Sorting Array of Pointers to Pointers

Question

I have a struct holding an int.

typedef struct n {
    int cars;
    struct n *next;
} node;

I make nodes such as node *temp; This is because I start with a linked list, and then go through it and add them to an array.

Which are then added to an array node **arr;

So I have an array of pointers to pointers. My question is: how can I do a qsort on this? My attempts up to now have resulted in:

qsort(arr, numberCars, sizeof(node), sortCars);

int sortCars(const void *i1, const void *i2){
    node *a = (node*)i1, *b = (node*)i2;
    return (a->cars - b->cars);
}

I'm having difficulty figuring out what qsort is actually pointing to.

asaelr · Accepted Answer · 2012-02-12 22:18:21Z

1

Since you have an array of pointers, your function should compare between pointers:

int sortCars(const void *i1, const void *i2){
    node *a = *(node**)i1, *b = *(node**)i2;
    return (a->cars - b->cars);
}

Also, you should pass sizeof(node*) instead of sizeof(node).

BTW, qsort sort the elements in the array, but not effects next, so your linked list will remain unsorted.

answered Feb 12, 2012 at 22:18

asaelr

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Daniel Earwicker · Accepted Answer · 2012-02-12 22:15:42Z

0

If the nodes have next pointers, that suggests a linked list. That's not the same as an array. Are you sure you're allocating all the structs in a contiguous memory block? That's what a sort algorithm will need as the first argument.

typedef struct n {
    int cars;
} node;

node arr[10]; // ten nodes as an array

(How would a general sorting function know to update the next pointers in a struct that you defined?)

Also if arr is node **arr, that's not an array of pointers to pointers. It's a pointer to a pointer, or (roughly equivalent) an array of arrays, a pointer to an array or an array of pointers.

answered Feb 12, 2012 at 22:15

Daniel Earwicker

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1 Comment

RichInDreams Over a year ago

I have them set up as a linked list, and then made an array from that.

mikithskegg · Accepted Answer · 2012-02-12 22:23:20Z

0

The comparator function must return int:

int sortCars(const void *i1, const void *i2)

An also you may do this function more simple:

int sortCars(const void *i1, const void *i2){
    return ((*((node**) i1))->cars - (*((node**) i2))->cars);
}

edited Feb 12, 2012 at 22:23

answered Feb 12, 2012 at 22:17

mikithskegg

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