not valid\n", data ); 
	return 0;
}

Todd

Well, I think my regex is correct, but it reports 123e as being valid. Methinks the default RE support on a Mac might not be working properly.

Todd

It also reports --123 as being valid. I suppose this might be why folks recommend the perl compatible regular expression (pcre) library over the default lib.

Todd

Never mind, unless someone can explain the bogus results. It also says 123ee-4 is valid.

Originally Posted by matsp

Right, so what's wrong with using this:

Code:

int main()
{
   char buf[40];
   char *endPt;
   double x;
   fgets(buf, sizeof(buf), stdin);
   buf[strlen(buf)-1] = '\0;'
   x = strtod(buf, &endPt);
   if (endPt != '\0') printf("Not a valid number\n");
   else printf("x = %f\n", x);
   return 0;
}

It's fairly efficient compared to using regular expressions, and it validates that the entire string is accepted by strtod.

--
Mats

LOL. Other than it doesn't work, nothing! (I fixed the typo with the semi colon). I entered 123.45 and it says it's not valid.

Todd

This fixes it.

Code:

int main()
{
   char buf[40];
   char *endPt;
   double x;
   fgets(buf, sizeof(buf), stdin);
   buf[strlen(buf)-1] = '\0';
   x = strtod(buf, &endPt);
   if (*endPt != '\0') printf("Not a valid number\n");
   else printf("x = %f\n", x);
   return 0;
}

I think it is a clever approach - leveraging the second parm to strtod(). I found one case, that of just pressing enter, that didn't work. So, thanks to matsp, here's the final version.

Code:

int main()
{
   char buf[40];
   char *endPt;
   double x;
   fgets(buf, sizeof(buf), stdin);
   buf[strlen(buf)-1] = '\0' ; 
   x = strtod(buf, &endPt);
   if (*endPt != '\0' || strlen(buf)==0 ) printf("Not a valid number\n");
   else printf("x = %f\n", x);
   return 0;
}