What is the difference between [0] and [:1] in Go?

Question

I split a string by spaces:

splstr = strings.Split(str, " ")

Then I iterate each word, and look at the first character like this:

splstr[i][0] == "#"

But I got these errors from that line:

... : cannot convert "#" to type uint8

... : invalid operation: splstr[i][0] == "#" (mismatched types uint8 and string)

But then I spliced it:

splstr[i][:1] == "#"

And that works. I get why [:1] is of type string, but why is [0] of type uint8? (I'm using Go 1.1.)

Answer 1

Because the array notation on a string gives access to the string's bytes, as documented in the language spec:

http://golang.org/ref/spec#String_types

A string's bytes can be accessed by integer indices 0 through len(s)-1.

(byte is an alias for uint8)

Answer 2

[x:x] ([:x] is a form of [0:x]) will cut a slice into another slice while [x] will retrieve the object at index x. The difference is shown below:

arr := "#####"
fmt.Println(arr[:1]) // will print out a string
fmt.Println(arr[0]) // will print out a byte

If the string is converted into []byte:

arr := []byte("#####")
fmt.Println(arr[:1]) // will print out a slice of bytes
fmt.Println(arr[0]) // will print out a byte

You can try this yourself at http://play.golang.org/p/OOZQqXTaYK