How to check if a string is all upper or lower case in Go?

Question

What is an easy way in Golang to check if all characters in a string are upper case or lower case?

Also, how to handle a case where the string has punctuation?

See these examples:

package main

import (
    "fmt"
    "unicode"
)

func main() {
    s := "UPPERCASE"
    fmt.Println(s.IsUpper())  // Should print true

    s = "lowercase"
    fmt.Println(s.IsUpper())  // Should print false

    s = "lowercase"
    fmt.Println(s.IsLower())  // Should print true

    s = "I'M YELLING AT YOU!"
    fmt.Println(s.IsUpper())  // Should print true
}

Note: s.IsUpper() and s.IsLower() doesn't really exist, but would be nice to find an equivalent.

Solution 1

You can of course compare the upper and lower cased strings in their entirety, or you can short-circuit the comparisons on the first failure, which would be more efficient when comparing long strings.

func IsUpper(s string) bool {
    for _, r := range s {
        if !unicode.IsUpper(r) && unicode.IsLetter(r) {
            return false
        }
    }
    return true
}

func IsLower(s string) bool {
    for _, r := range s {
        if !unicode.IsLower(r) && unicode.IsLetter(r) {
            return false
        }
    }
    return true
}

Solution 2

One solution is to use strings.ToUpper()/ToLower() and compare with the original string. This works for the punctuation case as well.

Here's the solution:

package main

import (
    "fmt"
    "strings"
)

func main() {
    s := "UPPERCASE"
    fmt.Println(strings.ToUpper(s) == s)

    s = "lowercase"
    fmt.Println(strings.ToUpper(s) == s)

    s = "lowercase"
    fmt.Println(strings.ToLower(s) == s)

    s = "I'M YELLING AT YOU!"
    fmt.Println(strings.ToUpper(s) == s)
}

Comments

[jersey bean]

So I guess I'll state the obvious. I think your point is that using strings.ToUpper is a O(n) operation and is costly for really long strings (as n grows large). So your solution uses unicode.IsUpper and will short circuit as soon as your find a character which is not upper case. Good answer!

[jersey bean]

See my comment to @JimB solution. Its more efficient than this solution.

[Carson]

each has his strong point. consider the case 中文a1