stacks

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

• 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)

TYPE Stack DECLARE items : ARRAY[0:4] OF INTEGER DECLARE top : INTEGER DECLARE capacity : INTEGER
ENDTYPE // Initialise stack
DECLARE s : Stack
s.top ← -1
s.capacity ← 5 // Push 3 values onto the stack
CALL push(s, 10)
CALL push(s, 20)
CALL push(s, 30) // Peek at the top item
OUTPUT "Top item is ", peek(s) // Pop 2 values from the stack
DECLARE val : INTEGER
val ← pop(s)
OUTPUT "Popped value: ", val val ← pop(s)
OUTPUT "Popped value: ", val // Output current stack size
OUTPUT "Current size: ", size(s)

• 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

class Stack: def __init__(self, capacity): self.stack = [] self.capacity = capacity def isEmpty(self): return len(self.stack) == 0 def isFull(self): return len(self.stack) == self.capacity def push(self, value): if not self.isFull(): self.stack.append(value) else: print("Stack is full") def pop(self): if not self.isEmpty(): return self.stack.pop() else: return None def peek(self): if not self.isEmpty(): return self.stack[-1] else: return None def size(self): return len(self.stack) # Stack in operation
s = Stack(5) # Push 3 values
s.push(10)
s.push(20)
s.push(30) # Peek at top
print("Top item is", s.peek()) # Pop 2 values
print("Popped value:", s.pop())
print("Popped value:", s.pop()) # Output current size
print("Current size:", s.size())

Java

import java.util.Stack; public class StackExample { public static void main(String[] args) { Stack<Integer> s = new Stack<>(); // Push 3 values s.push(10); s.push(20); s.push(30); // Peek at top System.out.println("Top item is " + s.peek()); // Pop 2 values System.out.println("Popped value: " + s.pop()); System.out.println("Popped value: " + s.pop()); // Output current size System.out.println("Current size: " + s.size()); }
}