C qsort doesn't sort structure arrays

Question

I have a program that's designed to read in words and split them apart, separating each and counting them up individually (words with punctuation differences are counted as different words on purpose).

typedef struct word
{
    char letters[100];
    int count;
} Word;

int compare (const void *a, const void *b)
{
    return strcmp((*(struct word **) a)->letters,(*(struct word **) b)->letters);
}

int main()
{
    int sizeCheck = 0;
    Word ** collection = malloc(sizeof(Word*));
    Word * array = malloc(sizeof(Word));

    FILE *fptr, *fout;
    char fileName[80];
    char fileNameWords[80];
    char wordLine[100];
    strcpy(fileName,"data");
    strcpy(fileNameWords,fileName);
    strcat(fileNameWords,"data.out.txt");

Where the action starts, assuming everything's fine with opening files (removed for shortness sake):

            int wordExists = 0;
            int t1 = 0;
            char tw1[100];


            fscanf(fptr,"%s",wordLine);
            strcpy(array->letters,wordLine);
            array->count = 1;
            collection[sizeCheck] = array;
            sizeCheck++;

            while (!feof(fptr))
            {
                wordExists = 0;
                fscanf(fptr,"%s",wordLine);
                for (t1 = 0; (t1 < sizeCheck) && (wordExists == 0); t1++)
                {
                    strcpy(tw1,array[t1].letters);
                    if (strcmp(tw1,wordLine) == 0)
                    {
                        array[t1].count += 1;
                        wordExists = 1;
                    }
                }
                if (!wordExists)
                {
                    collection = realloc(collection,(sizeCheck+1)*sizeof(Word*));
                    array = realloc(array,(sizeCheck+1)*sizeof(Word));
                    strcpy(array[sizeCheck].letters,wordLine);
                    array[sizeCheck].count = 1;
                    collection[sizeCheck] = array;
                    sizeCheck++;
                }
            }

            qsort(collection,sizeCheck,sizeof(Word*),compare);

            for (t1 = 0; t1 < sizeCheck; t1++)
            {
                fprintf(fout,"%s - %d\n",array[t1].letters,array[t1].count);
            }
            free(collection);
        }
    }
    fclose(fptr);
    fclose(fout);
    return 0;
}

Using a pointer-to-pointer method, it works for the most part, except when it comes to either the qsort function, or the fprintf parts near the bottom. I'm a little stumped at this point. What am I doing wrong here that's preventing this from outputting a sorted file? (Sorting by alphabetical order with words)

Possible dupe: stackoverflow.com/questions/8451020/…

abligh
– abligh

2014-04-08 21:24:05 +00:00
Commented Apr 8, 2014 at 21:24 — abligh
– abligh, Commented Apr 8, 2014 at 21:24

nneonneo · Accepted Answer · 2014-04-08 21:26:27Z

3

Only the collection array (an array of pointers) is being sorted. The values they point to (the elements of array) are unchanged. Since you fprintf the elements of array, you won't see any changes.

If you want to sort array, you can do that with qsort:

qsort(array, sizeCheck, sizeof(Word), compareWord);

where compareWord is

int compareWord(const void *a, const void *b) {
    const Word *wa = a;
    const Word *wb = b;
    return strcmp(a->letters, b->letters);
}

Alternately, just print out the elements from collection instead of array:

fprintf(fout, "%s - %d\n", collection[t1]->letters, collection[t1]->count);

answered Apr 8, 2014 at 21:26

nneonneo

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