Stack Applications (Call Stack, Undo/Redo)

Stack Applications: Real-World Use Cases

Stacks are not just theoretical constructs; they are fundamental to how many computer systems and applications function. Their LIFO nature makes them ideal for scenarios where the most recently added item needs to be processed or accessed first. Let's explore some prominent real-world applications of stacks.

---

1. Function Call Stack (Call Stack)

Perhaps the most ubiquitous application of a stack is the call stack, which is an integral part of how programs execute functions. Every time a function is called, information about that function call is "pushed" onto the call stack. When a function finishes executing, its information is "popped" off the stack.

• Concept:

  • When Function A calls Function B, Function A is paused, and Function B's context is pushed onto the stack.
  • If Function B then calls Function C, Function B's context is paused, and Function C's context is pushed.
  • When Function C completes, its context is popped, and Function B resumes.
  • When Function B completes, its context is popped, and Function A resumes.

• What gets pushed onto the stack?

  • Return Address: The memory address of the instruction where the program should return after the function finishes.
  • Local Variables: Variables declared within the function's scope.
  • Function Arguments: The values passed to the function.
  • Current Machine State: Register values, etc.

• Example (Conceptual JavaScript):

  Consider this simplified execution flow:

  function multiply(x, y) {
    return x * y;
  }
  
  function calculate(a, b) {
    let result = multiply(a, b); // (1) Calls multiply
    return result + 1;
  }
  
  let finalValue = calculate(5, 10); // (2) Calls calculate
  console.log(finalValue); // (3) Continues after calculate returns
  

  Call Stack Behavior:

  1. [Empty]
  2. calculate(5, 10) is called: [calculate context]
  3. multiply(5, 10) is called from within calculate: [calculate context, multiply context]
  4. multiply finishes, its context is popped: [calculate context]
  5. calculate finishes, its context is popped: [Empty]
  6. Program continues.

• Key points:

  • This LIFO behavior ensures that the program always returns to the correct place after a function completes.
  • A "Stack Overflow" error occurs when the call stack runs out of memory, typically due to excessive recursion without a proper base case, or very deeply nested function calls.

---

2. Undo/Redo Functionality

Many applications, from text editors and image manipulation software to design tools, provide "undo" and "redo" features. Stacks are the perfect data structure for implementing this.

• Concept:

  • Maintain two stacks: an "undo stack" and a "redo stack".
  • Every time a user performs an action (e.g., typing text, drawing a line, applying a filter), the reverse of that action (or the state before the action) is pushed onto the undo stack. The redo stack is cleared.
  • When the user clicks "Undo," the action/state at the top of the undo stack is popped, applied (reversing the last change), and then pushed onto the redo stack.
  • When the user clicks "Redo," the action/state at the top of the redo stack is popped, applied (re-applying the undone change), and then pushed back onto the undo stack.

• Illustration:

  Initial: Undo Stack: [], Redo Stack: []

  1. User types "Hello": Push "delete Hello" onto Undo Stack. Undo: [delete "Hello"], Redo: []
  2. User types ", World!": Push "delete ', World!'" onto Undo Stack. Undo: [delete "Hello", delete ", World!"], Redo: []
  3. User clicks UNDO: Pop "delete ', World!'" from Undo, apply it, push "re-add ', World!'" onto Redo. Undo: [delete "Hello"], Redo: [re-add ", World!"]
  4. User clicks UNDO again: Pop "delete 'Hello'" from Undo, apply it, push "re-add 'Hello'" onto Redo. Undo: [], Redo: [re-add ", World!", re-add "Hello"]
  5. User clicks REDO: Pop "re-add 'Hello'" from Redo, apply it, push "delete 'Hello'" onto Undo. Undo: [delete "Hello"], Redo: [re-add ", World!"]

• Why a stack? The LIFO principle ensures that undoing always reverses the last action performed, and redoing always re-applies the last action undone.

---

3. Expression Evaluation (Postfix/Prefix Conversion)

Stacks are extensively used in compilers and interpreters for parsing and evaluating arithmetic expressions, especially when converting between infix (standard notation like A + B) and postfix (Reverse Polish Notation like AB+) or prefix notations.

• Concept:

  • When converting an infix expression to postfix, operators are temporarily stored on a stack, following precedence rules.
  • When evaluating a postfix expression, operands are pushed onto a stack, and when an operator is encountered, the top two operands are popped, the operation is performed, and the result is pushed back.

• Example (Evaluating Postfix 2 3 * 5 +):

  1. Read 2: Push 2 onto stack. Stack: [2]
  2. Read 3: Push 3 onto stack. Stack: [2, 3]
  3. Read *: Pop 3, Pop 2. Perform 2 * 3 = 6. Push 6. Stack: [6]
  4. Read 5: Push 5 onto stack. Stack: [6, 5]
  5. Read +: Pop 5, Pop 6. Perform 6 + 5 = 11. Push 11. Stack: [11]
  6. End of expression. Result: 11.

---

4. Browser History (Back/Forward Buttons)

While often implemented with two stacks (one for "back" history, one for "forward" history), the core idea leverages the LIFO principle.

• Concept:
  • When you navigate to a new page, the current page is "pushed" onto a "back" stack. The "forward" stack is cleared.
  • Clicking "Back" pops a page from the "back" stack and pushes the current page onto the "forward" stack.
  • Clicking "Forward" pops a page from the "forward" stack and pushes the current page onto the "back" stack.

---

Other Applications:

• Parenthesis Checking/Balancing: Stacks are used to verify if parentheses, brackets, and braces in an expression or code are correctly matched and nested.
• Syntax Parsing: Compilers use stacks to parse the syntax of programming languages.
• Depth-First Search (DFS): While not always explicitly stated, DFS graph traversal algorithms can be implemented using a stack (either explicitly or implicitly through recursion, which uses the call stack).

---

Key Takeaway: Stacks are incredibly versatile due to their simple yet powerful LIFO behavior. From managing the execution flow of your code to enabling common software features like undo/redo, their applications are pervasive in computer science.