CODEWITHSUNDEEP
go
fluffy
fluffy

Reputation: 5314

Assign-and-compare in go's while-equivalent?

In Go you can do:

if foo := bar() ; foo != nil {
    ...
}

In C/C++ you can do:

while ((foo = bar()) != NULL) {
    ...
}

However, Go's syntax does not seem to provide any equivalent way of doing assign-and-compare in a while loop; Go has replaced while with a specific invocation of for (e.g. for a is equivalent to for ; a ;). Simply trying to use the if version syntax confuses the parser, as it's expecting there to be a third statement.

I could just say:

for foo := bar() ; foo != nil ; foo = bar() {
    ....
}

but in this case, the bar() call is fairly long, complex, and not easy to break out into its own function (although I could do something like declaring a local func to call, but that still reduces the clarity of the code).

For now I am doing:

for {
    foo := bar();
    if foo == nil { break; }
    ...
}

but this seems unclean, both because it separates the loop criteria from the loop statement itself, and because it relies on break.

So, is there a clean, idiomatic way of doing an assign-and-compare in a while loop in Go? This is such a common use case I can't imagine that there's no way of doing it.

Upvotes: 21

Views: 4908

Answers (2)

Zombo
Zombo

Reputation: 1

You can create an iterator, similar to bufio#Scanner.Scan:

package main

type bar struct { foo int }

func (b *bar) next() bool {
   b.foo++
   return b.foo < 10
}

func main() {
   var b bar
   for b.next() {
      println(b.foo)
   }
}

Then in the Next function, you can put all the complicated logic you need.

Upvotes: 0

zzzz
zzzz

Reputation: 91203

No. Go has no while statement, only the special form of the for statement - and assignment is a statement, not an expression. Your examples are IMHO idiomatic Go.

Upvotes: 16

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