Reading numbers in C

Numbers can be complicated. For example, whether you accept scientific notation as floating point numbers can cause your program have a double number of lines.

Of course, the simplest way to grab any text is using the regular expressions. If you are using POSIX-capable systems, the natural way is to use the POSIX regex library. As shown in the program at the bottom.

The POSIX regex functions are declared in regex.h and the major functions are regcomp() to compile the regex, regexec() to perform a match, regerror() to collect error messages and regfree() to reclaim the memory occupied by regcomp().

However, if you just need simplest number-reading function in C, probably strtod() is good enough to use. For example, the following code excerpt:

  char *ptr, *ptrNext, buff[BUFSIZE];
  FILE* fpMatrix=fopen("matrix.txt","r");
  ....
  for(i=0;i<m;i++) {
      if(!fgets(buff, BUFSIZE, fpMatrix)) {
          perror("Failed to finish reading the whole matrix");
          exit(1);
      };
      ptr = buff;     /* We start at the beginning of *buf */
      for(j=0;j<m;j++) {
          R[i][j]=strtod(ptr, &ptrNext);
          if(ptr==ptrNext) {
              perror("Failed to finish reading a row of matrix");
              exit(1);
          };
          ptr = ptrNext;  /* Shift to next number */
      };
  };

The above excerpt is to read in a m-by-m matrix with each element is a double-precision float. The file is presented in rows as lines and each element in a row separated by some word-separating characters like spaces or tab. The validity of the above excerpt is due to the use of second parameter to strtod() function.

regex-example.c:

/* regex-example2.c
 * Tue, 11 May 2010 12:35:47 -0400
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <regex.h>

/* Example on how to use POSIX Regex calls
 * The regex calls supported in *nix can be those provided by GNU, from BSD, or
 * as in POSIX standard.
 *
 * Here the POSIX version is used and the following are the API:
 *   int regcomp(regex_t *preg, const char *regex, int cflags);
 *   int regexec(const regex_t *preg, const char *string, size_t nmatch, regmatch_t pmatch[], int eflags);
 *   size_t regerror(int errcode, const regex_t *preg, char *errbuf, size_t errbuf_size);
 *   void regfree(regex_t *preg);
 */

int main (int argc, char** argv)
{
	/* First example: Match a string multiple time */
	char* string = "   0.1234  1.2345  2.34567 	8 9   10.111213";
	char* pstr = string;
	int errcode;
	regex_t *preg = (regex_t*)malloc(sizeof(regex_t));
	regmatch_t *pmatch;

	/* Compile regex */
	if ((errcode=regcomp(preg, "([0-9]+\\.[0-9]+)",REG_EXTENDED|REG_NEWLINE))) {
		char* errbuf;
		size_t errbuf_size;

		errbuf_size = regerror(errcode, preg, NULL, 0);
		if (!(errbuf=(char*)malloc(errbuf_size))) {
			perror("malloc error!");
			exit(255);
		};
		regerror(errcode, preg, errbuf, errbuf_size);
		fprintf(stderr, "Regex compilation error: %s\n", errbuf);
		exit(1);
	};

	/* Allocate substrings buffer (1 substring) */
	pmatch = (regmatch_t*)malloc(sizeof(regmatch_t)*preg->re_nsub);

	/* Match regex to string */
	if(regexec(preg, pstr, preg->re_nsub, pmatch, 0)) {
		printf("Nothing matched with ""%s""\n",pstr);
		exit(0);
	};
	
	/* Loop to print all the matches */
	do {
		if (pmatch[0].rm_so != -1) {	/* The regex is matching part of a string */
			char *submatch;
			size_t matchlen = pmatch[0].rm_eo - pmatch[0].rm_so;
			submatch = (char*)malloc(matchlen+1);
			strncpy(submatch, pstr+pmatch[0].rm_so, matchlen+1);
			submatch[matchlen]='\0';
			printf("%s;\n", submatch);
			free(submatch);
		};
		pstr += pmatch[0].rm_eo;	/* Restart from last match */
	} while(!regexec(preg,pstr,preg->re_nsub,pmatch,0));
	regfree(preg);
	free(pmatch);

	/* Second example: Match the whole string once and extract multiple captures */
	string = "I have 20 apples and 34 oranges.";
	pstr = string;
	regcomp(preg, "([0-9]+) apples.* ([0-9]+) oranges",REG_EXTENDED|REG_NEWLINE);
	pmatch = (regmatch_t*)malloc(sizeof(regmatch_t)*(1+preg->re_nsub));
	printf("There can be %d captures\n",(int)preg->re_nsub);
	if(regexec(preg, pstr, 1+preg->re_nsub, pmatch, 0)) {
		printf("Nothing matched with ""%s""\n",pstr);
		exit(0);
	};
	if (pmatch[0].rm_so != -1) {	/* Regex matched */
		/* The whole matched part */
		char *submatch;
		size_t matchlen = pmatch[0].rm_eo - pmatch[0].rm_so;
		submatch = (char*)malloc(matchlen+1);
		strncpy(submatch, pstr+pmatch[0].rm_so, matchlen+1);
		submatch[matchlen]='\0';
		printf("Matched part = %s;\n", submatch);
		free(submatch);
		/* First capture bracket */
		matchlen = pmatch[1].rm_eo - pmatch[1].rm_so;
		submatch = (char*)malloc(matchlen+1);
		strncpy(submatch, pstr+pmatch[1].rm_so, matchlen+1);
		submatch[matchlen]='\0';
		printf("Apple = %s;\n", submatch);
		free(submatch);
		/* Second capture bracket */
		matchlen = pmatch[2].rm_eo - pmatch[2].rm_so;
		submatch = (char*)malloc(matchlen+1);
		strncpy(submatch, pstr+pmatch[2].rm_so, matchlen+1);
		submatch[matchlen]='\0';
		printf("Orange = %s;\n", submatch);
		free(submatch);
	};

	free(preg);
	exit(0);
};