[R] Please help translate this program in C++ to R

[Richard O'Keefe]

As a C implementation of merge sort, that is the very reverse of impressive.
I would not translate *that* code into anything.
There is a fundamental difference between between arrays in C and arrays in R,
and it is the same as the difference between Python and R.

You are MUCH better to start from high level pseudocode and express that in R
than to start from code tangled up with the presuppositions and peculiarities of
another language with quite different presuppositions and peculiarities.


On Sun, 15 Dec 2019 at 14:57, Александр Дубровский
<dubrovvsskkyy using gmail.com> wrote:
>
> /* Iterative C program for merge sort */
> #include<stdlib.h>
> #include<stdio.h>
>
> /* Function to merge the two haves arr[l..m] and arr[m+1..r] of array arr[]
> */
> void merge(int arr[], int l, int m, int r);
>
> // Utility function to find minimum of two integers
> int min(int x, int y) { return (x<y)? x :y; }
>
>
> /* Iterative mergesort function to sort arr[0...n-1] */
> void mergeSort(int arr[], int n)
> {
>    int curr_size;  // For current size of subarrays to be merged
>                    // curr_size varies from 1 to n/2
>    int left_start; // For picking starting index of left subarray
>                    // to be merged
>
>    // Merge subarrays in bottom up manner.  First merge subarrays of
>    // size 1 to create sorted subarrays of size 2, then merge subarrays
>    // of size 2 to create sorted subarrays of size 4, and so on.
>    for (curr_size=1; curr_size<=n-1; curr_size = 2*curr_size)
>    {
>        // Pick starting point of different subarrays of current size
>        for (left_start=0; left_start<n-1; left_start += 2*curr_size)
>        {
>            // Find ending point of left subarray. mid+1 is starting
>            // point of right
>            int mid = min(left_start + curr_size - 1, n-1);
>
>            int right_end = min(left_start + 2*curr_size - 1, n-1);
>
>            // Merge Subarrays arr[left_start...mid] &
> arr[mid+1...right_end]
>            merge(arr, left_start, mid, right_end);
>        }
>    }
> }
>
> /* Function to merge the two haves arr[l..m] and arr[m+1..r] of array arr[]
> */
> void merge(int arr[], int l, int m, int r)
> {
>     int i, j, k;
>     int n1 = m - l + 1;
>     int n2 =  r - m;
>
>     /* create temp arrays */
>     int L[n1], R[n2];
>
>     /* Copy data to temp arrays L[] and R[] */
>     for (i = 0; i < n1; i++)
>         L[i] = arr[l + i];
>     for (j = 0; j < n2; j++)
>         R[j] = arr[m + 1+ j];
>
>     /* Merge the temp arrays back into arr[l..r]*/
>     i = 0;
>     j = 0;
>     k = l;
>     while (i < n1 && j < n2)
>     {
>         if (L[i] <= R[j])
>         {
>             arr[k] = L[i];
>             i++;
>         }
>         else
>         {
>             arr[k] = R[j];
>             j++;
>         }
>         k++;
>     }
>
>     /* Copy the remaining elements of L[], if there are any */
>     while (i < n1)
>     {
>         arr[k] = L[i];
>         i++;
>         k++;
>     }
>
>     /* Copy the remaining elements of R[], if there are any */
>     while (j < n2)
>     {
>         arr[k] = R[j];
>         j++;
>         k++;
>     }
> }
>
> /* Function to print an array */
> void printArray(int A[], int size)
> {
>     int i;
>     for (i=0; i < size; i++)
>         printf("%d ", A[i]);
>     printf("\n");
> }
>
> /* Driver program to test above functions */
> int main()
> {
>     int arr[] = {12, 11, 13, 5, 6, 7};
>     int n = sizeof(arr)/sizeof(arr[0]);
>
>     printf("Given array is \n");
>     printArray(arr, n);
>
>     mergeSort(arr, n);
>
>     printf("\nSorted array is \n");
>     printArray(arr, n);
>     return 0;
> }
>
>         [[alternative HTML version deleted]]
>
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