doushun4666
doushun4666
2015-11-20 09:30

在for循环中未定义外部变量

已采纳

I am wondering why the following piece of code is not working:

package main

import (
    "fmt"
)

func main() {
    for i := 0; i < 10000; i++ {
        var randomString = fmt.Sprintf("a%sa
", "test")
    }
    fmt.Printf("Made 10000 random strings like", randomString);
}

I've stripped some unrelevant code (as this is obviously not really random).

The issue i'm having is that just under the for-loop, "randomString" is undefined.

I've tried setting it using randomString := fmt.Sprintf() and with the var you've seen above.

I'm quite sure this is a scoping issue (the randomString variable is not in the scope outside of the for-loop), but as a PHP / JS developer, i'm not used to this and would say that variable is also available after the for loop.

How can I access that variable from that point? Basically just displaying the last generated string.

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2条回答

  • douzheyo2617 douzheyo2617 6年前

    See the relevant section from the spec: Declarations and scope:

    The scope of a constant or variable identifier declared inside a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl for short variable declarations) and ends at the end of the innermost containing block.

    Define it in the scope in which you want to access it: before the for (in the scope of the main() function).

    Also note that fmt.Sprintf() requires an additional parameter besides the ones to be printed: a format string. Either provide a format string (e.g. include a %s verb for the randomString parameter) or alternatively you may use fmt.Sprintln().

    func main() {
        var randomString string
        for i := 0; i < 10000; i++ {
            randomString = fmt.Sprintf("a%sa
    ", "test")
        }
        fmt.Println("Made 10000 random strings like", randomString)
    }
    

    Output:

    Made 10000 random strings like atesta
    

    Try it on the Go Playground.

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  • douhensheng1131 douhensheng1131 6年前

    This is scoping issue, in Go the scope of randomString is the loop body, in JS it would be the whole function. Scoping rules differ in the different languages.

    Check the Go spec:

    https://golang.org/ref/spec#Declarations_and_scope

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