Original artwork, not a still
Beware of bugs in the above code; I have only proved it correct, not tried it.
Behind the line
What it means
Our interpretation.
It is the programmer's version of a mathematician's joke: a proof shows the idea is sound, but only a running machine shows you the typo, the off-by-one and the forgotten edge case. Knuth, who paid reward cheques for bugs found in his books, made the warning famous. It now turns up in code comments, conference slides and sign-offs wherever someone shares code they have reasoned about but never executed.
Context reviewed
In context
May contain spoilersShow the scene
Donald Knuth wrote the line as the closing sentence of a five-page memo titled "Notes on the van Emde Boas construction of priority deques: An instructive use of recursion". He sent it on March 29, 1977, to the Dutch computer scientist Peter van Emde Boas, with copies to Bob Tarjan and John Hopcroft. The memo described a data structure that Knuth had worked out on paper rather than run, so the final sentence warned readers that the code had only been proved correct, never tried.
Sources (2)
- Donald Knuth, Frequently Asked QuestionsPassage about the van Emde Boas memoReference checked
- Wikiquote: Donald KnuthQuotes section, first entryReference checked
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