loop 3

package main

type Book struct {
	Pages int
}

func f() int {
	var books = []Book{{555}}
	for _, book := range books {
		book.Pages = 999
	}
	return books[0].Pages
}

func g() int {
	var books = []*Book{{555}}
	for _, book := range books {
		book.Pages = 999
	}
	return books[0].Pages
}

func main() {
	println(f(), g())
}

Choices:

• 999 999
• 555 999
• 999 555
• 555 555

Answer: 555 999

Run it on Go play.

Key points:

• In a loop, each container element is copied to the second iteration variable, modifying the iteration variable doesn't change the values of container elements.
• A struct value owns its fields. When a struct value is copied, all of its fields are also copied.
• A pointer to a struct value doesn't own the fields of the struct value. It just references the struct value, a.k.a. it just references the fields of the struct value. When a pointer is copied, the values referenced by the pointer are not copied. After copying, the copy of the pointer references the same values as the original pointer.

• 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, 4
• operator: 1, 2, 3
• function call: 1
• scope: 1, 2
• nil: 1
• embedding: 1

(more are coming ...)