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For example, I want to replace abc in 123abc890 with xyz. How would I do this without using #include <string.h>.

I tried attempting this by creating a new string, and with the help of a for loop, I would copy every character into the new string. However, I couldn't seem to find out how to go about doing this. Any method is fine, however I cannot use #include <string.h>.

char str1[] = "123abc890";
char str2[] = "abc";
char str3[] = "xyz";
char new_string[50];

for (int i = 0; str1[i] != '\0'; i++) {
    if (str1[i] == str2[i]) {
        for (int j = 0; str2[j] != '\0'; j++) {
            if (str1[i+j] != str1[i+j]) {
                break:
            }
            new_string[i] = str3[i];
        }
    }
    new_string[i] = str1[i];
}

FYI I am very new to C, so beware of some very clumsy code.

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  • 1
    Please show a minimal reproducible example. This is just a code fragment that cannot be compiled. Commented Feb 4, 2021 at 20:54
  • 1
    Break the problem up into smaller steps. Write functions for those where it makes sense. For example, start by writing a function that takes str1 and str2 and then returns the first index (if any) in str1 that matches str2 - i.e, basically rewrite strstr. Commented Feb 4, 2021 at 20:54
  • 5
    Also if you are not allowed to use functions in string.h you can write your own versions of them. Many of them are one or two liners. Commented Feb 4, 2021 at 20:55
  • 2
    Does this answer your question? What function is to replace a substring from a string in C? Commented Feb 4, 2021 at 20:56
  • Try working out the algorithm on paper first. The loop and indices here don't make much sense. Just because str1[i] == str2[0] doesn't mean you have a full substring, so if you're going to rush ahead and start copying that substring as if you did, be prepared to undo it when the entire substring doesn't match. Write a helper that checks the full substring match between str2 and str and try it on every index of str1. When it returns true, you're safe to copy str2 over to the destination, else copy str1's character. Commented Feb 4, 2021 at 21:00

1 Answer 1

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2

Your code is a good start, but there are many problems:

  • the test if (str1[i+j] != str1[i+j]) is always true
  • you copy incomplete fragments and fail to skip it after copying.
  • you forgot to null terminate the destination.

Here is a modified version:

#include <stdio.h>

char *str_replace(char *dest, const char *str1, const char *str2, const char *str3) {
    size_t i = 0, j, k = 0;
    
    // replacing substring `str2` with `str3`, assuming sufficient space
    while (str1[i] != '\0') {
        for (j = 0; str2[j] != '\0'; j++) {
            if (str1[i + j] != str2[j]) {
                break;
            }
        }
        if (str2[j] == '\0' && j > 0) {
            // we have a match: copy the replacement and skip it
            i += j;
            for (j = 0; str3[j] != '\0'; j++) {
                dest[k++] = str3[j];
            }
        } else {
            // copy the byte and skip it.
            dest[k++] = str1[i++];
        }
    }
    dest[k] = '\0';  // null terminate the destination
    return dest;
}

int main() {
    char new_string[50];
    printf("%s\n", str_replace(new_string, "123abc890", "abc", "abc"));
    printf("%s\n", str_replace(new_string, "123abc890", "abc", "xyz"));
    printf("%s\n", str_replace(new_string, "123abc890", "a", "xyz"));
    printf("%s\n", str_replace(new_string, "123abc890", "abc", ""));
    return 0;
}

Output:

123abc890
123xyz890
123xyzbc890
123890
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1 Comment

Good call, that works.

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