Qsort struct of strings

Question

please, could you help me with qsort of struct of strings? I need to sort alphabeticaly words in dictionary. Problem is, that it throws me Segmentation fault... Here are my structs:

typedef struct {
    int length;
    char *data;
} Word;

typedef struct {
    int length;
    int index;
    Word *data;
} Dictionary;

Here is compare function:

int compare(const void *a, const void *b) 
{ 
    return strcmp (((Word *)a)->data, ((Word *)b)->data);
}

And here is qsort implementation:

qsort(&dictionary, dictionary.index, sizeof (Word *), compare);

Thank you very much for your help.

length is max length of dictionary and index is actual length — Jan Kožušník
– Jan Kožušník, Commented Dec 14, 2014 at 20:56
this qsort parameter: sizeof (Word *) is not correct, you do not want the size of a pointer, you want the size of the word struct. I.E. sizeof (Word ). There also seems to be something wrong with the first couple of parameters to qsort, but I'm not sure just what the problem is. — user3629249
– user3629249, Commented Dec 15, 2014 at 3:47

GWW · Accepted Answer · 2014-12-14 20:08:22Z

1

You are trying to qsort your dictionary structure rather than the dictionaries data.

qsort(dictionary.data, dictionary.index, sizeof (Word *), compare);

You should also double check to make sure dictionary.index is the length of the dictionary and not dictionary.length

answered Dec 14, 2014 at 20:08

GWW

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9 Comments

Iharob Al Asimi Over a year ago

and you need a better compare function.

Jan Kožušník Over a year ago

After I add dictionary.data, it still throws segmentation fault. How to improve my compare function? Thank you

GWW Over a year ago

Have you tried running it in a debugger to see what is causing the segmentation fault? It would also be nice if you could edit your question and include a minimal compilable example.

Jan Kožušník Over a year ago

I'm in C newbie and I'm not using any debugger yet. Could you be more specific about compilable example?

David C. Rankin Over a year ago

There are a number of good cheat-sheets and tutorials (e.g. gdb Command Cheatsheet). If you are going to program in C it is critical that you make friends with gdb. Go ahead, pay the price of admission, and spend several hours learning gdb. It will save you thousands of hours in the long run...

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user411313 · Accepted Answer · 2014-12-14 21:03:53Z

0

You should sort your Word-list of your Dictionary and not your Dictionary. If length is the number of all elements in your Word-list, you should use:

qsort(dictionary.data, dictionary.length, sizeof (dictionary.data[0]), compare);

and, you are right, your compare function is wrong, eg.

int compare(const void *a, const void *b) 
{ 
  const Word *x = a, *y =b;
  return strcmp (x->data,y->data); /* if data always contains a C string */
}

answered Dec 14, 2014 at 21:03

user411313

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2 Comments

Jan Kožušník Over a year ago

Thank you for your answer. But however I edit it, it still throws me segmentation fault

Jan Kožušník Over a year ago

Sorry, you were right. You just were probably confused about index and length. After I rewrite length to index, it works. Thank you!

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