parse input to rational in Julia

Question

I want to read the user input and store it as a Rational, whatever the type: integer, float ot rational. For instance:

5 --> store it as 5//1
2.3 --> store it as 23//10
4//7 --> store it as 4//7

At the moment I wrote the following:

a = convert(Rational,parse(Float64,readline(STDIN)))

which is fine if I input an integer, like 5.

But if I input 2.3, a stores 2589569785738035//1125899906842624.

And if I input a fraction (whether in the form 4/7 or the form 4//7) I get an ArgumentError: invalid number format for Float64.

How to solve the Float&Rational problems?

This *should* be available by ``parse(Rational{Int},string)``, but currently this is not implemented. — Moritz Schauer, Feb 15 '16 at 21:18
@mschauer Yeah, I tried but didn't work. Will it be so in 0.5? — Pigna, Feb 15 '16 at 21:38

Gomiero · Accepted Answer · 2020-11-29T20:53:10.203

One way is to parse the raw input to an Expr (symbols), eval the expression, convert it to a Float64 and use rationalize to simplify the rational generated:

julia> rationalize(convert(Float64, eval(parse("5"))))
5//1

julia> rationalize(convert(Float64, eval(parse("2.3"))))
23//10

julia> rationalize(convert(Float64, eval(parse("4/7"))))
4//7

julia> rationalize(convert(Float64, eval(parse("4//7"))))
4//7

rationalize works with approximate floating point number and you could specify the error in the parameter tol.

_{tested with Julia Version 0.4.3}

Update: The parse method was deprecated in Julia version >= 1.0. There are two methods that should be used: Base.parse (just for numbers, and it requires a Type argument) and Meta.parse (for expressions):

julia> rationalize(convert(Float64, eval(parse(Int64, "5"))))
5//1

julia> rationalize(convert(Float64, eval(parse(Float64, "2.3"))))
23//10

score 0 · Answer 2 · answered Feb 17 '16 at 04:58

Multiplication (then division) by a highly composite integer works pretty well.

julia> N = 2*2 * 3*3 * 5*5 * 7 * 11 * 13
900900

julia> a = round(Int, N * parse(Float64, "2.3")) // N
23//10

julia> a = round(Int, N * parse(Float64, "5")) // N
5//1

julia> a = round(Int, N * parse(Float64, "9.1111111111")) // N
82//9

score 0 · Answer 3 · answered Sep 01 '16 at 19:20

You could implement your own parse:

function Base.parse(::Type{Rational{Int}}, x::ASCIIString)
    ms, ns = split(x, '/', keep=false)
    m = parse(Int, ms)
    n = parse(Int, ns)
    return m//n
end

Base.parse(::Type{Rational}, x::ASCIIString) = parse(Rational{Int}, x)

parse input to rational in Julia

3 Answers3