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math/rand: add a comment for the i=0 iteration

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Fixes #13215

Change-Id: I126117d42e7c1e69cbc7fad0760e225b03ed15bd
Reviewed-on: https://go-review.googlesource.com/16852
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Jingguo Yao 2015-11-12 21:59:30 +08:00 committed by Keith Randall
parent 7af0839e11
commit fb01ad21c2
1 changed files with 5 additions and 0 deletions
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5
src/math/rand/rand.go
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@ -148,6 +148,11 @@ func (r *Rand) Float32() float32 {
// Perm returns, as a slice of n ints, a pseudo-random permutation of the integers [0,n). // Perm returns, as a slice of n ints, a pseudo-random permutation of the integers [0,n).
func (r *Rand) Perm(n int) []int { func (r *Rand) Perm(n int) []int {
m := make([]int, n) m := make([]int, n)
// In the following loop, the iteration when i=0 always swaps m[0] with m[0].
// A change to remove this useless iteration is to assign 1 to i in the init
// statement. But Perm also effects r. Making this change will affect
// the final state of r. So this change can't be made for compatibility
// reasons for Go 1.
for i := 0; i < n; i++ { for i := 0; i < n; i++ {
j := r.Intn(i + 1) j := r.Intn(i + 1)
m[i] = m[j] m[i] = m[j]