在Golang中，为什么这样的类型转换会导致运行时错误：索引超出范围？
I was doing the exercise of "A Tour of Go", the page I was on is https://tour.golang.org/moretypes/15
And following is my code:
package main
import "golang.org/x/tour/pic"
func Pic(dx, dy int) [][]uint8 {
var ret [][]uint8;
var row []uint8;
for i:=uint8(0);i<uint8(dy);i++ {
row = []uint8 {}
for j:=uint8(0);j<uint8(dx);j++ {
row = append(row, i+j)
}
ret = append(ret, row)
}
return ret
}
func main() {
pic.Show(Pic)
}
When I run these codes, the console throws an error:
panic: runtime error: index out of range
goroutine 1 [running]:
panic(0x18b820, 0x1040a010)
/usr/local/go/src/runtime/panic.go:464 +0x700
golang.org/x/tour/pic.Show(0x1d7948, 0x104000e0)
/go/src/golang.org/x/tour/pic/pic.go:24 +0x540
main.main()
/tmp/sandbox969725880/main.go:19 +0x20
Does anyone have any ideas why does the error occur for this int->uint8 type conversion?Thanks!
uint8 has a max value of 255 (only 8 bits, max 2^8) but dx, dy passed into Pic can have values greater than that (since they're int's, likely 64 bits). Values greater than 255 might get cast to 0 during conversion to uint8. If dy is 256 and it's cast to 0 as i, the outer for loop doesn't execute at all and no items get pushed into the array. Subsequently, when whatever mechanism in "golang.org/x/tour/pic" tries to access the values in the matrix after it's returned, it looks like it's generating an 'index out of range' error because there are literally no indexes in the matrix to access.
Working code:
package main
import "golang.org/x/tour/pic"
func Pic(dx, dy int) [][]uint8 {
var ret [][]uint8
for i := 0; i < dy; i++ {
row := []uint8{}
for j := 0; j < dx; j++ {
row = append(row, uint8(i+j))
}
ret = append(ret, row)
}
return ret
}
func main() {
pic.Show(Pic)
}