Stack and queue simulator

Pick a stack, queue, circular queue or dequeue, give the size of its array and the operations to run, and see the array, the top (or front and rear) and what happened after every step - overflow and underflow included. The table ISC Computer Science data-structure questions ask you to draw.

How to use it

1Choose what to run: a stack, a queue, a circular queue or a dequeue, and the size of its array.
2Type the operations in order, e.g. push 10, push 20, pop, or tap an example.
3Press Run it and read the array, the pointers and what happened after every step.

Not sure what to type? Tap an example:

stack: push, pop, overflow stack underflow linear queue wastes cells circular queue wraps round dequeue at both ends

Worked example

stack: push, pop, overflow

This is the kind of answer you get. Put these values in the boxes or type your own.

Step by step

Operation	0	1	2	3	top	What happens
start	·	·	·	·	-1	an empty stack of 4 cells
push 10	10	·	·	·	0	10 is pushed: top becomes 0
push 20	10	20	·	·	1	20 is pushed: top becomes 1
push 30	10	20	30	·	2	30 is pushed: top becomes 2
pop	10	20	·	·	1	30 is popped: top becomes 1
peek	10	20	·	·	1	the top element is 20
push 40	10	20	40	·	2	40 is pushed: top becomes 2
push 50	10	20	40	50	3	50 is pushed: top becomes 3
push 60	10	20	40	50	3	OVERFLOW: the stack is full (top is at the last cell), so nothing is pushed

The numbers above the cells are their positions, counted from 0. A red row is an operation that could not be done.

At the end

4 elements, top to bottom: 50, 40, 20, 10

The rules followed

top starts at -1 (empty). Push: if top is the last cell it is OVERFLOW, otherwise top goes up by 1 and the value is stored there.
Pop: if top is -1 it is UNDERFLOW, otherwise the value at top is removed and top goes down by 1.

About the Stack and queue simulator

What it does

Runs push, pop, insert and delete on an array-based stack, linear queue, circular queue or dequeue, one operation at a time, and shows the array, the pointers (top, or front and rear) and the result after each one. Overflow and underflow are reported instead of changing anything. It is the table that ISC Computer Science data-structure questions ask you to draw.

The conventions it follows

Stack: top starts at -1. Push is OVERFLOW when top is the last cell, otherwise top goes up by 1. Pop is UNDERFLOW when top is -1.
Queue: front and rear start at -1. A linear queue never reuses the cells before front, so it can be full while cells at the start are free. When the last value leaves, front and rear go back to -1.
Circular queue: rear and front wrap from the last cell to cell 0, so every cell can be used; it is full only when every cell is filled.
Dequeue: a circular array that you can add to and remove from at both ends: addfront, addrear, removefront, removerear.

How to write the operations

Separate them with commas. Stack: push 10, pop, peek. Queue and circular queue: insert 10 (or enqueue 10), delete (or dequeue), peek. Dequeue: addfront 10, addrear 10, removefront, removerear.

Common questions

What is the difference between overflow and underflow?

Overflow is trying to add to a structure that is full; underflow is trying to remove from one that is empty. In both cases nothing changes.

Why does my linear queue say it is full when there is space at the front?

In a linear queue rear only moves forward, so once rear reaches the last cell no more values can be inserted, even after deletions have freed the cells before front. A circular queue fixes this by wrapping rear round to cell 0 (try the circular queue example).

How is a circular queue different from a dequeue?

A circular queue adds only at the rear and removes only at the front. A dequeue (double-ended queue) can add and remove at both ends.

Practise on real ISC questions

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