Printing Stack of Arrays

Question

Some operation is performed on an array in function fillStackWithArray() and after the operation the array is pushed into the stack and is repeated again.

But the issue occurs when I try to print the stack. It gives me wrong answer.

Output:

Expected Output:

Code:

import java.util.Stack;

public class ArraysOnStack {
    public static void main(String[] args) {
        int sizeOfArray = 5;
        Stack<int[]> stack = fillStackWithArray(5);
        printStack(stack);
    }

    private static void printStack(Stack<int[]> stack) {
        while (!stack.empty()) {

            int[] arr = stack.pop();
            for (int i = 0; i < arr.length; i++) {
                System.out.print(arr[i] + " ");
            }
            System.out.println();
        }
    }

    private static Stack<int[]> fillStackWithArray (int size) {
        Stack<int[]> stack = new Stack<>();
        int[] arr = new int[size];

        // Some Operation that fills Stack with Arrays.
        for (int i = 0; i < size; i++) {
            for (int j = 0; j < size; j++) {
                arr[j] = size - i;
            }
            // Pushing the array into stack on which some operation
            // is performed.
            stack.push(arr);
        }

        return stack;
    }
}

PS: The operation is random to just fill the array. But my question is related to such a situation.

Please read: [How to debug small programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/) — Turing85, Aug 08 '20 at 08:22
You are editing same array. Move the array initialization inside the 1st `for loop`. — thuva4, Aug 08 '20 at 08:26

score 1 · Answer 1 · answered Aug 08 '20 at 08:26

1

You are pushing same int[] in stack in fillStackWithArray. An int array in Java is a subclass of Object, therefore it's an object type. Create int[] array inside loop.

for (int i = 0; i < size; i++) {
   int[] arr = new int[size];
   ...
}

answered Aug 08 '20 at 08:26

Eklavya

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score 1 · Accepted Answer · answered Aug 08 '20 at 08:57

Try this.

private static Stack<int[]> fillStackWithArray (int size) {
    Stack<int[]> stack = new Stack<>();
    int[] arr = new int[size];

    // Some Operation that fills Stack with Arrays.
    for (int i = 0; i < size; i++) {
        for (int j = 0; j < size; j++) {
            arr[j] = i + 1;                // changed
        }
        // Pushing the array into stack on which some operation
        // is performed.
        stack.push(arr.clone());           // changed
    }

    return stack;
}

output

You can explain also why you are using `arr.clone()` :) – Eklavya Aug 08 '20 at 10:54 — Eklavya, Aug 08 '20 at 10:54

score 0 · Answer 3 · answered Aug 08 '20 at 08:30

The int[] being pushed onto the stack is initialized once outside the loop. The same array is being updated in subsequent iterations. You need to declare the int[] array inside the loop.
The value assigned to the variable i should begin from 1 to i <= size since the output needs to be printed from 5 to 1 and as a Stack is being used, the insertion order needs to be reversed as the first element that is pushed will be printed last. Hence, the value assigned to arr[j] should be i.

for (int i = 1; i <= size; i++) {
    int[] arr = new int[size];
    for (int j = 0; j < size; j++) {
        arr[j] = i;
    }
    // Pushing the array into stack on which some operation
    // is performed.
    stack.push(arr);
}

score 0 · Answer 4 · answered Aug 08 '20 at 08:31

In the fillStackWithArray() function you have created a single array arr and you are changing the same arr in each iteration. To fix this, create a new array for each iteration of the outer loop. To understand the issue, read about deep-copy and passing by reference.

Printing Stack of Arrays

4 Answers4