Stacks (Cambridge International A-Level Computer Science)

Exam board Cambridge International · A-LevelUpdated 2025-06-03

Implementing a stack

How do you program a stack?

To recap the main operations of stacks and how they work, read the ADT overview
When programming a stack, the main operations listed below must be included in your program.
The main operations for stacks are:

Main operations

Operation	Pseudocode	Python	Java
isEmpty()	`FUNCTION isEmpty(s : Stack) RETURNS BOOLEAN RETURN s.top = -1 ENDFUNCTION`	`def isEmpty(self): return len(self.stack) == 0`	`stack.isEmpty();`
isFull()	`FUNCTION isFull(s : Stack) RETURNS BOOLEAN RETURN s.top = s.capacity - 1 ENDFUNCTION`	`def isFull(self): return len(self.stack) == self.capacity`	`public boolean isFull() { return top == capacity - 1; }`
push(value)	`PROCEDURE push(s : REF Stack, value : INTEGER) IF NOT isFull(s) THEN s.top ← s.top + 1 s.items[s.top] ← value ELSE OUTPUT "Stack is full" ENDIF ENDPROCEDURE`	`stack.push(1)`	`stack.push(1);`
pop()	`FUNCTION pop(s : REF Stack) RETURNS INTEGER IF NOT isEmpty(s) THEN DECLARE value : INTEGER value ← s.items[s.top] s.top ← s.top - 1 RETURN value ELSE RETURN -1 ENDIF ENDFUNCTION`	`popped_item = stack.pop()`	`int poppedItem = stack.pop();`
peek()	`FUNCTION peek(s : Stack) RETURNS INTEGER IF NOT isEmpty(s) THEN RETURN s.items[s.top] ELSE RETURN -1 ENDIF ENDFUNCTION`	`def peek(self): if not self.isEmpty(): return self.stack[-1] else: return None`	`int topItem = stack.peek();`
size()	`FUNCTION size(s : Stack) RETURNS INTEGER RETURN s.top + 1 ENDFUNCTION`	`size = len(stack)`	`int size = stack.size();`

In pseudocode, we assume a record-based stack with fields like items, top, and capacity
Python uses built-in lists or a class with .push() and .pop() methods
Java uses Stack<E> from java.util.Stack

Stack in operation (Pseudocode)

Defines a Stack structure with a fixed-size array of 5 items, a top pointer, and a capacity
Initialises the stack with top ← -1 to indicate it is empty
Pushes 3 values (10, 20, 30) onto the stack one by one:
- Each push increases top and adds the value to the items array
Peeks at the current top of the stack:
- Returns 30 without removing it
Pops two values off the stack:
- First pop returns 30, second returns 20
- Each pop decreases top by 1
Outputs the current size of the stack:
- After popping twice, only one value (10) remains → size is 1

Python

Java

Worked Example Study the pseudocode function PushAnimal():

Write program code for the function PushAnimal()[3]

Final answer: Answer

Function header (and close where appropriate) with parameter, checking if full (AnimalTopPointer = 20) and returning false [1 mark]

If not full, inserting parameter value into AnimalTopPointer [1 mark]

…incrementing pointer and returning true [1 mark]

Example program code:

Java

VB.NET

Python

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Exam Questions Flashcards

Operation

Pseudocode

Python

Java

isEmpty()

FUNCTION isEmpty(s : Stack) RETURNS BOOLEAN RETURN s.top = -1 ENDFUNCTION

def isEmpty(self): return len(self.stack) == 0

stack.isEmpty();

isFull()

FUNCTION isFull(s : Stack) RETURNS BOOLEAN RETURN s.top = s.capacity - 1 ENDFUNCTION

def isFull(self): return len(self.stack) == self.capacity

public boolean isFull() { return top == capacity - 1; }

push(value)

PROCEDURE push(s : REF Stack, value : INTEGER) IF NOT isFull(s) THEN s.top ← s.top + 1 s.items[s.top] ← value ELSE OUTPUT "Stack is full" ENDIF ENDPROCEDURE

stack.push(1)

stack.push(1);

pop()

FUNCTION pop(s : REF Stack) RETURNS INTEGER IF NOT isEmpty(s) THEN DECLARE value : INTEGER value ← s.items[s.top] s.top ← s.top - 1 RETURN value ELSE RETURN -1 ENDIF ENDFUNCTION

popped_item = stack.pop()

int poppedItem = stack.pop();

peek()

FUNCTION peek(s : Stack) RETURNS INTEGER IF NOT isEmpty(s) THEN RETURN s.items[s.top] ELSE RETURN -1 ENDIF ENDFUNCTION

def peek(self): if not self.isEmpty(): return self.stack[-1] else: return None

int topItem = stack.peek();

size()

FUNCTION size(s : Stack) RETURNS INTEGER RETURN s.top + 1 ENDFUNCTION

size = len(stack)

int size = stack.size();