I am taking the "Tour of Go", and had a question regarding the Exercise: Slices example. Currently I can create the picture by iterating over each index using the the [] operator, just like you could in C.
func Pic(dx, dy int) [][]uint8 {
pic := make([][]uint8, dy)
for i := range pic {
pic[i] = make([]uint8, dx)
for j := range pic[i] {
pic[i][j] = uint8(1)
}
}
return pic
}
However, when I try to do something like below, I get an panic: runtime error: index out of range error. I tried adding print statements and calling Pic(3, 3), which printed out a 3x3 array just fine.
func Pic(dx, dy int) [][]uint8 {
pic := make([][]uint8, dy)
for _, y := range pic {
y = make([]uint8, dx)
for _, x := range y {
x = uint8(1)
_ = x // x has to be used
//fmt.Print("1")
}
//fmt.Print("
")
}
return pic
}
Any thoughts on what I am doing wrong?
The main problem is your attempt to do assignment. Check my example using your code; https://play.golang.org/p/lwoe79jQ70
What you actually get out of the latter implementation is a 3x0 array, all of the inner arrays are empty. The reason for this is because you're using the range variable for assignment which doesn't work. If the current index is 0, y != pic[0], pic[0] is assigned to y however, y is temporary storage, it typically is the same address and is over written on each iteration. So after the latter example executes, all your x direction arrays are empty, indexing into one causes a panic.
Basically you should just be using your first implementation because it works fine and is the way you would typically do this. But the take away is, when you do a, b := range Something b != Something[a], it is it's on instance, it goes out of scope at the bottom of the loop and assigning to it will not cause a state change to the collection Something, instead you must assign to Something[a] if you want to modify Something[a].
range copies the values from the slice you're iterating over.
See: http://golang.org/ref/spec#RangeClause
To clarify what happens see this simple code example and its output:
package main
import "fmt"
func main() {
s := "hi"
//s[0] = 'H' // cannot assign to s[0]
for _, v := range s {
fmt.Printf("%T, %[1]v, %X
", v, &v)
v = 'H' // has no effect: this is local var not ref
}
fmt.Println(s)
}
The output is:
int32, 104, C0820042D4
int32, 105, C0820042D4
hi
As you see the address of variable v is not changing (C0820042D4) and v is local variable and range copies value to it, so changing v has no effect. Here v is rune (int32 alias), A rune is an integer value identifying a Unicode code point, and you cannot assign to s[0] and this won’t compile: s[0] = 'H'
so v = 'H' has no effect on s, it is just local variable.