Sorting Customer Struct In C - c

Question

I have structure with most frequent words in huge text file, the field pointer array to char are words, and field count are their frequencies. My question is how to sort them from the longest word length to lowest - to nicely display it to user? Code:
typedef struct pair {
char * a[20000];
int count[32000];
} Pair;
Example print:
printf("%d, %d, %d\n", bag.count[0], bag.count[1], bag.count[2]); // -> 8, 7, 3
printf("%s, %s, %s\n", bag.a[0], bag.a[1], bag.a[2]); // -> abbes, abbey, abhor

Answer 1

I'd suggest to turn the structure/array inside-out.
Having your arrays inside the struct does not feel right. Because you primarily have a pair of things, and secondarily you want one array of these things. Do you see what I mean?
It would look like this:
typedef struct pair
{
char* word;
int count;
} Pair;
Pair pairs[32000];
You'd also need to know how many pairs are filled. (You would have needed this anyway.):
int index; // Index of next free pair.
Then use C standard qsort():
#include <stdlib.h>
...
int comparePairs(const void *pairA, const void *pairB)
{
Pair* a = (Pair*)pairA;
Pair* b = (Pair*)pairB;
return strlen(a->word) - strlen(b->word);
}
qsort(pairs, index, sizeof(Pair), comparePairs);
The index would start at 0, which indicates the next free Pair is at that index. Adding an element would be:
pairs[index].word = someWord; // someWord must be allocated elsewhere!
pairs[index].count = 1;
index++;
Note that, because your structure only has a char pointer, that the someWord must be allocated elsewhere. Without automatic memory management this is going to be rather cumbersome. A better alternative would be to strcpy() the word in by using the following structure:
typedef struct pair
{
char word[50]; // Assumes a word is NEVER longer than 49 characters.
int count;
} Pair;
Adding a new element would then become:
strncpy(pairs[index].word, someWord, 50 - 1);
pairs[index].count = 1;
index++;
The strncpy() above copies at most 49 characters. You need to make sure you chose this 50 or whatever size wisely to make sure strncpy() never starts chopping off ends of your very long words.
But of course to know if you have to add a new or simply increment the count of an existing one, you'd first need to search through the existing Pairs with a simple loop.