Question

How to get substring in C

Asked by: priyashree

How to get substring in C

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Asked On
2005-01-04 at 03:08:18ID21260896
Tags

c

,

substring

Topic

C Programming Language

Participating Experts
5
Points
50
Comments
12

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Answers

 

by: Jase-CoderPosted on 2005-01-04 at 03:26:37ID: 12951168

hi try
strchr(char *str, int ch)

for example

#inlucde <string.h>

void main()
{
  *p;

   p = strchr("This is a test", '  ')     // p holds "is a test"
}

 

by: Jase-CoderPosted on 2005-01-04 at 03:27:37ID: 12951169

better still use strstr(char *str1, char *str2)

 

by: msjammuPosted on 2005-01-04 at 03:59:27ID: 12951310

I can use strncpy() somehow

 

by: rstaveleyPosted on 2005-01-04 at 04:47:28ID: 12951588

char* points to an array of characters. Conventionally in C, there is an ASCII 0 ('\0') character in character arrays to denote the end.

If, say, you declare,

        char *p = "abcd";

your p character pointer points to an array of 5 (yes 5) characters, the last of which is '\0'.

If you wanted to point to "bc", you might be tempted to do something like...

        char p2 = p+1;

However, most C library functions expect you to '\0'-terminate the string, which means that p2 would be pointing to "bcd".

That's why you generally need to copy your substring into a separate character array and then copy a '\0'-terminator into it.

This illustrates the point:

--------8<--------
#include <stdio.h>
#include <string.h>

int main()
{
      char *src  = "abcd";
      char *dst = "01234";

      strncpy(dst,src+1,2);  /* Copy a substring of two characters from src into dst, starting from the 2nd character in src */
      printf("dst is \"%s\"\n",dst);
}
--------8<--------

See the output?

--------8<--------
dst is "bc234"
--------8<--------

 

by: Jase-CoderPosted on 2005-01-04 at 04:51:43ID: 12951613

I think   strncpy()  is a c++ function that has a defult parameter, which is the length of the string.

 

by: rstaveleyPosted on 2005-01-04 at 04:51:53ID: 12951614

NB: you'll get a core dump with that on Linux though, because you are copying data to read-only memory.

Better to write:
--------8<--------
#include <stdio.h>
#include <string.h>

int main()
{
        char src[]  = "abcd";
        char dst[] = "01234";

        strncpy(dst,src+1,2);
        printf("dst is \"%s\"\n",dst);
}
--------8<--------

That way the src and dst strings are copied into the stack, which you are allowed to write to.

Windows is less discriminating :-)

 

by: rstaveleyPosted on 2005-01-04 at 04:52:53ID: 12951623

>  I think   strncpy()  is a c++ function that has a defult parameter, which is the length of the string

Nope, strncpy is part of the C library. You are thinking of the C function strcpy, which copies the entire string.

 

by: MysidiaPosted on 2005-01-04 at 07:21:07ID: 12952776

You can copy the right pieces to a new buffer space to get a substring:

Where 'buffer' is of length greater than start-end+1

char* substring_r(char* buffer, char* str, int start, int end)
{
      int i, x = 0;
     
      for(i = start ; i <= end; i++)
            buffer[x++] = str[i];
      buffer[x] = '\0';

      return temp;
}

 

by: madhurdixitPosted on 2005-01-04 at 08:22:33ID: 12953548

String in C is simply array of characters terminated by '/0' .

If you are searching for a particular substring you can compare them through indexing.

If you want a substring say , from length 5 to 10  ,
you can start indexing from 5 and go on 10 and every time read it on to another character array.
This is pretty much done in answer above by "Mysidia".

 

by: MysidiaPosted on 2005-01-04 at 10:25:14ID: 12954795

return temp;

would really be
   return buffer;

to use this approach you need the offsets, and you need to set aside a buffer, i.e.

int main()
{
    char temp[25];

    substring_r(temp, 1, 6, "!Hello there...");
    printf("%s\n", temp);
}


if you're willing to destroy the original string, then it's even easier..

just null terminate where you want it to stop, and move the start of
the substring to the start of the array.

int main()
{
     char temp[25] = "!Hello there...";

     memmove(temp, temp+1, 6);  /* the 6 is how many bytes to copy starting from the source string
                                                     and ending at (temp+1)[5] */
     temp[6] = '\0'; /* End the result*/

     printf("%s\n", temp);
}

20120131-EE-VQP-002

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