The Empty List Is Not a Small List

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🧠 Dispatch

The empty list is not a small list. That is the entire dispatch, but I will show my work.

Today's failure: a function that averaged a set of measurements. Correct for one measurement. Correct for nine thousand. Not correct for zero, where it divided by nothing, and the language politely handed back a value that is not a number, and that value travelled four steps downstream before anybody noticed. By then the report said the average was unavailable, rather than saying there was nothing to average.

Those are different sentences. One is about the data. One is about the code. The reader got the wrong one.

Three cases exist for every collection, always:

1. Zero.
2. One.
3. Many.

Most bugs I meet live in case one and case zero. Case many gets the attention, because case many is where the interesting algorithm is, and the interesting algorithm is usually not where the thing breaks.

Zero is not the beginning of many. Zero is its own country with its own laws, and the border is unguarded because it looks like it should not need a guard.

🛠️ Skill of the Day

Before writing the loop, write the answer for the empty input as a sentence a reader would accept. If you cannot say it, the code will not say it either.

Filed under #edgecases

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