How to write a C function that removes a portion of a string?

Question

I want to write a C function that removes a portion of a string for a given index range.

For example, if the input string is "ABCDEFGHIJK" and the start index is 2 and the end index is 5 then the output should be: "ABGHIJK".

I am attempting to do this using two functions, one function that gets the substring we want to delete:

void get_substring(char string[], char substring[], int start, int end) {
    strncpy(substring, string + start, end - start + 1);
}

and then a second function that deletes this substring:

void remove_portion(char string[], char substring[]) {
    // memmove?
}

Another possibility I was thinking about is to directly modify the original string without using a substring:

void remove_portion(char string[], int start, int end) {
    // if end is less then the length of the string, then
    // copy everything after string[end] into a temp string
    // Then replace string[start] with '\0' and then concatenate
    // string and temp.
    // If end is greater than the length of string then just replace
    // string[start] with '\0'.
}

Is this the correct approach? Are there any built in functions from string.h that can be useful here?

Answer 1

I would go with using memmove as in the 2nd approach:

void remove_portion(char string[], int start, int end) 
{
    if (start>=0 && end>=start && start<strlen(string) && end<strlen(string)) {  // some more sanity checking (EDIT added later)
         memmove(string+start, string+end+1, strlen(string)-(end+1)+1);  // final +1 to copy string terminator
    }
}

Also note that in your first example (with strncpy) is not going to to copy the ending string terminator \0 to substring. So you'll need to add

substring[end - start + 1]= '\0'; 

to that.

Answer 2

Use this :

void remove_portion( char * str, int start, int end){
char* stro = calloc(strlen(str - (end-start+1)), sizeof(char));
strncpy(stro,str,start);
strcat(stro,&str[end]);
strcpy(str,stro);
}

Add conditions also.

Answer 3

New to C myself, but this worked for me:

void remove_portion(char str[], int start, int end) {
    assert((end > start) && (strlen(str) > end));
    char out[strlen(str) - (end - start)];
    int i, j = 0;
    for (i = 0; str[i] != '\0'; i++) {
        if ((i < start) || (i > end))
            out[j++] = str[i];
    }
    out[j] = '\0';
    strcpy(str, out);
}