data structure used to implement UNDO and REDO option

Question

I want to implement UNDO and REDO option(as we see in MS word etc). Can you suggest me a data structure for it, and how can i implement it.?

Answer 1

Objective-C Cocoa has a well documented anwser named NSUndoManager.

Answer 2

You can use Command Pattern to achive Undo/Redo

Check these samples:

http://www.codeproject.com/csharp/undoredobuffer.asp

http://www.dofactory.com/Patterns/PatternCommand.aspx

Answer 3

It isn't a data structure but a design pattern. You're looking for the Command Pattern.

The standard is to keep the Command objects in a stack to support multi level undo. In order to support redo, a second stack keeps all the commands you've Undone. So when you pop the undo stack to undo a command, you push the same command you popped into the redo stack. You do the same thing in reverse when you redo a command. You pop the redo stack and push the popped command back into the undo stack.

Answer 4

This is a classic case of Command Pattern. Following is a sample implementation of undo feature in Python :

from os import rename
class RenameFileCommand(object):
    def __init__(self, src_file, target_file):
        self.src_file=src_file
        self.target_file=target_file


    def execute(self):
        rename(self.src_file, self.target_file)

    def undo(self):
        rename(self.target_file,self.src_file)



class History(object):
    def __init__(self):
        self.commands=list()
    def execute(self, command):
        command.execute()
        self.commands.append(command)

    def undo(self):
        self.commands.pop().undo()


if __name__=='__main__':
    hist=History()
    hist.execute(RenameFileCommand( 'test1.txt', 'tmp.txt', ))
    hist.undo()
    hist.execute(RenameFileCommand( 'tmp2.txt', 'test2.txt',))

Answer 5

Actually, the standard pattern for this functionality (Gang of Four, even) is Memento.

Also, while most programs use Undo/Redo stacks, afficionados of certain text editors prefer Undo/Redo trees so that they don't lose their entire history if they undo a few commands, try a new one, and change their minds.