loop 1

package main

func main() {
	x := []int{7, 8, 9}
	y := [3]*int{}
	for i, v := range x {
		defer func() {
			print(v)
		}()
		y[i] = &i
	}
	print(*y[0], *y[1], *y[2], " ")
}

Choices:

• 012 789
• 012 987
• 222 789
• 222 999

Answer: 222 999

Run it on Go play.

Key points:

• In Go, the iteration variables (here they are i and v) are shared by all loop steps. In other words, each iteration variable only has one instance during the execution of the loop. That means all the elements of y store the same address (of the value v).
• In the end, the variable i is set as 2, and the variable v is set as 9.

Different from the above program, the following one prints 012 987.

package main

func main() {
	x := []int{7, 8, 9}
	y := [3]*int{}
	for i, v := range x {
		defer print(v)
		
		i := i
		y[i] = &i
	}
	print(*y[0], *y[1], *y[2], " ")
}

• slice: 1, 2, 3
• map: 1
• channel: 1
• loop: 1, 2, 3, 4
• switch: 1
• defer: 1, 2
• panic/recover: 1, 2
• reflect: 1, 2
• const: 1, 2, 3
• operator: 1, 2, 3
• function call: 1
• scope: 1, 2
• nil: 1
• embedding: 1

(more are coming ...)