Undo stack
- Algorithms course
- Intermediate
- Lesson 46/80
- 4 min
Push completed actions to an undo stack.
Undo stack
Now we write a function that pushes an action onto the stack. init_undo comes from the previous lesson and is used here.
The push operation: stack.append(action) adds the action to the end of the stack. Because of LIFO the last added stays on top and is undone first.
We can add several actions and watch the stack grow and its state; this tracks the undo history.
Action list
Actions are represented as strings: 'add', 'delete', 'change'. Each is pushed onto the stack and builds the undo history.
If several actions are added the stack grows. The last added is on top; it is undone first and removed from the history.
The stack size shows how many undos are possible; an empty stack means nothing to undo.
Practice Push action
Write push_undo(stack, action). Push onto stack, return stack. Print ['ekle', 'sil'] with s = init_undo(); push_undo(s, 'ekle'); push_undo(s, 'sil'); print(s).
Starter code
def init_undo():
# Empty undo stack
return []
def push_undo(stack, action):
# Push to undo stackSample solution
def init_undo():
# Empty undo stack
return []
def push_undo(stack, action):
# Push to undo stack
stack.append(action)
return stack
s = init_undo()
push_undo(s, 'add')
push_undo(s, 'del')
print(s)Expected output
['ekle', 'sil']
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which method does stack push use?
- pop
- insert
- append Correct answer
- remove
Explanation: append
2 Push return
stack.append(action)
return ____stack.append(action)
return stackExplanation: stack is right.
3 Undo stack: match the terms
| Term | Meaning |
|---|---|
append |
Push |
pop |
Extract |
LIFO |
Last first |
Stack |
Undo |