How to replace the last index of a string using C language? - c

Question

The code below tries to increment the last index in a string, eg: if label = "1_1_9", find_next_label (label ) will return "1_1_10".
This works. However, I also want to alter the original label, increment it as well. eg: if label = "1_1_9", find_next_label(label) will return "1_1_10" and during this procedure, label also becomes "1_1_10".
This code below is unable to do this. The result from main() function shows that label is still "1_1_9".
Could anyone help find where the problem is?
char * find_next_lable(char * label)
{
int length = strlen(label);
char * last_index = label + length - 1;
int num = atoi(last_index);
num = num + 1;
char * next_lable = malloc(sizeof(label));
strncpy(next_label, label, length-1);
*(next_label + length - 1) = '\0';
sprintf(next_label, "%s%d", next_label, num);
label = next_label;
return label;
}
int main()
{
char * s = malloc(6);
strcpy(s, "1_1_9");
char * n = find_next_label(s);
printf("%s\n", s);
printf("%s\n", n);
return 0;
}

Answer 1

The last_index() and atoi() code block assumes that the final number is only one digit long; clearly this is not very general. You could search for the last underscore instead, and convert a number from the character following that. Use strrchr() to look for the last underscore.
Also you must think a lot about buffer sizes and overruns, you should probably make the function accept the available buffer size as an additional argument especially if you want to modify the input. If you want that, there's of course no point in allocating additional space either, just return the input.

Answer 2

If you don't need to create a new string you can just do:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_LEN 20
int main()
{
char *s = malloc(MAX_LEN); /* You must have enough memory if the number of chars grows! */
char *n;
int i;
strcpy(s, "1_1_9");
printf("%s\n", s);
n = strrchr(s, '_'); /* find the last '_' */
n++; /* and move to the number */
i = atoi(n);
sprintf(n, "%d", i+1); /* write the new value instead of the old one */
printf("%s\n", s);
free(s);
return 0;
}
else you can have the function:
char * find_next_lable(char *label)
{
char *n, *next_lable = malloc(sizeof(MAX_LEN));
int i;
strcpy(next_lable, label);
n = strrchr(next_lable, '_');
n++;
i = atoi(n);
sprintf(n, "%d", i+1);
return next_lable;
}

Answer 3

The result from main() function shows that lable is still 1_1_9.
That's because you are not changing the dynamically allocated array pointed to by s in main. Instead, you allocate new memory in the function find_next_lable. Also,
sprintf(next_lable, "%s%d", next_lable, num);
won't work since %s conversion specifier means that sprintf will read from the buffer pointed to by next_lable till and including the terminating null byte.
You must allocate enough memory so as to contain the incremented integer part.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
// make sure MAX is large enough to
// contain the modified string
#define MAX 20
void find_next_lable(char *label);
int main(void)
{
char *s = malloc(MAX);
strcpy(s, "1_1_90");
printf("%s\n", s);
find_next_lable(s);
printf("%s\n", s); // prints 1_1_91
free(s);
return 0;
}
void find_next_lable(char *label)
{
// strrchr returns a pointer to the last
// occurrence of the character _ in label
char *last_index = strrchr(label, '_');
if(last_index == NULL)
{
last_index = label;
}
else
{
last_index++;
}
int num = atoi(last_index);
num = num + 1;
sprintf(last_index, "%d", num);
}