every_array_element_is_multiplication_of_previous_and_next_103.c

/*

    every_array_element_is_multiplication_of_previous_and_next_103.c

https://www.w3resource.com/c-programming-exercises/array/c-array-exercise-103.php

103. Write a program in C to update every array element
with multiplication of previous and next numbers in array.
Expected Output:
The given array is:
1 2 3  4  5  6
The new array is:
2 3 8 15 24 30


    You can find all my C programs at Dragan Milicev's pastebin:

    https://pastebin.com/u/dmilicev

*/

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

// Displays the array arr[n] of n integers with text before and after the array
void print_array(char *text_before, int arr[], int n, char *text_after){
    printf("%s", text_before);
    for(int i=0;i<n;i++)
        printf("%3d",arr[i]);
    printf("%s", text_after);
}

// Fills array arr[n] of n integers with ordinal numbers
void fill_array(int arr[], int n){
    for(int i=0;i<n;i++)
        arr[i] = i+1;
}

void newArrPrevNext_v1(int arr[], int n){
    if (n <= 1)
      return;

    int pre_elem = arr[0];
    int cur_elem;

// new first element is multiplication of first and second element.
    arr[0] = pre_elem * arr[1];

// every new inner element is multiplication of previous and next element.
    for (int i=1; i<n-1; i++){
        cur_elem = arr[i];
        arr[i] = pre_elem * arr[i+1];
        pre_elem = cur_elem;
    }

// new last element is multiplication of previous and last element.
    arr[n-1] = pre_elem * arr[n-1];
}

void newArrPrevNext_v2(int *arr, int n){
    if (n <= 1)
      return;

    int *tmp = (int *)malloc(n*sizeof(int));    // temporary array of n integers

    if (tmp == NULL) {  // Check if the memory has been successfully allocated by malloc
        printf("\n Memory not allocated.\n");
        return;
    }

    for (int i=0; i<n; i++)     // copy elements from arr in tmp
        *(tmp+i) = *(arr+i);

// new first element is multiplication of first and second element.
    tmp[0] = *(arr+0) * *(arr+1);

// every new inner element is multiplication of previous and next element.
    for (int i=1; i<n-1; i++)
        tmp[i] = *(arr+i-1) * *(arr+i+1);

// new last element is multiplication of previous and last element.
    tmp[n-1] = *(arr+n-2) * *(arr+n-1);

    for (int i=0; i<n; i++)     // copy elements from arr in tmp
        *(arr+i) = *(tmp+i);

    free(tmp);                  // keep it clean
}

void newArrPrevNext_v3(int arr[], int n){
    if (n <= 1)
      return;

    int *tmp = (int *)malloc(n*sizeof(int));    // temporary array of n integers

    if (tmp == NULL) {  // Check if the memory has been successfully allocated by malloc
        printf("\n Memory not allocated.\n");
        return;
    }

    for (int i=0; i<n; i++)     // copy elements from arr in tmp
        *(tmp+i) = arr[i];

// new first element is multiplication of first and second element.
    tmp[0] = arr[0] * arr[1];

// every new inner element is multiplication of previous and next element.
    for (int i=1; i<n-1; i++)
        tmp[i] = arr[i-1] * arr[i+1];

// new last element is multiplication of previous and last element.
    tmp[n-1] = arr[n-2] * arr[n-1];

    for (int i=0; i<n; i++)     // copy elements from arr in tmp
        *(arr+i) = *(tmp+i);

    free(tmp);                  // keep it clean
}

void newArrPrevNext_v4(int arr[], int n){
    if (n <= 1)
      return;

    int tmp[n];                 // temporary array of n integers

    for (int i=0; i<n; i++)     // copy elements from arr in tmp
        tmp[i] = arr[i];

// new first element is multiplication of first and second element.
    tmp[0] = arr[0] * arr[1];

// every new inner element is multiplication of previous and next element.
    for (int i=1; i<n-1; i++)
        tmp[i] = arr[i-1] * arr[i+1];

// new last element is multiplication of previous and last element.
    tmp[n-1] = arr[n-2] * arr[n-1];

    for (int i=0; i<n; i++)     // copy elements from arr in tmp
        arr[i] = tmp[i];
}


int main(){
    int arr[] = {1, 2, 3, 4, 5, 6};
    int n = sizeof(arr)/sizeof(arr[0]);

    print_array("\n The given array is:  \n\n",arr,n,"\n");

    newArrPrevNext_v1(arr, n);
    print_array("\n After function newArrPrevNext_v1(), new array is:  \n\n",arr,n,"\n");

    fill_array(arr,n);      // Let's fill the array elements again with ordinal numbers
    newArrPrevNext_v2(arr, n);
    print_array("\n After function newArrPrevNext_v2(), new array is:  \n\n",arr,n,"\n");

    fill_array(arr,n);      // Let's fill the array elements again with ordinal numbers
    newArrPrevNext_v3(arr, n);
    print_array("\n After function newArrPrevNext_v3(), new array is:  \n\n",arr,n,"\n");

    fill_array(arr,n);      // Let's fill the array elements again with ordinal numbers
    newArrPrevNext_v4(arr, n);
    print_array("\n After function newArrPrevNext_v4(), new array is:  \n\n",arr,n,"\n");

    return 0;
}