From 2bba267120ad8ce7e9ecd97f3ce4b08bce80b41b Mon Sep 17 00:00:00 2001 From: Tom Levy Date: Tue, 22 Aug 2017 17:10:46 +1200 Subject: [PATCH] sort: fix mix-up between "!less" and "greater" in examples If Less(a, b) returns true when a is less than b, the correct way to check if a is greater than b is to use Less(b, a). It is wrong to use !Less(a, b) because that checks if a is greater than *or equal to* b. 1. The decreasingDistance function in Example_sortKeys makes this mistake. Fix it. 2. The documentation of multiSorter.Less says it loops along the less functions until it finds a comparison "that is either Less or !Less". This is nonsense, because (Less(a, b) or !Less(a, b)) is always true. Fix the documentation to say that it finds a comparison "that discriminates between the two items (one is less than the other)". The implementation already does this correctly. Change-Id: If52b79f68e4fdb0d1095edf29bdecdf154a61b8d Reviewed-on: https://go-review.googlesource.com/57752 Reviewed-by: Ian Lance Taylor Run-TryBot: Ian Lance Taylor TryBot-Result: Gobot Gobot --- src/sort/example_keys_test.go | 2 +- src/sort/example_multi_test.go | 9 +++++---- 2 files changed, 6 insertions(+), 5 deletions(-) diff --git a/src/sort/example_keys_test.go b/src/sort/example_keys_test.go index a8e47e4926..648f919e68 100644 --- a/src/sort/example_keys_test.go +++ b/src/sort/example_keys_test.go @@ -73,7 +73,7 @@ func Example_sortKeys() { return p1.distance < p2.distance } decreasingDistance := func(p1, p2 *Planet) bool { - return !distance(p1, p2) + return distance(p2, p1) } // Sort the planets by the various criteria. diff --git a/src/sort/example_multi_test.go b/src/sort/example_multi_test.go index 40d12152ce..de6ec142d1 100644 --- a/src/sort/example_multi_test.go +++ b/src/sort/example_multi_test.go @@ -49,10 +49,11 @@ func (ms *multiSorter) Swap(i, j int) { } // Less is part of sort.Interface. It is implemented by looping along the -// less functions until it finds a comparison that is either Less or -// !Less. Note that it can call the less functions twice per call. We -// could change the functions to return -1, 0, 1 and reduce the -// number of calls for greater efficiency: an exercise for the reader. +// less functions until it finds a comparison that discriminates between +// the two items (one is less than the other). Note that it can call the +// less functions twice per call. We could change the functions to return +// -1, 0, 1 and reduce the number of calls for greater efficiency: an +// exercise for the reader. func (ms *multiSorter) Less(i, j int) bool { p, q := &ms.changes[i], &ms.changes[j] // Try all but the last comparison.