In perforce, is it possible to subtract the changes of one changelist from another? - perforce

Question

Suppose, I have two changelist A and B (which is on top of A).
Now, I want only the changes done in B and not the one which are included in A, i.e. B-A
Is it possible to do so?

Answer 1

Yes. Do:
p4 undo #A,A
p4 sync #B
This will create a new pending changelist that is "B minus A". If A and B touched the same file(s), syncing to #B will require a resolve to merge B's changes into the file (without A's changes). The merge will be set up in such a way as to make everything happen as automatically as is possible.
If there are other, later, changes to the same file(s), you will need to sync and resolve those just as you would any time you're working on an older changelist.
This is theoretically also doable in P4V (I think it's the "back out" command), but I've had bad experiences with that and do not recommend it. It's very straightforward from the command line.
Example
C:\Perforce\test\undo>p4 annotate -c foo
//stream/main/undo/foo#3 - edit change 137 (text)
135: this
136: is **CHANGE A**
135: a
137: test **CHANGE B**
135: file
Change A is #136, Change B is #137.
C:\Perforce\test\undo>p4 undo #136,136
//stream/main/undo/foo#2 - opened for integrate
... undid //stream/main/undo/foo#2
C:\Perforce\test\undo>p4 sync #137
//stream/main/undo/foo#3 - is opened and not being changed
... //stream/main/undo/foo - must resolve #3 before submitting
C:\Perforce\test\undo>p4 resolve -am
c:\Perforce\test\undo\foo - merging //stream/main/undo/foo#3
Diff chunks: 1 yours + 1 theirs + 0 both + 0 conflicting
//compy386/undo/foo - merge from //stream/main/undo/foo
C:\Perforce\test\undo>p4 submit -d "undid A"
Submitting change 138.
Locking 1 files ...
integrate //stream/main/undo/foo#4
Change 138 submitted.
Change 136 has now been undone by change 138:
C:\Perforce\test\undo>p4 annotate -c foo
//stream/main/undo/foo#4 - integrate change 138 (text)
135: this
138: is
135: a
137: test **CHANGE B**
135: file
C:\Perforce\test\undo>p4 filelog foo
//stream/main/undo/foo
... #4 change 138 integrate on 2020/02/08 by Samwise#compy386 (text) 'undid A'
... ... undid //stream/main/undo/foo#2
... #3 change 137 edit on 2020/02/08 by Samwise#compy386 (text) 'change B'
... #2 change 136 edit on 2020/02/08 by Samwise#compy386 (text) 'change A'
... ... undone by //stream/main/undo/foo#4
... #1 change 135 add on 2020/02/08 by Samwise#compy386 (text) 'test file'