CODEWITHSUNDEEP
cstring
Giga Tocka
Giga Tocka

Reputation: 191

Extracting a number from a string

I am trying to write a function in C that takes a string, for example: "abc123def" and returns a number in string form: "123".

I have little experience with C so I was wondering if I am using the isDigit() function correctly. My code is below, if there is a better way to solve the problem I would appreciate help. Thanks!

char findNumber(char *str1)
{
    char num1[] = "";
    int i = 0;
    int j = 0;
    while(str1[i] != '\0') {
            if(isDigit(str1[i])) {
                    num1[j] = str1[i];
                    j++;
            }
            i++;
    }
    num1[j] = '\0';
    return num1;
}

int main(int argc, const char* argv[])
{
    char str2[] = "!3254";
    printf(findNumber(str2));
    return 0;
}

I'm getting errors such as:

undefined reference to `isDigit'

and

return makes integer from pointer without a cast

What could be causing these?

Upvotes: 0

Views: 242

Answers (2)

Johnny Mnemonic
Johnny Mnemonic

Reputation: 3912

This should work:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

char* findNumber(char *str1)
{
    char* num1=malloc(strlen(str1)+1);//allocate memory for the number
    int i = 0;
    int j = 0;
    while(str1[i] != '\0') {
            if(isdigit(str1[i])) {//isdigit() is in ctype.h
                    num1[j] = str1[i];
                    j++;
            }
            i++;
    }
    num1[j] = '\0';
    return num1;
}

int main(int argc, const char* argv[])
{
    char str2[] = "!3254";
    char* number=findNumber(str2);
    printf("%s\n",number);
    free(number);//free the allocated memory
    return 0;
}

Upvotes: 2

Bernhard Barker
Bernhard Barker

Reputation: 55609

Your function needs to return char * since you're not just returning a single character, but a bunch of characters.

After a quick Google search I found that isdigit is defined in ctype.h, so lower-case D and include ctype.h.

Also, you've got yourself some undefined behaviour there, since you only allocate memory to num1 for a length 0 string. char *num1 = malloc(someSize) is one option, which should have a corresponding free somewhere if the program were to execute for anything more than a few seconds / minutes.

Code after fixes:

#include <ctype.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define MAX_SIZE 100

char *findNumber(char *str1)
{
    char *num1 = malloc(MAX_SIZE);
    // using "strlen(str1)+1" instead of MAX_SIZE might be preferred
    int i = 0, j = 0;
    while(str1[i] != '\0') {
            if(isdigit(str1[i])) {
                    num1[j] = str1[i];
                    j++;
            }
            i++;
    }
    num1[j] = '\0';
    return num1;
}

int main(int argc, const char* argv[])
{
    char str2[] = "!3254";
    printf(findNumber(str2));
    return 0;
}

Test.

Upvotes: 3

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