# Mutate a Dates.Date

**URL:** <https://discourse.julialang.org/t/mutate-a-dates-date/97936>\
**Category:** New to Julia\
**Created:** [April 26, 2023, 9:07am UTC](https://discourse.julialang.org/t/mutate-a-dates-date/97936 "2023-04-26T09:07:48Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![nsslh](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nsslh/32/27447_2.png) [@nsslh](https://discourse.julialang.org/u/nsslh)\
**Post date:** [April 26, 2023, 9:07am UTC](https://discourse.julialang.org/t/mutate-a-dates-date/97936/1 "2023-04-26T09:07:48Z")

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Is there a way to mutate a Dates.Date without allocation, such that I don’t put pressure on the garbage collector?

I am reusing an array of structs to reduce GC pressure in an RPC service. I would like to mutate the dates in these structs. I can’t find a function for this in the Dates package. I woiuld prefer to not have to resort to unpacked integers for year, month, day.

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**Author:** ![Sukera](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@Sukera](https://discourse.julialang.org/u/Sukera)\
**Post date:** [April 26, 2023, 9:19am UTC](https://discourse.julialang.org/t/mutate-a-dates-date/97936/2 "2023-04-26T09:19:53Z")

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`Date` is not mutable:

```julia
julia> using Dates

julia> ismutabletype(Date)
false

```

The allocations you’re seeing are likely from something other than the `Date` object itself. Are the structs in your arrays mutable?

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**Author:** ![jules](https://avatars.discourse-cdn.com/v4/letter/j/41988e/32.png) [@jules](https://discourse.julialang.org/u/jules)\
**Post date:** [April 26, 2023, 10:36am UTC](https://discourse.julialang.org/t/mutate-a-dates-date/97936/3 "2023-04-26T10:36:21Z")

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If you have some immutable struct that has a `Date` typed field, and you store those structs in an array, then you can replace a whole entry of this array with a new struct instance of which only the date field has been changed. For that you do not need mutation of a `Date` or your struct. Packages like [GitHub - jw3126/Setfield.jl: Update deeply nested immutable structs.](https://github.com/jw3126/Setfield.jl) or [GitHub - JuliaObjects/Accessors.jl: Update immutable data](https://github.com/JuliaObjects/Accessors.jl) make creating new instances with changed fields easier. This usually does not allocate and is faster than using mutable data structures.

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**Author:** ![nsslh](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nsslh/32/27447_2.png) [@nsslh](https://discourse.julialang.org/u/nsslh)\
**Post date:** [May 3, 2023, 8:07am UTC](https://discourse.julialang.org/t/mutate-a-dates-date/97936/4 "2023-05-03T08:07:38Z")

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Interesting packages, thanks. But it looks like that approach will still pressure the garbage collector. Whether I’m creating a new struct or a new date, I’m creating new objects that must be garbage collected.

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**Author:** ![nsslh](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nsslh/32/27447_2.png) [@nsslh](https://discourse.julialang.org/u/nsslh)\
**Post date:** [May 3, 2023, 8:09am UTC](https://discourse.julialang.org/t/mutate-a-dates-date/97936/5 "2023-05-03T08:09:39Z")

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Replacing one date in a mutable struct with another date in a mutable struct will result in an allocation – for the new Date object. I checked it out in a repl.

If the structs immutable… well then I’m just replacing one struct with another struct, which will result in an allocation – for the new Struct object.

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**Author:** ![jules](https://avatars.discourse-cdn.com/v4/letter/j/41988e/32.png) [@jules](https://discourse.julialang.org/u/jules)\
**Post date:** [May 3, 2023, 8:55am UTC](https://discourse.julialang.org/t/mutate-a-dates-date/97936/6 "2023-05-03T08:55:16Z")

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There’s a difference between stack-allocated memory and heap-allocated memory though. Stack memory will not pressure the GC. A DateTime needs memory, yes, but if you already have that memory in an array, the stack memory to instantiate the object doesn’t matter.

```julia
julia> using Setfield

julia> using Dates

julia> struct Container
           x::Float64
           d::DateTime
       end

julia> arr = [Container(rand(), now()) for _ in 1:10_000]
10000-element Vector{Container}:
 Container(0.45518384534376943, DateTime("2023-05-03T10:50:09.981"))
 Container(0.44183067111724206, DateTime("2023-05-03T10:50:09.981"))
 ⋮
 Container(0.21275325287594737, DateTime("2023-05-03T10:50:09.983"))
 Container(0.7248760274958712, DateTime("2023-05-03T10:50:09.983"))

julia> function increase_hour(container)
           @set container.d = container.d + Hour(1)
       end
increase_hour (generic function with 1 method)

julia> function increase_hours!(array)
           array .= increase_hour.(array)
       end
increase_hours! (generic function with 1 method)

julia> @time increase_hours!(arr) # compile once
  0.129312 seconds (126.60 k allocations: 8.550 MiB, 43.15% gc time, 99.95% compilation time)
10000-element Vector{Container}:
 Container(0.45518384534376943, DateTime("2023-05-03T11:50:09.981"))
 Container(0.44183067111724206, DateTime("2023-05-03T11:50:09.981"))
 ⋮
 Container(0.21275325287594737, DateTime("2023-05-03T11:50:09.983"))
 Container(0.7248760274958712, DateTime("2023-05-03T11:50:09.983"))

julia> @time increase_hours!(arr) # tada, no allocation
  0.000033 seconds
10000-element Vector{Container}:
 Container(0.45518384534376943, DateTime("2023-05-03T12:50:09.981"))
 Container(0.44183067111724206, DateTime("2023-05-03T12:50:09.981"))

```

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<div class="post-metadata">

**Author:** ![nsslh](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nsslh/32/27447_2.png) [@nsslh](https://discourse.julialang.org/u/nsslh)\
**Post date:** [May 3, 2023, 10:29am UTC](https://discourse.julialang.org/t/mutate-a-dates-date/97936/7 "2023-05-03T10:29:15Z")

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Woah. I hadn’t thought of that. Thanks, I’ll take a look!

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**Author:** ![aplavin](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/aplavin/32/222056_2.png) [@aplavin](https://discourse.julialang.org/u/aplavin)\
**Post date:** [May 4, 2023, 5:42am UTC](https://discourse.julialang.org/t/mutate-a-dates-date/97936/8 "2023-05-04T05:42:56Z")

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In addition to performance with immutable structs, `Accessors` are also very convenient — this is hard to beat even if you allow for mutability. I mean stuff like `@set year(container.d) = 2022`, involving functions in addition to properties.