I am experiencing weird and unexpected behaviour of the qsort() function. I have my list of nodes, and every of them contains two values based on which I want to sort my list (technically, an array).
For example:
If I have an original array that looks like this (1st element is p, 2nd element is t):
[0,10|1], [0.05|0], [0,10|0], [0,05|2], [0,10|2], [0,15|1], [0,05|1]
After it is sorted, it should look like this:
[0,05|0], [0,05|1], [0,05|2], [0,10|0], [0,10|1], [0,10|2], [0,15|1].
Struct node:
typedef struct node{
int t;
double p;
}node;
Compare function, called like qsort(nodes, num_of_node, sizeof(node), compare_pairs);
static int compare_pairs(const void *n1, const void *n2){
const node *na1= n1;
const node *na2= n2;
if(na1->p < na2->p) return -1;
if(na1->p > na2->p) return 1;
// At this point, value p is equal, so I am sorting based on t
if(na1->t < na2->t) return -1;
if(na1->t > na2->t) return 1;
return 0;
Problem
The unwanted behaviour starts at 3. STEP which looks like this:
List: [0.10 | 2] [0.10 | 999] [0.10 | 999] [0.15 | 999] [0.15 | 999] [0.15 | 1] [0.25 | 999]
And should look like this:
List: [0.10 | 2] [0.10 | 999] [0.10 | 999] [0.15 | 1] [0.15 | 999] [0.15 | 999] [0.25 | 999]
Initial list:
[0.25 | 999] [0.15 | 999] [0.15 | 999] [0.10 | 999] [0.10 | 999] [0.05 | 999] [0.05| 999] [0.05 | 999] [0.05 | 999] [0.05 | 999]
- STEP:
Erasing (min) node 0.050000...
Erasing (min) node 0.050000...
Creating (new) node 0.100000...
List: [0.05 | 999] [0.05 | 999] [0.05 | 999] [0.10 | 1] [0.10 | 999] [0.10 | 999] [0.15 | 999] [0.15 | 999] [0.25 | 999]
- STEP:
Erasing (min) node 0.050000...
Erasing (min) node 0.050000...
Creating (new) node 0.100000...
List: [0.05 | 999] [0.10 | 1] [0.10 | 2] [0.10 | 999] [0.10 | 999] [0.15 | 999] [0.15 | 999] [0.25 | 999]
- STEP:
Erasing (min) node 0.050000...
Erasing (min) node 0.100000...
Creating (new) node 0.150000...
List: [0.10 | 2] [0.10 | 999] [0.10 | 999] [0.15 | 999] [0.15 | 999] [0.15 | 1] [0.25 | 999]
- STEP:
Erasing (min) node 0.100000...
Erasing (min) node 0.100000...
Erasing (new) node 0.200000...
List: [0.10 | 999] [0.15 | 999] [0.15 | 999] [0.15 | 1] [0.20 | 1] [0.25 | 999]
- STEP:
Erasing (min) node 0.100000...
Erasing (min) node 0.150000...
Creating (new) node 0.250000...
List: [0.15 | 999] [0.15 | 1] [0.20 | 1] [0.25 | 1] [0.25 | 999]
- STEP:
Erasing (min) node 0.150000...
Erasing (min) node 0.150000...
Erasing (new) node 0.300000...
List: [0.20 | 1] [0.25 | 1] [0.25 | 999] [0.30 | 1]
- STEP:
Erasing (min) node 0.200000...
Erasing (min) node 0.250000...
Erasing (new) node 0.450000...
List: [0.25 | 999] [0.30 | 1] [0.45 | 1]
- STEP:
Erasing (min) node 0.250000...
Erasing (min) node 0.300000...
Creating (new) node 0.550000...
List: [0.45 | 1] [0.55 | 1]
- STEP:
Erasing (min) node 0.450000...
Erasing (min) node 0.550000...
Creating (new) node 1.000000...
List: [1.00 | 1]
General idea*
In every step two minimal nodes get deleted from list, and one new node gets inserted into the list. The node that gets inserted has the value of t for 1 bigger then the greatest in the list, except that it does not compare itself to the t value of 999. If the greatest in the list has t = 999, then the one inserted will have 1.
Find greatest t:
int max_t(node *nodes, int num, double p){
int max_t= 0;
int i;
for(i=0; i<num; i+=1){
if(nodes[i].p== p && nodes[i].t != 999){
if(nodes[i].t > max_t){
max_t = nodes[i].t;
}
}
}
return max_t;
Main code:
node *nodes = malloc(num_of_nodes*sizeof(node));
int i;
for(i=0; i<num_of_nodes; i+=1){
node n;
n.t = 999;
n.p = *(probabs+ i);
*(nodes+i) = n;
}
qsort(nodes, num_of_nodes, sizeof(node), compare_pairs);
while(num_of_nodes> 1){
printf("\n%d. STEP:\n", z);
z += 1;
// 2) Find two min nodes
node *min_n1 = malloc(sizeof(node));
node *min_n2 = malloc(sizeof(node));
*min_n1 = nodes[0];
printf("Erasing (min) node %lf...\n", min_n1->p);
nodes= erase_node(nodes, min_n1, num_of_nodes);
num_of_nodes -= 1;
*min_n2 = nodes[0];
printf("Erasing (min) node %lf...\n", min_n2->p);
nodes= erase_node(nodes, min_n2, num_of_nodes);
num_of_nodes-= 1;
// 3) Create new node, add it to the list
node *new_node = malloc(sizeof(node));
new_node->p= min_n1->p + min_n2->p;
double p = new_node->p;
int max_t = max_t(nodes, num_of_nodes, p);
new_node->t = max_t + 1;
printf("Creating (new) node %lf...\n", new_node->p);
nodes = add_node(nodes, new_node, num_of_nodes);
num_of_nodes += 1;
qsort(nodes, num_of_nodes, sizeof(node), compare_pairs);
printf("List: ");
int k;
for(k=0; k<num_of_nodes; k+=1){
printf("[%.2lf | %d] ", nodes[k].p, nodes[k].t);
}
printf("\n");
Add / Remove node...
node *add_node(node *nodes, node *n, int num){
nodes = realloc(nodes, (num+1)*sizeof(node));
nodes[num] = *n;
return nodes;
node *erase_node(node *nodes, node *n, int num){
int i;
int index = 0;
for(i=0; i<num; i+=1){
if(nodes_equal(&nodes[i], n)){
index = i;
break;
}
}
for(i=index; i<num-1; i+=1){
nodes[i] = nodes[i+1];
}
nodes= realloc(nodes, (num-1)*sizeof(node));
return nodes;
}
int nodes_equal(node *n1, node *n2){
return !memcmp(n1, n2, sizeof(node));
}