Welcome! Today, we will delve into managing a document's editing history using stacks. Imagine building a text editor; you would need to handle actions like adding text, undoing, and redoing those changes. We will see how these features can be efficiently implemented using stacks. By the end of this lesson, you will possess an in-depth understanding of applying stacks in practical scenarios.
Before starting the coding portion, let's dissect the methods we will implement. These methods will manage a document's edit history, allowing us to apply changes, undo them, and redo them effectively.
apply_change(self, change: str) -> None
: This method applies a change to the document. The change, represented as a string, is stored in a way that allows us to remember the order of applied changes. Any previously undone changes are discarded.
undo(self) -> str | None
: This method undoes the most recent change and allows us to store it for possible redo. It returns the change that was undone. If there are no changes available to undo, it returns None
.
redo(self) -> str | None
: This method redoes the most recent undone change, making it active again. It returns the change that was redone. If there are no changes available to redo, it returns None
.
get_changes(self) -> list[str]
: This method returns a list of all applied changes in the order they were applied.
To implement these methods efficiently, we'll use two stacks. We use stacks to keep track of applied changes and undone changes because of their LIFO (Last In, First Out) property. The most recent change is always at the top, making it easy to undo and redo.
Let's break down each method's implementation and study how they interact with the stacks.
Let's implement the solution step-by-step. Firstly, we define our class and set up the initial state.
Python1class DocumentHistory: 2 def __init__(self): 3 self.changes_stack = [] 4 self.redo_stack = []
Here, DocumentHistory
is the class, and we initialize an empty list named changes_stack
to act as our stack of applied changes and another empty list named redo_stack
to act as our stack for undone changes.
Next, we put into effect the method to apply changes.
Python1class DocumentHistory: 2 def __init__(self): 3 self.changes_stack = [] 4 self.redo_stack = [] 5 6 def apply_change(self, change: str) -> None: 7 self.changes_stack.append(change) 8 self.redo_stack.clear()
With apply_change
, we take a string change
and append it to the changes_stack
list. Additionally, we clear the redo_stack
to ensure that once a new change is applied, any previously undone changes cannot be redone.
Now, we will implement the method to undo the most recent change.
Python1class DocumentHistory: 2 def __init__(self): 3 self.changes_stack = [] 4 self.redo_stack = [] 5 6 def apply_change(self, change: str) -> None: 7 self.changes_stack.append(change) 8 self.redo_stack.clear() 9 10 def undo(self) -> str | None: 11 if not self.changes_stack: 12 return None 13 change = self.changes_stack.pop() 14 self.redo_stack.append(change) 15 return change
Here, undo
checks if the changes_stack
list is empty. If it is, it returns None
. Otherwise, it pops the last item from the list, simulating a stack pop operation, pushes it to the redo_stack
, and returns the undone change.
Now we'll create the method to redo the most recent undone change.
Python1class DocumentHistory: 2 def __init__(self): 3 self.changes_stack = [] 4 self.redo_stack = [] 5 6 def apply_change(self, change: str) -> None: 7 self.changes_stack.append(change) 8 self.redo_stack.clear() 9 10 def undo(self) -> str | None: 11 if not self.changes_stack: 12 return None 13 change = self.changes_stack.pop() 14 self.redo_stack.append(change) 15 return change 16 17 def redo(self) -> str | None: 18 if not self.redo_stack: 19 return None 20 change = self.redo_stack.pop() # Redo the last undone change 21 self.changes_stack.append(change) 22 return change
The redo
method checks if the redo_stack
is empty. If it is, it returns None
. Otherwise, it pops the last item from the stack, simulating a LIFO operation, pushes it back to the changes_stack
, and returns the redone change.
Lastly, we implement the method to retrieve all applied changes.
Python1class DocumentHistory: 2 def __init__(self): 3 self.changes_stack = [] 4 self.redo_stack = [] 5 6 def apply_change(self, change: str) -> None: 7 self.changes_stack.append(change) 8 self.redo_stack.clear() 9 10 def undo(self) -> str | None: 11 if not self.changes_stack: 12 return None 13 change = self.changes_stack.pop() 14 self.redo_stack.append(change) 15 return change 16 17 def redo(self) -> str | None: 18 if not self.redo_stack: 19 return None 20 change = self.redo_stack.pop() # Redo the last undone change 21 self.changes_stack.append(change) 22 return change 23 24 def get_changes(self) -> list[str]: 25 return self.changes_stack[:]
The get_changes
method simply returns a copy of the changes_stack
list, which includes all changes applied to the document in the order they were applied.
Here is the final implementation of our DocumentHistory
class, combining all the methods we've discussed:
Python1class DocumentHistory: 2 def __init__(self): 3 self.changes_stack = [] 4 self.redo_stack = [] 5 6 def apply_change(self, change: str) -> None: 7 self.changes_stack.append(change) 8 self.redo_stack.clear() 9 10 def undo(self) -> str | None: 11 if not self.changes_stack: 12 return None 13 change = self.changes_stack.pop() 14 self.redo_stack.append(change) 15 return change 16 17 def redo(self) -> str | None: 18 if not self.redo_stack: 19 return None 20 change = self.redo_stack.pop() # Redo the last undone change 21 self.changes_stack.append(change) 22 return change 23 24 def get_changes(self) -> list[str]: 25 return self.changes_stack[:]
Let's test this with an example:
Python1doc_hist = DocumentHistory() 2 3# Apply changes 4doc_hist.apply_change("Added header") 5doc_hist.apply_change("Added footer") 6print(doc_hist.get_changes()) # Output: ['Added header', 'Added footer'] 7 8# Undo last change 9print(doc_hist.undo()) # Output: "Added footer" 10print(doc_hist.get_changes()) # Output: ['Added header'] 11 12# Redo last undone change 13print(doc_hist.redo()) # Output: 'Added footer' 14print(doc_hist.get_changes()) # Output: ['Added header', 'Added footer'] 15 16# Undo all changes 17print(doc_hist.undo()) # Output: 'Added footer' 18print(doc_hist.undo()) # Output: 'Added header' 19print(doc_hist.get_changes()) # Output: [] 20 21# Try undoing when no changes are left 22print(doc_hist.undo()) # Output: None 23 24# Redo changes 25print(doc_hist.redo()) # Output: 'Added header' 26print(doc_hist.redo()) # Output: 'Added footer' 27print(doc_hist.get_changes()) # Output: ['Added header', 'Added footer'] 28 29# Try redoing when no changes are left to redo 30print(doc_hist.redo()) # Output: None
In today's lesson, we learned how to manage a document's editing history using stacks. We implemented methods to apply changes, undo the last change, redo the most recent undone change, and retrieve all applied changes. This exercise provided us with practical experience in using stacks to efficiently track and revert operations in a real-life scenario. Keep practicing similar challenges to deepen your understanding of data structures in various applications. Fantastic job today, and keep up the good work!