# Cast \`Real\` to the closest representable value by default (or excess)

**URL:** <https://discourse.julialang.org/t/cast-real-to-the-closest-representable-value-by-default-or-excess/82811>\
**Category:** General Usage\
**Tags:** numbers\
**Created:** [June 15, 2022, 1:32pm UTC](https://discourse.julialang.org/t/cast-real-to-the-closest-representable-value-by-default-or-excess/82811 "2022-06-15T13:32:36Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![etienne\_dg](https://avatars.discourse-cdn.com/v4/letter/e/fbc32d/32.png) [@etienne\_dg](https://discourse.julialang.org/u/etienne_dg)\
**Post date:** [June 15, 2022, 1:32pm UTC](https://discourse.julialang.org/t/cast-real-to-the-closest-representable-value-by-default-or-excess/82811/1 "2022-06-15T13:32:36Z")

</div>

I want to get the closest representation by default of a value `x` of type `U<:Real` by a value `y` of type `T<:Real`.

If `T<:Integer`, I can do `y = floor(T, x)`  
If `T` is a Float, I can do `T(x)`.

But none of these methods cover both `Integer` and `Float`. Is it safe to assume that if `T` is not an `Integer`, I will not get an `InexactError`? Is there a clean way to do that ?

For context, this question arise du to [this issue](https://github.com/JuliaGraphs/Graphs.jl/issues/143). I have a function with as input a matrix `distmx` of type `Int64` representing the distance matrix of a graph, and a function `f` taking as input two vertices `s` and `t` and returning a `Float`. `f` is assumed to return a heuristic value always greater than the length of the shortest path between `s` and `t`.  
Generally, the type of the matrix and the type of `f` will be identical, but it is not to crazy to see this type of scenario: For example, the distances on the graph are known to be integer, but the heuristic is based on the computation of an euclidean distance, giving `Float64`.  
Of course, the user can provide a function returning the truncation of the output of `f` if he knows that his distance matrix is Integer (He can also cast the matrix to Float, though it is a bit less satisfying in terms of performance), but I think it seems reasonable to assume that he can provide these inputs ?

The problem arise when I store the values of the heuristic in a PriorityQueue. Here, we initialize a `PriorityQueue` to hold values of the `eltype` of `distmx` (so `Int64` here), but it returns `InexactError` when inputting a `Float`. A first solution would be to answer the question of the post, another would be to allows Float64, but that would be limitating if the input types are both `BigFloat`, and not satisfying in terms of performance if input types are `Int64`.

Maybe I can gather the output type of `f` by taking a random output or using Base.return\_types (and assuming `f` is type stable), but it does not seems really clean…

---

<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [June 15, 2022, 4:21pm UTC](https://discourse.julialang.org/t/cast-real-to-the-closest-representable-value-by-default-or-excess/82811/2 "2022-06-15T16:21:43Z")

</div>

> [@etienne\_dg](#):
>
> The problem arise when I store the values of the heuristic in a PriorityQueue.

Can’t you create the `PriorityQueue` lazily, when you need to actually evaluate the heuristic? Then you can use the actual return value to determine the required type.

---

<div class="post-metadata">

**Author:** ![Keno](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/keno/32/285_2.png) [@Keno](https://discourse.julialang.org/u/Keno)\
**Post date:** [June 15, 2022, 5:37pm UTC](https://discourse.julialang.org/t/cast-real-to-the-closest-representable-value-by-default-or-excess/82811/3 "2022-06-15T17:37:53Z")

</div>

It seems reasonable to me to extend floor(T, x) to floating point types where x is higher precision than T.

---

<div class="post-metadata">

**Author:** ![etienne\_dg](https://avatars.discourse-cdn.com/v4/letter/e/fbc32d/32.png) [@etienne\_dg](https://discourse.julialang.org/u/etienne_dg)\
**Post date:** [June 16, 2022, 7:33am UTC](https://discourse.julialang.org/t/cast-real-to-the-closest-representable-value-by-default-or-excess/82811/4 "2022-06-16T07:33:35Z")

</div>

Yes, this was one of my thoughts, I find it not very clean to get a type by analyzing the output of `f`, but maybe it is the most reasonable solution.