Why is this code written in C running slow?

Question

I have been practicing DS using C and feel sorry for my unreadable code.

i tried to implement linked list using C and i am beginner to learn data structures

Please help me why this is running slow.

#include <stdio.h>
#include <stdlib.h>
typedef struct node{
    int data;
    struct node* link;
}NODE;
int main(void){
    NODE* A;
    A = NULL;
    NODE* temp = (NODE*)malloc(sizeof(NODE));
    temp->data = 2;
    temp->link = NULL;
    A = temp;
    temp = (NODE*)malloc(sizeof(NODE));
    temp->data = 3;
    temp->link = NULL;
    A->link = temp;
    temp = (NODE*)malloc(sizeof(NODE));
    temp->data = 5;
    temp->link = NULL;
    NODE* temp1 = A;
    while(temp1->link != NULL){
        printf("%d\n",temp1->data);
        temp1 = temp1->link;
        printf("%d\n",temp1->data);
        
    }
    temp1->link = temp;
    temp1 = temp1->link;
    printf("%d\n",temp1->data);
    temp1 = A;
    printf("%d\n",temp1->data);
    printf("after inserting 100 in to the second\n");
    int count = 1;
    NODE* new = (NODE*)malloc(sizeof(NODE));
    new->data = 100;
    new->link = NULL;
    NODE* before = (NODE*)malloc(sizeof(NODE));
    NODE* after = (NODE*)malloc(sizeof(NODE));
    while(temp1->link != NULL){
        if(count == 2){
            new->link = temp1;
            break;
        }
        if(count == 1){
            before = temp1->link;
            temp1->link = new;
            temp1 = before;
            count++;
            continue;
        }
        temp1 = temp1->link;
        count++;
    }
    temp1 = A;
     while(temp1->link != NULL){
        printf("%d\n",temp1->data);
        temp1 = temp1->link;
    }
    printf("%d\n",temp1->data);
}

Welcome to Stack Overflow! [dont cast malloc](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — Barmar, Aug 13 '20 at 15:48
I think it is time you learned subroutines. Start by moving the repeated code into InsertNode. — stark, Aug 13 '20 at 16:02
"running slow" isn't something we can necessarily recognize or quantify. How long does the program take for you? Measured how? Running in what environment? What makes you characterize that as slow? — John Bollinger, Aug 13 '20 at 16:02

score 0 · Accepted Answer · answered Aug 13 '20 at 15:56

So as per @Barmar's remark, I see that you are allocating some memory but never freeing that memory which is a big NO-NO. Always make sure you are freeing any dynamically allocated memory.

Regarding your question, try to use gettimeofday() in order to find how long it is taking your code to execute. I assume it is very quick but maybe there is an issue with your computer and is giving the illusion that it is slow.

    #include <sys/time.h>

...
int main(..)
{
struct timeval tv_start, tv_end;
gettime ofday(&tv_start, NULL);

....
....

gettimeofday(&tv_end, NULL);
double run_time = ( double )( tv_end.tv_usec - tv_start.tv_usec ) / 1000000 + ( double )( tv_end.tv_sec - tv_start.tv_sec);

printf("Total runtime %f \n", run_time);

score 0 · Answer 2 · answered Aug 13 '20 at 16:41

Your code ran 10,000 times in 25 seconds on my ubuntu virtual box (On Windows 10). Do you expect faster than that for this particular code, or your actual use-case is much larger?

#!/bin/bash

ts=$(date +%s%N)

for i in {1..10000}
do
   ./a.out  >/dev/null
done

echo "Elapsed"
echo $((($(date +%s%N) - $ts)/1000000000))

Elapsed 25

Why is this code written in C running slow?

2 Answers2