Recursion With Array

Question

Recently i was introduced into the concept of "Recursion" and it seemed fun untill i came upon an array in which i was asked to get an array from a user with it's size and Get the negative numbers and the odd numbers Count in the array , i have thought of some ways to do this but none worked , i tried making it with different conditions or loops but everytime i found myself resettings something either negative counter or odd counter or just an infinite loop , i can't seem to understand how passing an array through this recursive procedure works for some reason it keeps giving me wrong outputs so i resetted it and started all over with the base case , Is there like a general way to follow when making recursive functions , i know its about making your problem into smaller sub problems and creating the base case but i wasn't able to figure it out here , if anyone could guide me through it would be appreciate .

Down below is the code i made for getting the array and passing it to the recursive function with the conditions that worked with the iterative function , and tried putting the base case as if (Count < 0) because Count is the size of the array so i thought about starting with it and decreasing it by one everytime i want to call the function , but that didn't seem to work either , any ideas ?

Thanks in advance !

 #define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
int SortArray(int* Array, int Size, int* Negative, int* Odd)
{
    if (Size < 0)   
    {
        return;
    }
            if (Array[Size] % 2 == 1 || Array[Size] % 2 == -1)
            {
                return SortArray(*Array, Size - 1, *Negative, *Odd);
            }
            if (Array[Size] < 0)
            {

            }
}
int main()
{
    int Size;
    int Negative = 0;
    int Odd = 0;
    printf("Enter The Size Of The Array: \n");
    scanf("%d", &Size);
    int* Array = malloc(Size * sizeof(int));
    if (Array ==  NULL)
    {
        printf("Malloc Failure ! \n");
        return 0;
    }
    printf("Enter The Elements Of Your Array: \n");
    for (int i = 0; i < Size; i++)
    {
        scanf("%d", &Array[i]);
    }
    SortArray(Array, Size,&Negative,&Odd);
    printf("Odd Count:%d \n", Odd);
    printf("Negative Count:%d \n", Negative);
    free(Array);
    return 0;
}

If you have to count, first you should initialize or set to zero the counters and then update their values. Do you know how to change the value of a variable passed to a function as a pointer? Why this function has to be recursive? Note that you can always check the value of the first element of an array and then pass a pointer to the next element in the array (along with a size reduced by one). — Bob__
– Bob__, Commented Jan 9, 2021 at 10:15
it's gotta be recursive because the lecturer specified that in the question ;) , i do know how to change the value of a passed variable through a pointer , my question roamed about "how" should i base my conditions in the recursive function itself . My idea was to pass a pointer-to-integer that points to the counters i need and change ithe values once my base condition is met , in my case it was if (Counter<0) but that was only an idea and i am trying to get it to work — Dark
– Dark, Commented Jan 9, 2021 at 10:34

MarcoLucidi · Accepted Answer · 2021-01-09 12:28:34Z

tried putting the base case as if (Count < 0) because Count is the size of the array so i thought about starting with it and decreasing it by one everytime i want to call the function

this is a good start and the function parameters are the right ones for what you want to accomplish. the name SortArray is a bit misleading, you are not sorting anything, you are counting, I think Count would be more appropriate. also Size is not really the size, but rather our "index":

void Count(int *Array, int Index, int *Negative, int *Odd)

the base case is right, we are scanning the array backwards, so when Index is less than 0 there is nothing more left.

now, you want to update both your counters before calling the Count function again. the update part is missing from your code, that is why you get wrong results.

I would do it this way:

int n = Array[Index];       /* current number */
if (n % 2 != 0)             /* is it odd? */
        *Odd += 1;          /* if so, update odd counter by one */
if (n < 0)                  /* is it negative? */
        *Negative += 1;     /* if so, update negative counter by one */

at the end we call the Count function again decreasing Index by one, as you already guessed:

Count(Array, Index - 1, Negative, Odd);

putting all together:

void Count(int *Array, int Index, int *Negative, int *Odd)
{
        if (Index < 0)
                return;

        int n = Array[Index];
        if (n % 2 != 0)
                *Odd += 1;
        if (n < 0)
                *Negative += 1;

        Count(Array, Index - 1, Negative, Odd);
}

the function should be called the first time with the last index and counters should be set to 0 (as you already did). from main() the first call would be:

int Negative = 0;
int Odd = 0;
Count(Array, Size - 1, &Negative, &Odd);

you could make a wrapper function to "hide" this (important) particulars.

This Is truely wonderful , thanks ! i was one step away from getting the right combination of steps my mistake was not updating my counters before calling the function i was updating them during the call and that what gave me errors , thanks again !

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