Kiểu dữ liệu map trong Go không có thứ tự khi duyệt qua:
A map is an unordered group of elements of one type, called the element type, indexed by a set of unique keys of another type, called the key type. https://go.dev/ref/spec#Map_types
When iterating over a map with a range loop, the iteration order is not specified and is not guaranteed to be the same from one iteration to the next. https://go.dev/blog/maps#iteration-order
package main
func main() {
m := map[string]string{
"name": "FAMILUG",
"since": "2010",
"loc": "Vietnam",
}
for k, v := range m {
println(k, v)
}
fmt.Printf("%v\n", m)
Output
// lần 1
name FAMILUG
since 2010
loc Vietnam
map[loc:Vietnam name:FAMILUG since:2010]
// lần 2
since 2010
loc Vietnam
name FAMILUG
map[loc:Vietnam name:FAMILUG since:2010]
Vậy print(m) kết quả liệu có khác nhau mỗi lần chạy? Từ Go 1.12 (2018) thì không, output print(map) luôn cố định và được sắp xếp theo thứ tự key:
fmt¶
Maps are now printed in key-sorted order to ease testing. The ordering rules are:
When applicable, nil compares low
ints, floats, and strings order by <
NaN compares less than non-NaN floats
bool compares false before true
Complex compares real, then imaginary
Pointers compare by machine address
Channel values compare by machine address
Structs compare each field in turn
Arrays compare each element in turn
Interface values compare first by reflect.Type describing the concrete type and then by concrete value as described in the previous rules.
When printing maps, non-reflexive key values like NaN were previously displayed as <nil>. As of this release, the correct values are printed.
Kết luận
Output print hay format map type được sắp xếp theo key.
Hết.
HVN at https://pymi.vn and https://www.familug.org.