Sorting 3 strings Lexicographically using conditionals in C

Question

I cannot wrap my mind around doing this logically. I have mapped out all possibilities for sorting and based my algorithm on that.

ABC

ACB

BAC

BCA

CAB

CBA

Only six possible combinations for 3 strings, and so, I thought it a very quick and easy solution to hardcode this with conditionals. See code;

  char str1[x]; // where x is some constant value
  char str2[x];
  char str3[x];
  char temp[x];
  if (strcmp(str1, str2) > 0)
  {
    strcpy(temp, str1);
    strcpy(str1, str2);
    strcpy(str2, temp);
  }
  else if (strcmp(str2, str3) > 0)
  {
    strcpy(temp, str2);
    strcpy(str2, str3);
    strcpy(str3, temp);
  }
  else if (strcmp(str1, str3) > 0)
  {
    strcpy(temp, str1);
    strcpy(str1, str3);
    strcpy(str3, temp);
  }

Essentially I would like to take any variation of ABC for example and sort it to ABC. i.e. BAC -> ABC, CAB -> ABC.

1) What's your question? 2) As you say, there are six different possibilities, but your code can only take four different execution paths (it runs the first if block, the second, the third, or none of them). That's not a good sign. — hobbs, Sep 29 '17 at 02:05
removing `else` makes it work for everything but CBA, and BCA — compute, Sep 29 '17 at 02:15

score 1 · Accepted Answer · answered Sep 29 '17 at 02:18

You're very close. Your biggest problem is those elses. By using else if you're ensuring that at most one of your if blocks can run, which means that there are four possible paths you can take through the code — the first block runs, or the second block runs, or the third block runs, or none of them run. But that can't be correct, since you pointed out yourself that there are six cases to account for.

Second, you have a slight ordering problem, which can be solved by exchanging your second and third blocks.

If you make both of those changes, you should have something that works. By first swapping str1 with str2 if needed, then swapping str1 and str3 if needed, you ensure that the smallest value ends up in str1, and only the two higher values remain in str2 and str3 (in unknown order). Then, by swapping str2 with str3 if needed, you ensure that those two are in the right order as well, therefore the whole thing must be in order.

That makes sense, well explained. – compute Sep 29 '17 at 02:23 — compute, Sep 29 '17 at 02:23

score 0 · Answer 2 · answered Sep 29 '17 at 02:34

Are you trying to implement bubble sort with strings? Take a look here

Here's a small code that does it for strings:

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

#define strSize 20
int main() {
  char strings[][strSize] = {"CBA", "CAB", "BCA", "BAC", "ACB", "ABC"};
  char tmp[strSize];
  size_t len = sizeof(strings)/sizeof(strings[0]);
  for (size_t j = 1; j < len; ++j) {
    for (size_t i = 1; i < len; ++i) {
      strcpy(tmp, strings[i - 1]);
      if (strcmp(strings[i], tmp) < 0) {
        strcpy(strings[i - 1], strings[i]);
        strcpy(strings[i], tmp);
      }
    }
  }

  for (size_t i = 0; i < len; ++i) {
    printf("%s ", strings[i]);
  }
}

Sorting 3 strings Lexicographically using conditionals in C

2 Answers2