Note Editor Undo/Redo Stack

A small C program demonstrating the classic undo/redo pattern with two stacks.

How it works

Two Stack instances are maintained:

• undoStack — holds actions that can be undone
• redoStack — holds actions that can be redone

Each stack is a fixed-size array (MAX = 100) of strings (up to 50 characters each). A top index tracks the current position; -1 means empty.

API

Function	Description
push(Stack *s, char item[50])	Pushes a string onto the top of the stack. No-op if full.
pop(Stack *s)	Removes and returns the top string. Returns NULL if empty.
isEmpty(Stack *s)	Returns true when the stack holds no items.
isFull(Stack *s)	Returns true when the stack has reached MAX.

Example

push(&undoStack, "Name1");
push(&undoStack, "Name3");
push(&undoStack, "Name5");

pop(&undoStack);   // "Name5"

Every new edit is pushed onto undoStack. When the user hits undo, the last action is popped off and pushed onto redoStack; redo reverses that movement.

Build & run

gcc undo_redo_stack.c -o undo_redo_stack
./undo_redo_stack

Limitations

• Educational example, not a full editor.
• redoStack is not yet wired to undoStack; undo does not currently move the popped action onto the redo stack.
• Stack capacity is fixed at compile time.