C: insert element in empty heap - c

Question

i'm supposed to write a code, that inserts numbers from stdin into an at first empty max-heap. my code just doesn't get the order of elements right, i found out, that it doesnt even enter the while loop before the third number. Anybody willing to help? Thanks in advance!
int heap_insert(heap* h, int key) {
if (h->size==MAX_HEAP_SIZE){
return(-1);
}
h->size=h->size+1;
int i=h->size-1;
h->array[i]=key;
int parent=(i-1)/2;
while (i>1 && h->array[parent]< key) {
h->array[i]= h->array[parent];
i = parent;
h->array[i]=key;
}
return(0);
}

Answer 1

it doesnt even enter the while loop before the third number
That part can be answered. Your loop won't go until i is 2 or greater...
while (i > 1 && h->array[parent]< key) {
^^^^^
Here's the code that sets i.
h->size = h->size+1;
int i = h->size-1;
That code is easier to understand like so:
int i = h->size;
h->size++;
First time through, i will be 0 (assuming h->size is initialized to 0, you didn't show your heap init code). Second time it will be 1. Third time it will be 2 and then finally the loop can run.
I'm guessing you want i >= 1 in the while loop so it will go on the second call.
As for why it's not working, the primary problem is you're forgetting to change parent in the loop.
/* i and parent initialized */
int i=h->size-1;
...
int parent=(i-1)/2;
while (i>1 && h->array[parent]< key) {
h->array[i]= h->array[parent];
/* i is changed, but where's parent? */
i = parent;
h->array[i]=key;
}
Here's what it should look like. I've changed i, which should only be used in loop indexes, to the more descriptive new.
/* new and parent initialized */
int new = h->size;
...
int parent = (new-1)/2;
while( new != 0 && h->array[parent] < key ) {
h->array[new] = h->array[parent];
h->array[parent] = key;
/* new AND parent changed */
new = parent;
parent = (new-1)/2;
}
Here's the complete code, plus I made the heap size dynamic because fixed size structures are a crutch best avoided.
#include <stdio.h>
#include <stdlib.h>
typedef struct {
int size;
int max_size;
int *array;
} heap;
#define INIT_HEAP_SIZE 4
static heap *heap_init() {
heap *h = calloc(1, sizeof(heap));
h->max_size = INIT_HEAP_SIZE;
h->array = calloc(h->max_size, sizeof(int));
return h;
}
static void heap_destroy(heap *h) {
free(h->array);
free(h);
}
static void heap_grow(heap *h) {
h->max_size *= 2;
h->array = realloc( h->array, h->max_size * sizeof(int) );
}
static void heap_insert(heap* h, int key) {
if (h->size >= h->max_size) {
heap_grow(h);
}
int new = h->size;
h->size++;
h->array[new] = key;
int parent = (new-1)/2;
while( new != 0 && h->array[parent] < key ) {
h->array[new] = h->array[parent];
h->array[parent] = key;
new = parent;
parent = (new-1)/2;
}
return;
}
int main(void) {
heap *h = heap_init();
heap_insert(h, 23);
heap_insert(h, 11);
heap_insert(h, 42);
heap_insert(h, 5);
heap_insert(h, 99);
for( int i = 0; i < h->size; i++ ) {
printf("%d: %d\n", i, h->array[i]);
}
heap_destroy(h);
}

Answer 2

It doesn't enter the while loop before the 3rd number because your i is not greater than 1 until the 3rd number is entered. At 1st number i = 0, then 1 then 2.
For the loop, here's my advice on figuring out the problem: Suppose you enter the values 3, 5, 7. As soon as 5 is entered, you need a swap. 5 should become the new root, and 3 should be a child. (So maxheap property is kept) Then, when 7 is entered, another swap is in order. This time with 5. 7 becomes root, 3 and 5 are children. What does this tell you about the indexes? What happens if we insert 10, 16, 1 as well? More swaps? If you answer these properly the while loop should be easy to solve. (Hint: You need to keep swapping by starting from the child, and move to next parent until everything is in order)