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teivah / 100-go-mistakes

📖 100 Go Mistakes and How to Avoid Them
https://100go.co/
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Does range operator really copy the iterable? #24

Closed lootek closed 1 year ago

lootek commented 1 year ago

Hello, first of all, please accept many thanks for the great book you wrote! It's awesome to have all those gotchas collected in one place and so well explained :)

I would like to ask for a further explanation of the statement in the book that made me a bit confused though. It says that the range operator copies the slice/array/channel being iterated over (ie the range clause) on its first (and only) evaluation just before the iteration. I cannot find that in the relevant part of the Golang spec. My understanding was, on the other hand, the for-range behaves that way because it's the v (the local iterator variable) that is a copy of the slice element (not that the whole iterable thing gets copied). Please see the following snippet: https://go.dev/play/p/tEE8fW6Jxrk.

Referring to the code samples in this repo here, such a statement can be found in the book as the comment eg. to this one (or a couple of other snippets in the 4.2.x chapter): https://github.com/teivah/100-go-mistakes/blob/b2152d01bbe3397f7df3bc8af10a62a6f8e07aa9/04-control-structures/31-range-loop-arg-evaluation/arrays/main.go#L5-L13

teivah commented 1 year ago

You are right; this could be better phrased. Thanks, it'll be updated in the next version.