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?

Answer 1

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}

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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