The Go Cookbook

Properly Capitalizing a Title

How do I convert a string into a properly capitalized title?

Welcome To The Dollhouse?

Golang comes packaged with a good number of very useful string manipulation helpers in the strings package. Unfortunately, the strings.Title() method is clever but naive in implementation. Let’s look at the source for strings.Title() (this is from the 1.4.1 version of Go):

// Title returns a copy of the string s with all Unicode letters that 
// begin words mapped to their title case.
//
// BUG: The rule Title uses for word boundaries does not handle Unicode
// punctuation properly.
func Title(s string) string {
	// Use a closure here to remember state.
	// Hackish but effective. Depends on Map scanning in order and calling
	// the closure once per rune.
	prev := ' '
	return Map(
		func(r rune) rune {
			if isSeparator(prev) {
				prev = r
				return unicode.ToTitle(r)
			}
			prev = r
			return r
		},
		s)
}

strings.Map() iterates over each character of a string, replacing it with whatever character is returned from the func(r rune) rune method supplied by the caller. strings.Title() makes use of this in a clever way, by recording the current character as prev, so that the next run can tell whether it’s at a word boundary.

The flaw in this implementation is that it doesn’t account for small words. For example:

package main

import (
	"fmt"
	"strings"
)

func main() {
	fmt.Println(strings.Title("welcome to the dollhouse!"))
}

$ go run test_simple.go
Welcome To The Dollhouse!

Welcome to the Dollhouse!

Our English professors wouldn’t be happy with the results. Small words, such as “a” and “the” shouldn’t be capitalized in a title. In order to properly capitalize a string, we have to do a bit more work.

package main

import (
	"fmt"
	"strings"
)

func properTitle(input string) string {
	words := strings.Fields(input)
	smallwords := " a an on the to "

	for index, word := range words {
		if strings.Contains(smallwords, " "+word+" ") {
			words[index] = word
		} else {
			words[index] = strings.Title(word)
		}
	}
	return strings.Join(words, " ")
}

func main() {
	fmt.Println(properTitle("welcome to the dollhouse!"))
}

$ go run test_proper.go
Welcome to the Dollhouse!

In the implementation above, we first split the string into an array of words, loop through them, and only capitalize those who don’t match our hard-coded list of small words. Finally, we join the words up into a single string, and return the result.

Go doesn’t have anything like an array.Includes(element) predicate, so we get a bit clever when determining if the word is a small word by making use of spaces as word boundaries.

As you can see in the output above, the printed title is grammatically correct.