The letter `A` is easy to store and read. Text gets trickier when it includes characters from many languages or symbols such as emoji. Unicode assigns each character a number called a code point, so the character has an identity independent of how it is stored. section 11.0 section 11.1.1
A code point is not the same as a code unit. A code point identifies a character; code units store it. Julia strings use UTF-8, which takes a variable number of code units per code point. ASCII characters fit in one byte, while other characters can take more. That distinction matters when examining storage or moving through a string. section 11.1.1 section 11.5
Check a character in Julia
At the REPL, `Int(codepoint('A'))` returns `65`, while `ncodeunits('A')` returns `1`. For `'Æ'`, `codepoint('Æ')` returns `0x000000c6` and `ncodeunits('Æ')` returns `2`. The first call asks which character it is; the second asks how much UTF-8 storage it needs. section 11.1.1
julia> codepoint('Æ')
0x000000c6
julia> ncodeunits('Æ') #1
2
julia> isascii('Æ') #2
false
julia> codepoint('😏') #3
0x0001f60f
julia> ncodeunits('😏') #4
4
julia> isascii('😏')
false
Compare `A`, `Æ`, and an emoji: their code points identify them, but their UTF-8 code-unit counts differ. section 11.1.1
Listing 11.1 Iterating over characters in a string using a while loop
julia> s = "Þrúðvangar"
julia> i = firstindex(s)
1
julia> while i <= lastindex(s)
println((i, s[i]))
i = nextind(s, i)
end
(1, 'Þ')
(3, 'r') #1
(4, 'ú')
(6, 'ð') #2
(8, 'v')
(9, 'a')
(10, 'n')
(11, 'g')
(12, 'a')
(13, 'r')
The multilingual string’s printed indices skip positions because moving to the next character can cross multiple code units. section 11.1.1
