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Grades K–2 · Computing

How can Undo bring back an earlier drawing?

A drawing app can make a new line disappear when Undo is pressed. It has not rubbed real paper. It has remembered enough about earlier changes to bring an earlier drawing back.

D33 visual explanationOne Undo brings back the earlier page with the circle but no line.The app remembers earlier drawingsEmpty pageCircle addedShown nowLine addedUndoOne Undo goes back before the line.

In the diagramThe app remembers earlier drawingsEmpty pageCircle addedShown nowLine addedUndoOne Undo goes back before the line.

The three remembered pages show why one Undo removes the line, while a second Undo also removes the circle.

Our toy app remembers three pages: an empty page, a circle, then a circle with a line. One Undo brings back the circle without the line. Another brings back the empty page.

Undo follows the changes backward, starting with the latest one. Apps do not remember everything forever. Their rules decide which earlier changes can still be brought back.

About this explanation

Starting knowledge: Understand before, after and most recent.

Sources

  • GIMP — Undoing
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