Insertion Sort on Linked List in C

Question

I am trying to perform Insertion Sort on linked list in C using following function, it gets stuck in a infinite loop. I debugged the code and found out that it works for the first pass and gets stuck in infinite loop in the second pass.

void insertion_sort()//Insertion Sort function
{
    struct Node *p = root->next;

    struct Node *a = NULL, *b = NULL;

    while(p != NULL)
    {
        a = root;

        while(a != p)
        {
            b = a;
            a = a->next;
        }

        if(b != NULL)
            b->next = a->next;
        a->next = NULL;

        struct Node *q = root;
        struct Node* r = NULL;

        while(p->data > q->data)
        {
            r = q;
            q = q->next;
        }
        p->next = q;
        if(r != NULL)
            r->next = p;

        p = p->next;
    }

}

Answer 1

For starters it is a bad idea when a function depends on a global variable. In case of your program it means for example that you can not have two lists in one program.

I do not see where the pointer root is changed in the function insertion_sort. So even if all other code is valid nevertheless the function is incorrect because it does not change the pointer root when the value of the pointed node is unordered.

I can suggest the following solution shown in the demonstrative program below.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

struct Node
{
    int data;
    struct Node *next;
};

int push_front( struct Node **head, int data )
{
    struct Node *node = malloc( sizeof( struct Node ) );
    int success = node != NULL;
    
    if ( success )
    {
        node->data = data;
        node->next = *head;
        
        *head = node;
    }
    
    return success;
}

void insertion_sort( struct Node **head )
{
    for ( struct Node **current = head; *current != NULL; )
    {
        struct Node **sorted = head;
        
        while ( *sorted != *current && !( ( *current )->data < ( *sorted )->data ) )
        {
            sorted = &( *sorted )->next;
        }
        
        if ( *sorted != *current )
        {
            struct Node *tmp = *current;
            *current = ( *current )->next;
            tmp->next = *sorted;
            *sorted = tmp;
        }
        else
        {
            current = &( *current )->next;
        }
    }
}

FILE * output( struct Node *head, FILE *fp )
{
    for ( ; head != NULL; head = head->next )
    {
        fprintf( fp, "%d -> ", head->data );
    }
    
    fputs( "null", fp );
    
    return fp;
}

int main(void) 
{
    enum { N = 13 };
    
    struct Node *head = NULL;
    
    srand( ( unsigned int )time( NULL ) );
    
    for ( int i = 0; i < N; i++ )
    {
        push_front( &head, rand() % N );    
    }
    
    fputc( '\n', output( head, stdout ) );
    
    insertion_sort( &head );
    
    fputc( '\n', output( head, stdout ) );
    
    return 0;
}

The program output might look like

1 -> 12 -> 0 -> 4 -> 0 -> 12 -> 3 -> 7 -> 12 -> 2 -> 5 -> 9 -> 7 -> null
0 -> 0 -> 1 -> 2 -> 3 -> 4 -> 5 -> 7 -> 7 -> 9 -> 12 -> 12 -> 12 -> null