Find first occurrence of string and replace

Discussion in 'C Programming' started by Registered User, Oct 16, 2006.

  1. Hi experts,
    I'm trying to write a program to solve the following exercise:

    Accept three strings from the user. Find the first occurrence of the
    first string in the third string. If it is present replace the second
    string in its place.
    Note: First and Second string might not be of the same length.
    E.g.: First string: "cat"
    Second string: "camel"
    Third string: "concatenate"
    Output: "concamelenate"

    I've made a complete working program. The following is the main() part
    of my program.
    int main()
    char s1[80], s2[80], s3[80];
    int pos;

    /*Code for accepting 3 strings from user*/
    /*and storing in s1, s2 and s3*/

    pos=find(s1, s3); /*find() returns the position at which */
    /*s1 occurs in s3, -1 otherwise*/

    delete(pos, s3, strlen(s1)); /*Deletes strlen(s1) characters from s3
    starting at pos*/
    /*Does not delete if pos<0*/

    insert(pos, s3, s2);/*Inserts s2 in s3 at location specified by pos*/
    /*Does nothing if pos<0*/


    So, the three operations I've used for finding and replacing are:
    (2)delete and
    My question is: Is there a better way of doing this?
    Registered User, Oct 16, 2006
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  2. What if strlen(s3)+strlen(s2) >= 80? Does 'insert' behave differently
    then, or have you got a buffer overflow?

    Also, you could speed up some cases by deleting or inserting only
    as many characters as you need to, and writing over the others. For
    example, to get from "concatenate" to "concamelenate", you'd delete
    zero characters, insert two characters ("concatxxenate"), and then
    overwrite all five characters ("concamelenate"). To get from
    "concatenate" to "condogenate", you wouldn't insert or delete any
    characters; just write "dog" at the right point in "concatenate"
    and you're done.

    Arthur J. O'Dwyer, Oct 16, 2006
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  3. One could also merge the the three operations together by
    using strstr(), memmove() and memcpy() in a single function:

    #include <string.h>

    char *found = strstr(s3, s1);
    size_t len = strlen(s2);

    if (found) {
    /* Exercise: insert bounds checking code here */
    memmove(found + len,
    strlen(s3) - strlen(s1) - (found - s3));
    memcpy(found, s2, len);

    Tak-Shing Chan, Oct 16, 2006
  4. Sorry, the memmove() line above is wrong. It should be:

    memmove(found + len,
    found + strlen(s1),
    strlen(s3) - strlen(s1) - (found - s3));

    Tak-Shing Chan, Oct 16, 2006
  5. Oops, I forgot the terminating '\0':

    memmove(found + len,
    found + strlen(s1),
    strlen(s3) - strlen(s1) - (found - s3) + 1);

    Tak-Shing Chan, Oct 16, 2006
  6. Perhaps posting on Mondays is a bad idea :)
    Christopher Benson-Manica, Oct 16, 2006
  7. Registered User

    Ed Collins Guest

    Is there a better way of doing this?

    Your method looks fine. In the real world, time is money. An argument can
    be made that whatever method works for you, especially if it's simple to
    code, understand, and maintain, can be considered "better." Are there more
    elegant solutions? Possibly. Are the "better?" Not necessarily.

    I don't code in C but here's one way to do it PowerBASIC. (This probably
    won't help you, since C almost surely doesn't have its own EXTRACTS$
    function, but I couldn't resist.)

    first$ = "cat" : second$ = "dog"
    third$ = "Actually, I believe cat is man's best friend!"

    a$ = EXTRACT$(third$, first$)

    IF a$ <> third$ THEN
    a$ = a$ + second$ + RIGHT$(third$, LEN(third$) - LEN(a$) - LEN(first$))
    END IF

    In the above example, the variable a$ now is equal to "Actually, I believe
    dog is man's best friend!"

    Ed Collins
    Ed Collins, Oct 18, 2006
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