Counting substrings in C

Question

A friend of mine needed help counting the occurrences of a substring in a string, and I came up with the following code. Does anyone know a better method to do this?

#include "stdio.h"
#include "string.h"

int main(int argc, char *argv[])
{
    char str1[50], str2[50];
    int i, j, l1, l2, match, count;

    printf("String 1:\n");
    gets(str2);
    printf("String 2:\n");
    gets(str1);

    l1 = strlen(str1);
    l2 = strlen(str2);

    count = 0;

    for(i = 0; i < l1; i++)
    {
        match = 0;
        for(j = 0; j < l2; j++)
        {
            if(str1[i + j] == str2[j])
            {
                match++;
            }
        }

        if(match == l2)
        {
            count++;
        }
    }

    printf("Substrings: %d\n", count);
}

LOL it's not my homework, and you can see I've already done it... I was just looking for a better solution. — guhemama
– guhemama, Commented Sep 30, 2011 at 14:36
Use #include <stdio.h> with angle brackets instead of quotes; it's what the standard says you should do. Although the quotes work, they're aconventional. — Jonathan Leffler
– Jonathan Leffler, Commented Sep 30, 2011 at 14:46
Thanks for the info, but I only changed them here because StackOverflow wasn't showing them in its WYSIWYG editor. :) — guhemama
– guhemama, Commented Sep 30, 2011 at 14:55

Constantinius · Accepted Answer · 2014-07-26 14:10:19Z

4

how about this: (using the strstr function, reference here)

int count = 0;
char str1[50], str2[50];
char* tmp = str1;
int count;

printf("String 1:\n");
gets(str2);
printf("String 2:\n");
gets(str1);

while(*tmp != '\0' && (tmp = strstr(tmp, str2))) {
    ++count;
    ++tmp;
}

edited Jul 26, 2014 at 14:10

answered Sep 30, 2011 at 14:27

Constantinius

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2 Comments

guhemama Over a year ago

It worked flawlessly. I already knew strstr, but I did not realize how to use its return value. Simple and efficient solution, thanks!

Hernán Erasmo Over a year ago

I think you're missing a parenthesis in that while definition that encloses tmp = strstr(tmp, str2). This answer helped me a lot, thank you.

Jonathan Leffler · Accepted Answer · 2011-09-30 14:59:34Z

3

Please do not use or encourage the use of gets. Beyond the fact that it will introduce a point of failure in your code, it has been deprecated as of C99 and will be gone completely from C1X.

As others have said, strstr is your friend here:

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

int main(void)
{
  char s1[50], s2[50];
  char *p;
  size_t count = 0;
  size_t len1;

  printf("Gimme a string: ");
  fflush(stdout);
  fgets(s1, sizeof s1, stdin);
  p = strchr(s1, '\n');          // get rid of the trailing newline
  if (p)
    *p = 0;

  printf("Gimme another string: ");
  fflush(stdout);
  fgets(s2, sizeof s2, stdin);
  p = strchr(s2, '\n');          // get rid of the trailing newline
  if (p)
    *p = 0;

  p = s2;
  len1 = strlen(s1);

  while ((p = strstr(p, s1)) != NULL && p != s1)
  {
    count++;
    p += len1;
  }

  printf("Found %lu occurrences of %s in %s\n", count, s1, s2);
  return 0;
}

edited Sep 30, 2011 at 14:59

Jonathan Leffler

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answered Sep 30, 2011 at 14:54

John Bode

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3 Comments

Jonathan Leffler Over a year ago

Good point about not using gets(). Although it will not be standard in C1X, it will (sadly) be present in C libraries for another decade or three for reasons of backwards incompatibility. Personally, I think the correct implementation is char *gets(char *buffer) { abort(); return 0; } (where the return is optional if the compiler knows that abort() does not return).

guhemama Over a year ago

Thanks for the info mate. GCC always complains about gets(), so I guess I'll try to get acquainted to fgets() hehe. Just a simple question: are all those fflush's really needed?

John Bode Over a year ago

Standard output is (normally) buffered, so if you write a string without a terminating newline, it may not show up on the console without an explicit flush.

0xCAFEBABE · Accepted Answer · 2011-09-30 14:32:37Z

0

You might want to take a look at the strstr function (if you're not already familiar with it).

edited Sep 30, 2011 at 14:32

answered Sep 30, 2011 at 14:27

0xCAFEBABE

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2 Comments

Constantinius Over a year ago

I don't think that this function is defined in the C standard. I guess it is only available in non-standard extensions.

0xCAFEBABE Over a year ago

You're right. Changed my post from "instr" to "strstr". Thanks.

kriya · Accepted Answer · 2013-12-30 11:48:43Z

0

int main()
  {
          char *str = "This is demo";
          char *sub = "is";
          int i,j,count;
          i=j=count=0;
          while(str[i]!='\0')
          {
           if (str[i] == sub[j] && str[i+1] == sub[j+1])
          {
                  count++;
          }
          i++;
          }
          cout<<count;
          return 0;
  }

Above code works but this is static.

answered Dec 30, 2013 at 11:48

kriya

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Comments

Hadi Rasekh · Accepted Answer · 2015-02-24 06:25:20Z

0

You can use QString in QT library

QString t = "yourstring";
t.count("yoursubstring");

answered Feb 24, 2015 at 6:25

Hadi Rasekh

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