How to reduce or replace the else if statements in C language?

Question

I'm using the code as shown below

  if (strcmp(oid, ".1.3.6.1.4.1.53864.1.1.0") == 0) 
  {
      return 1;
  } 
  else if (strcmp(oid, ".1.3.6.1.4.1.53864.1.2.0") == 0) 
  {
      return 2;
  }
  ...
  else if (strcmp(oid, ".1.3.6.1.4.1.53864.1.n.0") == 0) 
  {
      return n;
  }

Now the problem is updated n value is 250. Can anyone please give me any idea to reduce this else if statement?

I tried to search on StackOverflow but those answers are related to the other languages and I want an idea that I can implement in the C language.

Answer 1

You can use strncmp() to match the prefix (".1.3.6.1.4.1.53864.1."), extract the text value say by using strchr() and convert it with strtol() for the return, and match the suffix (".0"):

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int oid_sub(const char *s) {
    const char prefix[] = ".1.3.6.1.4.1.53864.1.";
    if(strncmp(s, prefix, sizeof(prefix) - 1))
        return -1;
    char *end;
    // check n for under and overflow?
    int n = strtol(s + sizeof(prefix) - 1, &end, 10);
    const char suffix[] = ".0";
    if(end == s + sizeof(prefix) - 1 || strcmp(end, suffix))
        return -1;
    return n;
}

int main() {
    char *s = ".1.3.6.1.4.1.53864.1.42.0";
    printf("%d\n", oid_sub(s));
}

I would prefer a regex based solution:

#include <regex.h>

int oid_sub2(const char *s) {
    regex_t r;
    const char regex[] = "^\\.1\\.3\\.6\\.1\\.4\\.1\\.53864\\.1\\.([^.]+)\\.0$";
    if(regcomp(&r, regex, REG_EXTENDED))
        return -1;
    regmatch_t m[2];
    if(regexec(&r, s, 2, m, 0))
        return -1;
    return atoi(s + m[1].rm_so);
}

Note, atoi() fails with 0 on error, so you could tweak the regex to ensure it matches numbers or use strtol() as above (but no need to check the suffix as the regex already takes care of that).