[C]Reverse array with only pointers

Question

I tried to reverse an array using only pointers. The program runs without any errors but it doesn't reverse the array. What's wrong with my code?

#include <stdio.h>

#define SIZE 10
void arrayReverseOutput(int * arr);
void arrayInput(int * arr);
void printArray(int * arr);

int main(void)
{
    int arr[SIZE] = { 0 };
    arrayInput(arr);
    arrayReverseOutput(arr);
    printArray(arr);

    system("PAUSE");
    return 0;
}

void arrayInput(int * arr){
    int i = 0;
    for (i = 0; i < SIZE; i++){
        scanf("%d",(arr+i));
    }
}

void arrayReverseOutput(int * arr){
    int i = 0;
    int k = SIZE-1;
    int temp = 0;


    for (i = 0; i < SIZE; i++){
        temp = *(arr+i);
        *(arr+i) = *(arr + k);
        *(arr + k) = temp;
        k--;
    }
}

void printArray(int * arr){
    int i = 0;
    for (i = 0; i < SIZE; i++){
        printf("%d ", *(arr+i));
    }
}

What do you mean by "using only pointers"?

Ajay Brahmakshatriya
– Ajay Brahmakshatriya

2017-04-06 08:03:22 +00:00
Commented Apr 6, 2017 at 8:03 — Ajay Brahmakshatriya
– Ajay Brahmakshatriya, Commented Apr 6, 2017 at 8:03

Sourav Ghosh · Accepted Answer · 2017-04-06 07:57:25Z

3

The problem is, you are actually reversing the array twice, negating any changes achieved by swapping places.

Loop over only half of the array while swapping the elements, like

 for (i = 0; i < SIZE/2; i++)

answered Apr 6, 2017 at 7:57

Sourav Ghosh

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1 Comment

ThingyWotsit Over a year ago

'you are actually reversing the array twice' - you knew that before opening the question, yes? :)

LPs · Accepted Answer · 2017-04-06 08:03:29Z

You can use 2 pointers to do so

void arrayReverseOutput(int *head, int *tail)
{
    int temp = 0;

    do
    {
        temp = *tail;
        *tail = *head;
        *head = temp;
    }
    while (head++ < tail--);
}

Complete code

#include <stdio.h>

#define SIZE 10
void arrayReverseOutput(int *head, int *tail);
void arrayInput(int * arr);
void printArray(int * arr);

int main(void)
{
    int arr[SIZE] = { 0 };
    arrayInput(arr);
    arrayReverseOutput(arr, &arr[SIZE - 1]);
    printArray(arr);

    return 0;
}

void arrayInput(int * arr)
{
    int i = 0;
    for (i = 0; i < SIZE; i++)
    {
        scanf("%d", (arr + i));
    }
}

void arrayReverseOutput(int *head, int *tail)
{
    int temp = 0;

    do
    {
        temp = *tail;
        *tail = *head;
        *head = temp;
    }
    while (head++ < tail--);
}

void printArray(int * arr)
{
    int i = 0;
    for (i = 0; i < SIZE; i++)
    {
        printf("%d ", *(arr + i));
    }
}

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