Thread: Substr

  1. #1
    Registered User
    Join Date
    Aug 2006


    Is there no equivalent to the substr function in c++ in c???

  2. #2
    Registered User hk_mp5kpdw's Avatar
    Join Date
    Jan 2002
    Northern Virginia/Washington DC Metropolitan Area
    You can use the sprintf function to simulate that.
    "Owners of dogs will have noticed that, if you provide them with food and water and shelter and affection, they will think you are god. Whereas owners of cats are compelled to realize that, if you provide them with food and water and shelter and affection, they draw the conclusion that they are gods."
    -Christopher Hitchens

  3. #3
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    I think he means strstr(), which finds a string within a string.

    To extract a substring from a string, use strncpy, as in
    strncpy( dest, &src[start], len );
    dest[len] = '\0';
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  4. #4
    Just Lurking Dave_Sinkula's Avatar
    Join Date
    Oct 2002
    I took hk's reply in this manner:
    #include <stdio.h>
    char *substr(char *dst, const char *src, int start, int len)
       sprintf(dst, "%.*s", len, &src[start]);
       return dst;
    int main(void)
       const char src[] = "Here is some text.";
       char dst[sizeof src];
       printf("dst = \"%s\"\n", substr(dst,src,5,7));
       return 0;
    /* my output
    dst = "is some"
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  5. #5
    Mad OnionKnight's Avatar
    Join Date
    Jan 2005
    Umeň, Sweden
    There's no substr function in the C standard libraries. The standard libraries don't like using dynamic memory with strdup as the only exception I can think of. C++ has a bunch though, I don't know why but I suspect it's because they don't want to give people the burden of having to free memory themselves.

    I wrote a substr function once which was supposed to be like the Perl substr but there were some limitations since I couldn't replace text on a string of unknown size so I had the function make a copy instead and that 0 and NULL is used to signal unused parameters.

    #include <stdlib.h>
    char* substr (const char* string, int pos, int len, const char* replace)
    	char* substring;
    	int   i;
    	int   length;
    	if (string == NULL)
    		return NULL;
    	length = strlen(string);
    	if (pos < 0) {
    		pos = length + pos;
    		if (pos < 0) pos = 0;
    	else if (pos > length) pos = length;
    	if (len <= 0) {
    		len = length - pos + len;
    		if (len < 0) len = length - pos;
    	if (pos + len > length) len = length - pos;
    	if (replace != NULL) {
    		if ((substring = malloc(sizeof(*substring)*(length-len+strlen(replace)+1))) == NULL)
    			return NULL;
    		for (i = 0; i != pos; i++) substring[i] = string[i];
    		pos = pos + len;
    		for (len = 0; replace[len]; i++, len++) substring[i] = replace[len];
    		for (; string[pos]; pos++, i++) substring[i] = string[pos];
    		substring[i] = '\0';
    	else {
    		if ((substring = malloc(sizeof(*substring)*(len+1))) == NULL)
    			return NULL;
    		len += pos;
    		for (i = 0; pos != len; i++, pos++)
    			substring[i] = string[pos];
    		substring[i] = '\0';
    	return substring;
    substr("some string", 5, 0, NULL)
    returns "string"

    substr("some string", -5, 3, NULL)
    returns "str"

    substr("some string", 4, 0, "thing")
    returns "something"

    free() needs to be called on the string returned after it's use.

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