Thread: sorting alphabetically

oki need to sort a string alphabetically by the method of selection sort.

here's the piece of code i don't quit understand.

Code:

void sortString(char* s, int size) 
{
  int i, j;
  char current;
 
  for (j = 1; j < size; j++) {
    current = s[j];
    i = j;
 
    while (i > 0 && current < s[i-1]) {
      s[i] = s[i-1];
      i--;
    }
    s[i] = current;
  }
}

i think i understand the for loop. but the while function is confusing me. current is of char type and i guess so is the string 's[i-1]. i thought you couldn't compare char's or strings. unless it was done by ACSII, though i'm not how it all works.

i thought you couldn't compare char's or strings. unless it was done by ACSII

Not quite. A character is basically a tiny int. When you use something like 'A' (character in single quotes) it translates it to a number. In ASCII it would use 65 for 'A'. So if you do:

Code:

char c = 'A';

the c variable holds the value 65. So if you do c1 < c2 you're checking if the first character is less than second character. Since alphabetic characters are placed in the ASCII table in ascending order it makes it a convenient way to see if one character comes before another alphabetically.

In your example, since s is of type char *, then s[index] ( the same as *(s + index) ) is of type char.

Does that make a little more sense?

since there are no quotation marks around s[i] or current, how does it compare the two characters?

Because s[whatever_index] and current already hold the numerical value of the character. if you did strcpy(s, "hello"); and looked at s in memory it would look like this:

Code:

104 101 108 108 111 0

Doing current = 'A'; is exactly equivalent to doing current = 65; as long as you're on a machine using ASCII. Both of those statements store 65 in current.

Maybe this program will help you understand:

Code:

#include <stdio.h>

int main(void)
{
  char c = 'A';

  printf("char = %c, value = %d\n", c, c);

  return 0;
}

You get:

Code:

char = A, value = 65

Two ways of displaying the same variable.

Exactly.

But you're not comparing strings. You're comparing characters. For instance:

Code:

if("foo" < "bar")

is not correct. That compares the pointer to the first string with the pointer to the second string. In that case you need to use something like strcmp() instead which tests the actual characters in the string. Something like:

Code:

if(str1[0] == str2[0] && str1[1] == str2[1])

etc., etc.