# Generic way to turn mutating into nonmutating functions?

**URL:** <https://discourse.julialang.org/t/generic-way-to-turn-mutating-into-nonmutating-functions/9011>\
**Category:** New to Julia\
**Created:** [February 12, 2018, 12:05pm UTC](https://discourse.julialang.org/t/generic-way-to-turn-mutating-into-nonmutating-functions/9011 "2018-02-12T12:05:51Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![under-Peter](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/under-peter/32/3626_2.png) [@under-Peter](https://discourse.julialang.org/u/under-Peter)\
**Post date:** [February 12, 2018, 12:05pm UTC](https://discourse.julialang.org/t/generic-way-to-turn-mutating-into-nonmutating-functions/9011/1 "2018-02-12T12:05:51Z")

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Hi!  
I want to use functions like `deleteat!` or `insert!` without mutating whatever they work on.  
Concretely, I have vectors where I replace two neighboring values by their sum which I thought I’d achieve with

```julia
vec = [1:10...]
indices = [3;4]
newvec = deleteat!(insert!(vec,max(indices)+1,sum(vec[indices])),indices);

```

but I still need the original vector.  
If someone can tell me how to do it more readable, I’d appreciate that too.

Anyway, I guess a solution would be to use `copy`, i.e.

```julia
newvec = copy(vec)
newvec = deleteat!(insert!(newvec,max(indices)+1,sum(newvec[indices])),indices);

```

but that makes my code a lot less readable since I’m mapping this operation over an iterator of dictionary keys so the whole thing turns into a monster like

```julia
map(x->deleteat!(insert!(x,max(indices)+1,sum(x[indices])),indices), copy(keys(somedict)))

```

which doesn’t even work because I can’t copy KeyIterators and I’d rather not collect all keys beforehand.

Any ideas? I’m a bit irritated by all the functions that aren’t available side-effect free such as `push!`, `pop!`, `insert!`, etc. but maybe I’m just missing something.

Thanks!

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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:** [February 12, 2018, 12:30pm UTC](https://discourse.julialang.org/t/generic-way-to-turn-mutating-into-nonmutating-functions/9011/2 "2018-02-12T12:30:31Z")

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What about

```julia
sum_adjacent(v, i) = vcat(v[1:(i-1)], v[i] + v[i+1], v[(i+2):end])

sum_adjacent(1:10, 3)

```

---

<div class="post-metadata">

**Author:** ![under-Peter](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/under-peter/32/3626_2.png) [@under-Peter](https://discourse.julialang.org/u/under-Peter)\
**Post date:** [February 12, 2018, 2:39pm UTC](https://discourse.julialang.org/t/generic-way-to-turn-mutating-into-nonmutating-functions/9011/3 "2018-02-12T14:39:06Z")

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Thanks, that’s way more readable.  
I still prefer the `insert`/`deleteat` method since it allows easier generalisation for non-neighboring values but for the specification I gave in my post that’s great!

This leaves open the question of whether there’s a straightforward way to get mutating functions without side-effects (maybe by some Macro that inserts `copy` or `deepcopy` automatically in the mutating functions?)

Cheers

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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:** [February 12, 2018, 2:42pm UTC](https://discourse.julialang.org/t/generic-way-to-turn-mutating-into-nonmutating-functions/9011/4 "2018-02-12T14:42:23Z")

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I am at loss to understand why you would want to start with mutating functions then turn them into non-mutating ones. I don’t see any performance gains in doing so, and it will lead to really convoluted code, instead of a simple solution. Bringing macros into it does not look necessary (from what I have seen so far).

The solution with non-mutating forms should be easy to generalize. I am very happy to help if you describe what you need.

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<div class="post-metadata">

**Author:** ![under-Peter](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/under-peter/32/3626_2.png) [@under-Peter](https://discourse.julialang.org/u/under-Peter)\
**Post date:** [February 21, 2018, 10:07am UTC](https://discourse.julialang.org/t/generic-way-to-turn-mutating-into-nonmutating-functions/9011/5 "2018-02-21T10:07:53Z")

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Dear Tamas

My situation was like this: I needed a functionality such as e.g. remove an element at a certain position and I wanted it to be non-mutating since I prefer functions without side effects. In the documentation I only found functions such as `deleteat!` or `pop!` that mutate my array.

Of course I could always just copy the array I work on first, then use those functions and have the same functionality but I thought that since that must be a quite common use-case - working on data you don’t want to modify and instead save the modified data in another variable - there might be a _julian_ way to do it.

For the problem at hand I used your suggestion above which works fine and is very readable, thanks for that!