How to replace a specified number of occurrences of a cstring in C?

Question

Languages such as Python have the replace(s, old, new [,maxreplace]) method which not only replaces a substring but also allows one to limit the number of replacements made. How can I implement this in C without use of Regex? It is preferred that the cstring is not modified and can be of any length including 0. This was what I tried. I would like this function to have a parameter max_replacements such that if and only if it equates to 0, all occurrences are replaced.

char *strnrep(char *haystack, char needle, char replacement, int max_replacements) {
    char buf[strlen(haystack) + 1];
    int count = 0;
    strcpy(buf, haystack);
    for (unsigned j = 0; j < strlen(buf); ++j) {
        if (buf[j] == needle) count++;
    }
    int j = 0;
    if (max_replacements == 0) {
        max_replacements = count;
    }
    while (j < max_replacements) {
        for (int i = 0; buf[i] != '\0'; i++) {
            if (buf[i] == needle) {
                buf[i] = replacement;
                break;
            }
        }
        j++;
    }
    return strdup(buf);
}

But only works for char rather than char* or char[]...

NOTE This question is NOT duplicate to what one of the commenters is suggesting. I am asking for a specified number of occurrences rather than all.

I'm confused. You want to replace a substring, but not modify the string? How does that work? — William Pursell, Mar 09 '22 at 14:19
I think he means the haystack should not be modified, and returns a new allocated string — Ôrel, Mar 09 '22 at 14:20
Does this answer your question? [What function is to replace a substring from a string in C?](https://stackoverflow.com/questions/779875/what-function-is-to-replace-a-substring-from-a-string-in-c) — Ôrel, Mar 09 '22 at 14:24
@Ôrel I am interested in replacing a specified number of occurrences. I tried the code in your link before posting this question and it replaces all occurrences. — user14773854, Mar 09 '22 at 14:26
have you at least try to implement the limitation of occurrences ? What is the issue you get ? — Ôrel, Mar 09 '22 at 14:30
`char buf[strlen(haystack)];` ==> `char buf[strlen(haystack) + 1];` — pmg, Mar 09 '22 at 14:40

score 0 · Accepted Answer · answered Mar 09 '22 at 14:45

Updating previous replace solution

Don't look for all occurence, stop if you reach the limit with something like:

        if (count >= max)
            break;

// You must free the result if result is non-NULL.
char *str_replace(const char *orig, char *rep, char *with, int max) {
    char *result; // the return string
    const char *ins;    // the next insert point
    char *tmp;    // varies
    int len_rep;  // length of rep (the string to remove)
    int len_with; // length of with (the string to replace rep with)
    int len_front; // distance between rep and end of last rep
    int count;    // number of replacements

    // sanity checks and initialization
    if (!orig || !rep)
        return NULL;
    len_rep = strlen(rep);
    if (len_rep == 0)
        return NULL; // empty rep causes infinite loop during count
    if (!with)
        with = "";
    len_with = strlen(with);

    // count the number of replacements needed
    ins = orig;
    for (count = 0; tmp = strstr(ins, rep); ++count) {
        ins = tmp + len_rep;
        if (count >= max)
            break;
    }

    tmp = result = malloc(strlen(orig) + (len_with - len_rep) * count + 1);

    if (!result)
        return NULL;

    // first time through the loop, all the variable are set correctly
    // from here on,
    //    tmp points to the end of the result string
    //    ins points to the next occurrence of rep in orig
    //    orig points to the remainder of orig after "end of rep"
    while (count--) {
        ins = strstr(orig, rep);
        len_front = ins - orig;
        tmp = strncpy(tmp, orig, len_front) + len_front;
        tmp = strcpy(tmp, with) + len_with;
        orig += len_front + len_rep; // move to next "end of rep"
    }
    strcpy(tmp, orig);
    return result;
}

int main(void)
{
    const char* input = "foo bar one bar two bar three bar";
    char *replace = str_replace(input, "bar", "john", 2);
    printf("before: %s\nafter: %s\n", input, replace);
    free(replace);
    return 0;
}

will output

before: foo bar one bar two bar three bar
after: foo john one john two bar three bar

Ok got it. Changing `if (count >= max) break;` to `if (max_replacements != 0 && count >= max_replacements) break;` ensures that if `max_replacements` is set to 0, then all replacements are made. — user14773854, Mar 09 '22 at 15:09

How to replace a specified number of occurrences of a cstring in C?

1 Answers1