The Stack block

What it does

Uses a list as a stack, like a pile of plates: push puts one on top, pop takes the top one off. The last one in is the first one out.

Use it for

• Undo: push every change, pop to undo the last one.
• Reversing things: push them all, then pop them all.
• Checking brackets match: push each (, pop at each ).

What to fill in

What to do

Push a value on top, Pop the top one off, or Peek at the top one without taking it.

Which stack

A list, made first with Create Value: plates = [].

Push

The value to put on top, like "blue plate" (for Push).

Keep it in

A name for the one taken off or looked at, like top (for Pop and Peek).

Try it

Open it in the editor, press Run, then change it and run it again.

1plates = []2plates.append("red")3plates.append("blue")4plates.append("green")5top = plates.pop()6print("Took off: " + top)7print("Left: " + ", ".join(plates))