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Pivot map[key] reversal doesn't revert the correct value #39242

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goldsky opened this issue May 25, 2020 · 4 comments
Closed

Pivot map[key] reversal doesn't revert the correct value #39242

goldsky opened this issue May 25, 2020 · 4 comments
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@goldsky
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goldsky commented May 25, 2020

What version of Go are you using (go version)?

$ go version
go1.14.2 darwin/amd64

Does this issue reproduce with the latest release?

If play.golang.org has the latest release, then yes.

What operating system and processor architecture are you using (go env)?

go env Output
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/admin/Library/Caches/go-build"
GOENV="/Users/admin/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/usr/local/var/www/htdocs/gopath"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/c9/gwddgxzn4gqf_4qr4gdf68yw0000gn/T/go-build273093480=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I am trying to flatten inner slice by attaching the outer slice, then reverse the output of flatten the outer slice by attaching inner slice

https://play.golang.org/p/YwYC3BYwfyx

Inner slices

map[id:1]
map[id:2]
map[id:3]

Outer slice
[map[id:A inner:[map[id:1] map[id:2] map[id:3]]]]

Pivot Inner
[map[id:1 outer:map[id:A]] map[id:2 outer:map[id:A]] map[id:3 outer:map[id:A]]]

What did you expect to see?

Pivot Outer
[map[id:A inner:map[id:1]] map[id:A inner:map[id:2]] map[id:A inner:map[id:3]]]

What did you see instead?

Pivot Outer
[map[id:A inner:map[id:3]] map[id:A inner:map[id:3]] map[id:A inner:map[id:3]]]
@seankhliao
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https://github.com/golang/go/wiki/Range#gotchas

@goldsky
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goldsky commented May 25, 2020 •
edited

if you run the playground

		// dump row
		fmt.Println(pivotInnerMap)
		outerX["inner"] = pivotInnerMap

returns

map[id:1]
map[id:2]
map[id:3]

but when appended the value is lost

@seankhliao
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That is the point of the link. Ranges reuse the same variable for all iterations, and maps are reference types so your code all points to the same map.

@seankhliao
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see https://github.com/golang/go/wiki/Questions for places to ask questions

@goldsky goldsky closed this as completed May 25, 2020
@golang golang locked and limited conversation to collaborators May 25, 2021
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@goldsky @gopherbot @seankhliao