Linear Search using functions and dynamic memory allocation in C

Question

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

#define LIMIT 30000

void CreateArray(int *p, int N) {
    int i;
    p = (int *)malloc(N * sizeof(int));
    srand((long)210);
    for (i = 0; i < N; i++)
        *(p + i) = rand() % LIMIT;

    for (i = 0; i < N; i++)
        printf("%d ", p[i]);
}

void Search(int *p, int N, int key) {
    int comparisons = 0, success_search = 0;
    int i;

    clock_t start, end;
    double elapsed;
    start = clock();

    for (i = 0; i < N; i++) {
        if (key == p[i]) {
            comparisons++;
            success_search++;
            printf("\nFound!");
            break;
        } else {
            comparisons++;
            printf("\nNot found!");
        }
    }

    end = clock();
    elapsed = ((double)(end - start)) / CLOCKS_PER_SEC;

    printf("\nTotal comparisons: %d \n", comparisons);
    printf("Time elapsed: %f \n", elapsed);
    printf("Successful comparisons: %d \n\n", success_search);
}

int main() {
    int N, i, p, key;

    key = 1;
    CreateArray(&p, N = 7);
    Search(&p, N, key);
}

I'm trying to create a pseudo-random array and then try to search for a specific number in it and keep track of the total comparisons made and the total time needed to complete the search. I have manually inserted a number that is not in the array and it keeps saying that the number was found after 3 comparisons. Also the time elapsed always appears to be zero. I can't figure out what's wrong.

Answer 1

Make the following changes.

1) You need to allocate array and pass it to different functions. So "n" should be a pointer.

int *n = NULL;

2) You want CreateArray() to allocate memory and pass the pointer.

void CreateArray (int **p, int N)

3) You have to pass pointer to Search(). So call from main() becomes

Search(p, N, key);

I think it should work fine as expected now.

For time elapsed, you refer to Weather Vane's comment in your question.

Answer 2

There are multiple problems in your code:

• CreateArray should return the pointer to the allocated space. Passing it a pointer to a local int in main() makes no sense.

• you should test for malloc potential failure.

• Search should get the pointer to the allocated array, not the address of a local int.

• Search should print the Not found message just once at the end of the scan phase.

• If you want to count the number of successful comparisons, you should not break from the loop when you find the first one, but then the total number of comparisons is N.

• for better timing accuracy, you should avoid using printf inside the timed fragment.

• you should free the memory before exiting the program.

Here is a corrected version:

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

#define LIMIT 30000

void CreateArray(int N) {
    int i;
    int *p = (int *)malloc(N * sizeof(int));
    if (p != NULL) {
        srand((long)210);
        for (i = 0; i < N; i++)
            *(p + i) = rand() % LIMIT;

        for (i = 0; i < N; i++)
            printf("%d ", p[i]);
    }
    return p;
}

void Search(int *p, int N, int key) {
    int comparisons = 0, success_search = 0;
    int i;

    clock_t start, end;
    double elapsed;
    start = clock();

    for (i = 0; i < N; i++) {
        comparisons++;
        if (key == p[i])
            success_search++;
    }
    end = clock();
    elapsed = ((double)(end - start)) / CLOCKS_PER_SEC;

    if (success_search)
        printf("Found!\n");
    else
        printf("Not found!\n");

    printf("Total comparisons: %d\n", comparisons);
    printf("Successful comparisons: %d\n\n", success_search);
    printf("Time elapsed: %f\n", elapsed);
}

int main() {
    int N, i, key;
    int *p;

    key = 1;
    N = 7;
    p = CreateArray(N);
    if (p == NULL) {
        fprintf(stderr, "cannot allocate memory for %d elements\n", N);
        return 1;
    }
    Search(p, N, key);
    free(p);
    return 0;
}