Reverse an integer array in C [duplicate]

Question

Given an integer array with 5 elements [1,2,3,4,5], I am attempting to reverse the order of the elements in the array; e.g. the array would become [5,4,3,2,1].

int main(void) {
    int n = 5; //Num of elements
    int arr[5] = {1,2,3,4,5};
    for (int i = 0; i < n; i++) {
        printf("%d\n", arr[i]); //Print original vals
    }
    n--; //Decrement n by 1 for simplicity
    for (int i = n; i >= 0; i--) {
        int temp = arr[n - i]; //Set temp the max-index (4) - i
        printf("\nSmall: %d\nBig: %d\n", arr[n - i], arr[i]); //Print current temp & arr[i]
        arr[n - i] = arr[i]; //Set arr[max-index - i] to arr[i] (e.g arr[0] = arr[4]) 
        arr[i] = temp; //Set arr[i] to arr[max-index - 1] (e.g. arr[4] = arr[0])
        printf("\nBig: %d\nSmall: %d\n", arr[n - i], arr[i]); //Print new set
    }
    for (int i = 0; i < n + 1; i++) { //Print array in reverse order
        printf("%d\n", arr[i]);
    }
    return 0;

}

The first for loop should print:1 2 3 4 5 and the last: 5 4 3 2 1

However, it prints 1 2 3 4 5 both times, but the print statements in the loop that reverses the array prints the right numbers. Have I done something wrong?

Should I be dynamically allocating memory or something else not allowing me to change the array?

Trace out what your code is doing on paper. Believe it or not, your middle loop is reversing the array twice. — hobbs
– hobbs, Commented Dec 11, 2017 at 1:07
I see! Because I iterate through the whole list, after 3, the numbers are re-switching — Leshawn Rice
– Leshawn Rice, Commented Dec 11, 2017 at 1:11

Community · Accepted Answer · 2020-06-12 08:17:50Z

13

The problem is, you swap every element twice (unless it is the center one) which ultimately does not swap anything. At the first loop, you technically swap first and last item. At the last loop, you did the same. So, you reverse the first action, returning the item back to original position.

Another problem is you try to access arr[5] which is undefined as your array is of size 5 and thus the index should be from 0 to 4 only.

Following is a function reversing the array:

void ReverseArray(int arr[], int size)
{
    for (int i = 0; i < size/2; i++)
    {
        int temp = arr[i];
        arr[i] = arr[size - 1 - i];
        arr[size - 1 - i] = temp;
    }
}

edited Jun 12, 2020 at 8:17

CommunityBot

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answered Dec 11, 2017 at 1:11

lamandy

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Comments

Steephen · Accepted Answer · 2017-12-11 01:10:57Z

0

To reverse an array you have to do as follows:

for(int i = i; i<n/2; ++i)
{
    int temp = arr[i];
    arr[i] = arr[n-i-1];
    arr[n-i-1] = temp;        
}

edited Dec 11, 2017 at 1:10

answered Dec 11, 2017 at 1:04

Steephen

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2 Comments

Leshawn Rice Over a year ago

im not trying to print the array in reverse, but actually reverse the elements. e.g. the index 0 = 1; index 4 = 5; after reversing index 0 = 5; index 4 = 0;

Steephen Over a year ago

@LeshawnRice check the correction now

Absalon Castañon · Accepted Answer · 2017-12-11 01:15:30Z

0

You are reversing the array twice. On each iteration you swap two numbers to the same distance from the center.

So, on the first half of the iterations you have already reversed the array. As you keep iterating you swap again and the array stays unmodified.

To fix it just change the reversing loop to this (change i>=n for i>=n/2 )

  for (int i = n; i >= n/2; i--) {...swaping operations...}

edited Dec 11, 2017 at 1:15

answered Dec 11, 2017 at 1:09

Absalon Castañon

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