In C, sort array of strings by string length

Question

So I input strings into an array mydata[10][81]

while ((ct<=10) && gets(mydata[ct]) != NULL && (mydata[ct++][0] != '\0'))

I then use a for loop to create a second array of pointers

for (i=0;i<11;i++){
    ptstr[i] = mydata[i];
}

This is where I get stuck I know I need to use strlen somehow, but I can't even conceive of how to get the length of a pointer and then re-assign that pointer a new position based on a third additional value of length

Hopefully that makes sense, I'm so lost on how to do it or explain it, I'm just trying to sort strings by length using array positions (not using something like qsort)

I did some more work on it and came up with this: any idea why its not working?

void orderLength(char *ptstr[], int num){
int temp;
char *tempptr;
int lengthArray[10];
int length = num;
int step, i, j, u;
for (i=0; i<num;i++){
    lengthArray[i] = strlen(ptstr[i]);
}
for (step=0; step < length; step++){
    for(j = step+1; j < step; j++){
          if (lengthArray[j] < lengthArray[step]){
              temp = lengthArray[j];
              lengthArray[j] = lengthArray[step];
              lengthArray[step] =temp;
              tempptr=ptstr[j];
              ptstr[j]=ptstr[step];

              }
          }
    }
    for (u=0; u<num; u++){
        printf("%s \n", ptstr[u]);
        }    
} 

Answer 1

As suggested in the comments by Deduplicator, you can use qsort defined in stdlib.h.

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

#define ROWS 4
#define MAXLEN 20

int compare (const void * a, const void * b) {
    size_t fa = strlen((const char *)a);
    size_t fb = strlen((const char *)b);
    return (fa > fb) - (fa < fb);
}

int main(int argc, const char * argv[]) {
    char arr[ROWS][MAXLEN] = {
        "abcd",
        "ab",
        "abcdefgh",
        "abc"
    };
    qsort(arr, ROWS, MAXLEN, compare);
    return 0;
}

You can see it in action over here.

Answer 2

To avoid to call several time strlen() on the same strings, you can use a listed chain of structures like following :

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

typedef struct  t_elem
{
    char        data[81];
    int         length;
    t_elem      *next;
};

int     main(int ac, char **av)
{   
    t_elem      *head;
    t_elem      *recent;
    t_elem      *current;

    while (/* string have to be sorted */)
    {
        if (head == NULL) {
            head = (t_elem *)malloc(sizeof(t_elem));
            head->data = //readTheFirstString();
            head->length = strlen(head->data);
            head->next = NULL;
        }
        else {
            recent = (t_elem *)malloc(sizeof(t_elem));
            recent->data = //readTheNextString();
            recent->length = strlen(recent->data);
            recent->next = NULL;

            if (recent->length < head->length) {
                recent->next = head;
                head = recent;
            }
            else {
                current = head;
                while (current->next && current->next->length < recent->length) {
                    current = current->next;
                }
                recent->next = current->next;
                current->next = recent;
            }
        }
    }

    // print the sorted chained list
    current = head;
    while (current->next) {
        printf("%s\n", current->data);
        current = current->next;
    }

    // free the list
    current = head;
    while (current->next) {
        recent = current;
        current = current->next;
        free(recent);
    }
    return (0);
}