How many times len() runs?

Question

How many times will len(p) run? Only one and the result will be saved, or will it run for each iteration?

func main() {
    p := []int{2, 3, 5, 7, 11, 13}
    for i:=0;i<len(p);i++ {}
}

Answer 1

len() on a slice is optimized by the compiler, it's like accessing a local variable. It's not really a function call.

You can verify with:

$ cat x.go
package main
import "fmt"
func main() {
    a := []int{1,2, 3}
    fmt.Println(len(a))
}

And then look at the compiler output:

$ go tool 6g -S x.go

--- prog list "main" ---
0000 (x.go:3) TEXT    main+0(SB),$128-0
0001 (x.go:4) MOVQ    $0,autotmp_0002+-80(SP)
0002 (x.go:4) MOVQ    $0,autotmp_0002+-72(SP)
0003 (x.go:4) MOVQ    $0,autotmp_0002+-64(SP)
0004 (x.go:4) LEAQ    autotmp_0002+-80(SP),BX
0005 (x.go:4) MOVQ    BX,autotmp_0001+-56(SP)
0006 (x.go:4) MOVQ    autotmp_0001+-56(SP),BX
0007 (x.go:4) MOVQ    statictmp_0000+0(SB),BP
0008 (x.go:4) MOVQ    BP,(BX)
0009 (x.go:4) MOVQ    statictmp_0000+8(SB),BP
0010 (x.go:4) MOVQ    BP,8(BX)
0011 (x.go:4) MOVQ    statictmp_0000+16(SB),BP
0012 (x.go:4) MOVQ    BP,16(BX)
0013 (x.go:4) MOVQ    autotmp_0001+-56(SP),BX
0014 (x.go:4) MOVQ    $3,CX
0015 (x.go:5) LEAQ    autotmp_0005+-16(SP),DI
0016 (x.go:5) MOVQ    $0,AX
0017 (x.go:5) STOSQ   ,
0018 (x.go:5) STOSQ   ,
0019 (x.go:5) LEAQ    autotmp_0005+-16(SP),BX
0020 (x.go:5) MOVQ    BX,autotmp_0004+-48(SP)
0021 (x.go:5) MOVQ    autotmp_0004+-48(SP),BX
0022 (x.go:5) MOVQ    $1,SI
0023 (x.go:5) MOVQ    $1,DX
0024 (x.go:5) MOVQ    BX,autotmp_0003+-40(SP)
0025 (x.go:5) MOVQ    autotmp_0003+-40(SP),BX
0026 (x.go:5) MOVQ    $type.int+0(SB),AX
0027 (x.go:5) MOVQ    AX,(BX)
0028 (x.go:5) MOVQ    CX,8(BX)
0029 (x.go:5) MOVQ    autotmp_0003+-40(SP),BX
0030 (x.go:5) MOVQ    BX,(SP)
0031 (x.go:5) MOVQ    SI,8(SP)
0032 (x.go:5) MOVQ    DX,16(SP)
0033 (x.go:5) CALL    ,fmt.Println+0(SB)
0034 (x.go:6) RET     ,

Notice the CALL to fmt.Println, but no call to len.

Answer 2

Try an experiment! http://play.golang.org/p/Eksb6bQovC

Or read the docs: http://golang.org/ref/spec#For_statements (TL;DR: "The condition is evaluated before each iteration.")

In your example, len is part of the condition so it is evaluated before each iteration. There will be an iteration for the six times the condition evaluates to true, then one more evaluation where it evaluates to false and the for loop exits. So, seven times in all.