go/build: Why is replacing the map key not working as expected?

[shuqingzai]

Go version

go version go1.25.4 darwin/amd64

Output of go env in your module/workspace:

#╰─⠠⠵ go env
AR='ar'
CC='clang'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='0'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='clang++'
GCCGO='gccgo'
GO111MODULE='on'
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN='/Users/worker/Code/go/bin'
GOCACHE='/Users/worker/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/worker/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch x86_64 -m64 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/cf/29v27s5s3vb8_c62bstsjqnm0000gp/T/go-build3803336973=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='amd64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/Users/worker/Code/go-project/worker/stickercube-goframe/go.mod'
GOMODCACHE='/Users/worker/Code/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/worker/Code/go'
GOPRIVATE=''
GOPROXY='https://goproxy.cn,https://goproxy.io,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/worker/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/darwin_amd64'
GOVCS=''
GOVERSION='go1.25.4'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

source code

package main

import (
	"fmt"
)

func main() {
	m1 := map[string]string{
		"a1": "aa",
		"b1": "bb",
	}
	m2 := make(map[string]string, len(m1))
	for k, v := range m1 {
		m2[k] = v+v
	}

	// replace m1 key with m2 value
	for k, v := range m1 {
		m1[m2[k]] = v
		delete(m1, k)
	}
	fmt.Println(m1)
	// output:
	//
	// map[aaaa:aa bbbb:bb] -- This is the expected output
	// map[:aa bbbb:bb] -- This is the actual output
}

What did you see happen?

This is the expected output

map[aaaa:aa bbbb:bb]

What did you expect to see?

This is the actual output

map[:aa bbbb:bb]

I'm sorry, I don't know much about the underlying mechanism of map. Is this as expected?

[gabyhelp]

Related Issues

• Map keys are being all updated #62428 (closed)
• go/map #59917 (closed)
• map: a key-value pair may be GC #38976 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[ianlancetaylor]

You are ranging over a map as you add and delete entries to the map. The results are somewhat unpredictable, as map iteration permit newly added items to appear in the range loop, or not. When they do appear, you are looking up the newly added key in m2, not finding it, and adding an entry to m1 with an empty key.

In general we do not the issue tracker for questions about Go. If you have any follow-up, or any other questions, please do not use the issue tracker. Please use a forum instead. See https://go.dev/wiki/Questions. Thanks.