# JSON3 - How write(serialize) the specified number of significant digits?

**URL:** <https://discourse.julialang.org/t/json3-how-write-serialize-the-specified-number-of-significant-digits/47633>\
**Category:** General Usage\
**Created:** [October 2, 2020, 11:20am UTC](https://discourse.julialang.org/t/json3-how-write-serialize-the-specified-number-of-significant-digits/47633 "2020-10-02T11:20:55Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Martin\_Florek](https://avatars.discourse-cdn.com/v4/letter/m/f4b2a3/32.png) [@Martin\_Florek](https://discourse.julialang.org/u/Martin_Florek)\
**Post date:** [October 2, 2020, 11:20am UTC](https://discourse.julialang.org/t/json3-how-write-serialize-the-specified-number-of-significant-digits/47633/1 "2020-10-02T11:20:55Z")

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Hi all,  
I would like write short form number of digits. e.g as `round(x; sigdigits=10)`  
I parse vector of float with help [JSON3](https://quinnj.github.io/JSON3.jl/stable/) to json string and vice versa.  
Default `Float64` is NumberType `JSON3.StructType(::Type{<:Float64}) = JSON3.NumberType()`  
Any suggestions? Thx

NOTE: This is just part of the structure.

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**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [October 2, 2020, 2:14pm UTC](https://discourse.julialang.org/t/json3-how-write-serialize-the-specified-number-of-significant-digits/47633/2 "2020-10-02T14:14:04Z")

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You could round the values in place. It’s a little bit strange to have an option to change what values are serialize. Like if there was an option to apply a transform to each key before writing it out. If you’re going to do that, just transform the keys and then save the JSON.

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**Author:** ![Palli](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/palli/32/3380_2.png) [@Palli](https://discourse.julialang.org/u/Palli)\
**Post date:** [October 2, 2020, 2:38pm UTC](https://discourse.julialang.org/t/json3-how-write-serialize-the-specified-number-of-significant-digits/47633/3 "2020-10-02T14:38:41Z")

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> [@Martin\_Florek](#):
>
> I would like write short form number of digits. e.g as `round(x; sigdigits=10)`

I’m not sure you should get into the habit of doing that, or with digits=2 (what I’m guessing is what you rather want). [For numerical applications you want all the accuracy of your calculations saved.]

Is your problem that you should rather be using:

[GitHub - JuliaMath/DecFP.jl: Julia IEEE decimal floating-point via the Intel decimal-float library](https://github.com/JuliaMath/DecFP.jl) (there’s also a slower Julia-only decimal float format library). I.e. [decimal64 floating-point format - Wikipedia](https://en.wikipedia.org/wiki/Decimal64_floating-point_format)

thus avoiding problems you see in the docs:

```julia
  If the digits keyword argument is provided, it rounds to the specified number of digits after the decimal place (or before if negative), in base base.

  If the sigdigits keyword argument is provided, it rounds to the specified number of significant digits, in base base.
[..]
  │ Rounding to specified digits in bases other than 2 can be inexact when operating on binary floating point numbers. For example, the Float64 value
  │ represented by 1.15 is actually less than 1.15, yet will be rounded to 1.2.
  │
  │ Examples
  │ ≡≡≡≡≡≡≡≡≡≡
  │
  │ x = 1.15
  │ 1.15
  │  
  │ @sprintf "%.20f" x
  │ "1.14999999999999991118"
  │  
  │ x < 115//100
  │ true

```

Some trivia, Douglas Crockford who popularized the data format JSON format proposed one other format (Julia doesn’t support, but no problem as it never caught on)

> **[GitHub - douglascrockford/DEC64: Decimal floating point](https://github.com/douglascrockford/DEC64)**
>
> Decimal floating point. Contribute to douglascrockford/DEC64 development by creating an account on GitHub.

[http://blog.aventine.se/2014/03/09/a-silly-review-of-dec64.html](http://blog.aventine.se/2014/03/09/a-silly-review-of-dec64.html)

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<div class="post-metadata">

**Author:** ![Martin\_Florek](https://avatars.discourse-cdn.com/v4/letter/m/f4b2a3/32.png) [@Martin\_Florek](https://discourse.julialang.org/u/Martin_Florek)\
**Post date:** [October 2, 2020, 8:17pm UTC](https://discourse.julialang.org/t/json3-how-write-serialize-the-specified-number-of-significant-digits/47633/4 "2020-10-02T20:17:00Z")

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Thanks, guys @StefanKarpinski & @Palli for a quick response and recommendations. I found a mistake with constructor `obj[:w] ./ sum(obj[:w])` where GMMmodel expects exactly 1.0 and the SHA256 signature did not fit. Sorry 😃 I also applied round in the constructor of a struct.

However, I have a new problem with valid writing a vector or value using serialization (method `JSON3.write`). I would expect to write the vector as a valid JSON format (and a simple format for other parsers). For e.g vector `Foo([2.3, 4.5, NaN], NaN)` as

> { “a”: [2.3, 4.5, null], “b”: null} #This is expected 🙂

As I mentioned in the [link](https://discourse.julialang.org/t/ann-json3-jl-1-0-release-new-structtypes-jl-package/34700/2). Some suggestions for (de)serialize?

I tried to choose a number as a string.

> JSON3.StructType(::Type{\<:Float64}) = JSON3.StringType()

but I’ll just get only this `{"a": ["2.3", "4.5", "null"], "b": "null"}` ☹ duplicate quotes 🙄