# Extending Float64 Base.show

**URL:** <https://discourse.julialang.org/t/extending-float64-base-show/11265>\
**Category:** General Usage\
**Created:** [May 30, 2018, 3:18pm UTC](https://discourse.julialang.org/t/extending-float64-base-show/11265 "2018-05-30T15:18:31Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![iwelch](https://avatars.discourse-cdn.com/v4/letter/i/8c91f0/32.png) [@iwelch](https://discourse.julialang.org/u/iwelch)\
**Post date:** [May 30, 2018, 3:18pm UTC](https://discourse.julialang.org/t/extending-float64-base-show/11265/1 "2018-05-30T15:18:31Z")

</div>

Dear julia experts—I would like to print very small numbers in terms of eps(Float64). Something like

```julia
julia> Base.show(io::IO, p::Float64) =
          print(io, (abs(p)<1000*eps(Float64)) ? sprintf("%.3feps", (p/eps(Float64))) : p)

```

This runs into a “minor” difficulty, of course. I have overridden the original Base.show. I need to rescue it first.

An even more minor issue is that I would like to have it work later under 0.7 and 1.0, too. will it still work the same?

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<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:** [May 30, 2018, 3:40pm UTC](https://discourse.julialang.org/t/extending-float64-base-show/11265/2 "2018-05-30T15:40:42Z")

</div>

Just define your own `printsmall(x)` function; you generally shouldn’t overwrite `Base` methods on Base types.

Note that `print` by default calls `show`, so writing `show(x)` in terms of `print(x)` as you have done here will probably lead to a dispatch loop.

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<div class="post-metadata">

**Author:** ![iwelch](https://avatars.discourse-cdn.com/v4/letter/i/8c91f0/32.png) [@iwelch](https://discourse.julialang.org/u/iwelch)\
**Post date:** [May 30, 2018, 3:43pm UTC](https://discourse.julialang.org/t/extending-float64-base-show/11265/3 "2018-05-30T15:43:05Z")

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hi steven: yes, I know that the final term does cause the fatal loop. this is what I need to avoid.

but I also want to do this, because there is an advantage for overriding `Base.show()`. Even if Float64 then appear in vectors or structs, they still display how I want them to display.

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<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:** [May 30, 2018, 3:50pm UTC](https://discourse.julialang.org/t/extending-float64-base-show/11265/4 "2018-05-30T15:50:28Z")

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Realize that by changing `Float64` display your code may not be composable with other code that relies on the Base behavior, which is a big drawback.

One way to change the display of Base types without composability problems (i.e., without type piracy) would be to define your own `struct MyIO <: IO; io::IO; end` type wrapping another `io` stream, and then define your own `Base.show(io::MyIO, x)` methods, falling back to `Base.show(io::MyIO, x) = show(io.io, x)`. You can even redirect `stdout` to go to a `MyIO` stream.

That being said, if you _really_ want to do this you need to look at what `Base.show(io, x::Float64)` normally calls. Currently, it is a method on `Union{Float32,Float64}`, so by defining your own `show(io, ::Float64)` you are just providing a more specific method, not overwriting the existing method. Therefore, you can call the old method by using `invoke`.

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<div class="post-metadata">

**Author:** ![iwelch](https://avatars.discourse-cdn.com/v4/letter/i/8c91f0/32.png) [@iwelch](https://discourse.julialang.org/u/iwelch)\
**Post date:** [May 31, 2018, 1:08am UTC](https://discourse.julialang.org/t/extending-float64-base-show/11265/5 "2018-05-31T01:08:33Z")

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in case someone else ever needs this, the file is `.../julia/base/grisu/grisu.jl`

```julia
function Base.show(io::IO, x::Union{Float64,Float32})
    if get(io, :compact, false)
        _show(io, x, PRECISION, 6, true, true)
    else
        _show(io, x, SHORTEST, 0, true, false)
    end
end

```