CODEWITHSUNDEEP
carrayspointers
Anirban Nag 'tintinmj'
Anirban Nag 'tintinmj'

Reputation: 5762

Why array pointer is not printing properly?

I have a function(from a tutorial)

int* func(int *arr, int n){
    if(n==1 || n==0)
        return arr;
    else{
        int temp=arr[0];
        arr[0]=arr[n-1];
        arr[n-1]=temp;
        return func(++arr,n-2);
    } 
}

I dry run it and get that it will reverse the array, very good. I'm getting the result as expected when I using this piece of code

int x[]={1,2,3,4,5,6,7,8,9};
int i;
func(x,9);
for(i=0;i<9;i++) 
{
    printf("%d\n",x[i]);
}

But getting garbage value when using below code

int x[]={1,2,3,4,5,6,7,8,9};
int* p;
p = func(x,9);
for(i=0;i<9;i++) 
{
    printf("%d\n",*(p+i));
}

Weak in pointer please explain with you answer.

Upvotes: 1

Views: 142

Answers (3)

Wiingaard
Wiingaard

Reputation: 4302

So when the recursive algorithm is done, fund will return a pointer to the 5th element in arr. So your print-sentence: printf("%d\n",*(p+i));, will start printing from the 5th element, and 9 fields forward. When it tries print elements after the 9th element, you will get garbage values.

I tried to visualize it like this, maybe it helps:

arr = 0         // as non-relative pointer to element
temp = 1
arr[0] = 9
arr[8] = 1
func( arr=1, n = 7 )
    arr = 1
    temp = 2
    arr[1] = 8
    arr[7] = 2
    func ( arr=2, n = 5 )
        arr = 2
        temp = 3
        arr[2] = 7
        arr[6] = 3
        func ( arr=3, n = 3 )
            arr = 3
            temp = 4
            arr[3] = 6
            arr[5] = 4
            func ( arr=4, n = 1 )
                arr = 4
                n == 1
                return arr = 4
            return 4
        return 4
    return 4
return 4
p = 4

Upvotes: 1

Matt Eckert
Matt Eckert

Reputation: 1975

Your problem is with recursion, not pointers. Visualizing the recursive calls, the returned pointer points to the 5th element:

func([1, 2, 3, 4, 5, 6, 7, 8, 9], 9) ->
func([2, 3, 4, 5, 6, 7, 8, 1], 7) ->
func([3, 4, 5, 6, 7, 2, 1], 5) ->
func([4, 5, 6, 3, 2, 1], 3) ->
func([5, 4, 3, 2, 1], 1) ->
[5, 4, 3, 2, 1]

From the comments:

This might illustrate it better, I incremented the array before printing, so you can see what the array looks like in the recursive call: http://ideone.com/lzgEUX

Upvotes: 4

Broseph
Broseph

Reputation: 1753

Check out this code:

#include <stdio.h>

int* func(int* arr, int n){
     if (n==1 || n==0)
        return arr; //remember this is the pointer we've been incrementing
     int temp = *arr; //instead of using bracket, I'm translating to pointer
     *arr = *(arr + n - 1); //arithmetic so it is more clear that we are 
     *(arr + n - 1) = temp; //merely manipulating a pointer to an int.
     return func(arr + 1, n - 2); //the pointer is incremented!
}


int main(int argc, char** args){
    int x[]={1,2,3,4,5,6,7,8,9}; //initialize our array
    int i=-4; //this is how much we need to offset the returned pointer
    int* p = func(x,9); //get the returned pointer
    for(;i<5;i++)
        printf("%d\n", *(p+i)); //print out each element of the array
    return 0;
}

Upvotes: 1

Related Questions