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Inside the ComputerProgramming

The Fencepost Puzzle

A 10-meter fence with a post every meter needs 11 posts

A 10-meter fence, a post every meter. Easy: 10 posts, right? Programmers fall into this exact trap all the time.

You'll find out

  • Where the extra post comes from
  • Why code counts from zero
  • The one-symbol bug in a loop

Why it matters

Off-by-one slips are a classic bug in code, and they hide in calendars, page counts and hotel stays too.

About 3 min6 interactive slides · 5 questionsLesson 25 · Zone 3

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