Arithmetic with Chars in Julia

Question

Julia REPL tells me that the output of

'c'+2

is 'e': ASCII/Unicode U+0065 (category Ll: Letter, lowercase) but that the output of

'c'+2-'a'

is 4.

I'm fine with the fact that Chars are identified as numbers via their ASCII code. But I'm confused about the type inference here: why is the first output a char and the second an integer?

Answer 1

The reason is:

julia> @which 'a' - 1
-(x::T, y::Integer) where T<:AbstractChar in Base at char.jl:227

julia> @which 'a' - 'b'
-(x::AbstractChar, y::AbstractChar) in Base at char.jl:226

Subtraction of Char and integer is Char. This is e.g. 'a' - 1.

However, subtraction of two Char is integer. This is e.g. 'a' - 'b'.

Note that for Char and integer both addition and subtraction are defined, but for two Char only subtraction works:

julia> 'a' + 'a'
ERROR: MethodError: no method matching +(::Char, ::Char)

This indeed can lead to tricky cases at times that rely of order of operations, as in this example:

julia> 'a' + ('a' - 'a')
'a': ASCII/Unicode U+0061 (category Ll: Letter, lowercase)

julia> 'a' + 'a' - 'a'
ERROR: MethodError: no method matching +(::Char, ::Char)

Also note that when working with Char and integer you cannot subtract Char from integer:

julia> 2 - 'a'
ERROR: MethodError: no method matching -(::Int64, ::Char)

Motivation:

• subtraction of two char - this is sometimes useful if you want to get a relative position of a char with reference to other char, e.g. c - '0' to convert char to its decimal representation if you know char is a digit;
• adding or subtracting an integer and char - the same but in reverse e.g. you want to convert digit to char and write '0' + d.

I have been using Julia for years now, and I used this feature maybe once or twice, so I would not say it is super commonly needed.