Variable in for loop undefined outside

Question

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\n", "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.

icza · Accepted Answer · 2016-12-05 20:58:29Z

10

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\n", "test")
    }
    fmt.Println("Made 10000 random strings like", randomString)
}

Output:

Made 10000 random strings like atesta

Try it on the Go Playground.

edited Dec 5, 2016 at 20:58

answered Nov 20, 2015 at 9:35

icza

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2 Comments

Rob Over a year ago

Ah! That was so obvious I didn't see it. Great, thank you :)

Rob Over a year ago

And indeed, i forgot the %s in the formatted string. But Println works well for this too.

chill · Accepted Answer · 2015-11-20 09:35:45Z

2

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

answered Nov 20, 2015 at 9:35

chill

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3 Comments

Rob Over a year ago

Thanks, but I've checked that page before asking the question - it didn't clear things out for me. I knew I had scoping issues so thats why I asked.

chill Over a year ago

@RobQuist The relevant part for this case would be "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."

Rob Over a year ago

Exactly :) Thanks for the effort though!

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