clinked-listpre-increment

Preincrement operator resulting in different output than post increment operator in C code


In the code snippet below the pre-increment operator used in the main function results in values starting from 2 while the post increment values start from 1 when inserting to the list. I am unable to figure out why.

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

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

void insert_node(int new_data, Node **head_ref) {
    Node *node= (Node *)malloc(sizeof (Node));
    node->data = new_data;
    node->next = *head_ref;
    *head_ref = node;
}

void display(Node *head) {
    Node *traverse = head;
    while (traverse->next != NULL) {
        printf("\ndata=%d", traverse->data);
        traverse = traverse->next;
    }
}

void main() {
    Node *pdata;
    Node *list_head = NULL;
    int i = 0;
    while (i <= 10)
        insert_node(++i, &list_head);
    display(list_head);
}

Solution

  • First, you need to understand how pre and post increment work.

    Pre-Increment: increment the value of a variable before using it in an expression.

    Example:

    int i = 0;
    
    // first increment value of i by 1 and then print it
    print("%d", ++i); // output: 1
    print("%d", i);   // output: 1
    

    Post-Increment: increment the value of the variable after executing the expression completely, in which post-increment is used.

    Example:

    int i = 0;
    
    // first use the value of i in print() and after executing
    // the print(), increment value of i
    print("%d", i++); // output: 0
    print("%d", i);   // output: 1
    

    Now it depends on you which one you have to use.

    Second, as far as I understand there is a logical error in this function

    void display(Node* head)
    {
        Node* traverse = head;
        while (traverse->next != NULL)
        {
            printf("\ndata=%d", traverse->data);
            traverse = traverse->next;
        }
    }
    

    The while loop condition should be

    while (traverse != NULL)
    

    I hope, it will answer your question.