# Why do Rationals promote to Floats

**URL:** <https://discourse.julialang.org/t/why-do-rationals-promote-to-floats/28526>\
**Category:** New to Julia\
**Created:** [September 8, 2019, 2:31am UTC](https://discourse.julialang.org/t/why-do-rationals-promote-to-floats/28526 "2019-09-08T02:31:07Z")\
**Posts on this page:** 1\
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**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [September 8, 2019, 6:26am UTC](https://discourse.julialang.org/t/why-do-rationals-promote-to-floats/28526/2 "2019-09-08T06:26:37Z")

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> [@thelonious](#):
>
> I’m wondering, is there an explanation or discussion about this that I can read? If I wanted to reverse this behavior (convert the Float to a Rational in this case), is that something I can override?

I think the reason is that

1. calculations with eg `Rational{Int64}` would quickly overflow for the denominator,

2. the results of transcendental functions (`exp`) cannot be a rational number in general.

So it is safest to promote to `Float64`. Of course, the trade-off is floating point inaccuracy.

Generally, I would just _not_ promote but leave it to the operation. For a more specific workaround, please provide more context.

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>
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