# Reversing order of \`prod(itr)\`
**URL:** https://discourse.julialang.org/t/reversing-order-of-prod-itr/51613
**Category:** General Usage
**Created:** [December 10, 2020, 2:42pm UTC](https://discourse.julialang.org/t/reversing-order-of-prod-itr/51613 "2020-12-10T14:42:31Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@jlbosse](https://discourse.julialang.org/u/jlbosse)
#### Post date: [December 10, 2020, 2:42pm UTC](https://discourse.julialang.org/t/reversing-order-of-prod-itr/51613/1 "2020-12-10T14:42:31Z")
Is there a way to make the product built by `prod(itr)` from right to left instead from left to right? So I want
```julia
julia> prod("$i" for i in [1,2,3]) == "123"
```
instead of
```julia
julia> prod("$i" for i in [1,2,3]) == "321"
```
?
Note that in my case [using `Iterators.reverse`](https://discourse.julialang.org/t/is-there-a-version-of-enumerate-in-the-reverse-order/38374) does not work, because I get `MethodError: no method matching iterate(::Base.Iterators.Reverse{Dict{Int64,String}}`
---
### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [December 10, 2020, 2:52pm UTC](https://discourse.julialang.org/t/reversing-order-of-prod-itr/51613/2 "2020-12-10T14:52:49Z")
What collection are you actually trying to `prod` over? `reverse` on the input works fine for me:
```julia
julia> prod("$i" for i in Iterators.reverse([1,2,3]))
"321"
```
as does regular `reverse`:
```julia
julia> reverse(prod("$i" for i in [1,2,3]))
"321"
```
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [December 10, 2020, 2:53pm UTC](https://discourse.julialang.org/t/reversing-order-of-prod-itr/51613/3 "2020-12-10T14:53:27Z")
> [@jlbosse](#):
>
> Note that in my case [using `Iterators.reverse`](https://discourse.julialang.org/t/is-there-a-version-of-enumerate-in-the-reverse-order/38374) does not work, because I get `MethodError: no method matching iterate(::Base.Iterators.Reverse{Dict{Int64,String}}`
If you are using a dictionary, the iteration order is undefined and may change from release to release. So I think you need a different data structure if you want a specific order.
---
### Author:  [@jlbosse](https://discourse.julialang.org/u/jlbosse)
#### Post date: [December 10, 2020, 3:38pm UTC](https://discourse.julialang.org/t/reversing-order-of-prod-itr/51613/4 "2020-12-10T15:38:19Z")
I guess I am out of luck then and need to do
```julia
str = ""
for i in [1,2,3]
str = str * "$i"
end
```
because my iterator (a dict) happens to be ordered the way I want it. And since it is only for string representation this order is not a must, but more convenient to read.
---
### Author:  [@jlbosse](https://discourse.julialang.org/u/jlbosse)
#### Post date: [December 10, 2020, 3:40pm UTC](https://discourse.julialang.org/t/reversing-order-of-prod-itr/51613/5 "2020-12-10T15:40:07Z")
Over `pair(Dict(Int => String))` and because of the way I create it, it usually happens to be iterated over in the “correct” order.
---
### Author:  [@FPGro](https://discourse.julialang.org/u/FPGro)
#### Post date: [December 10, 2020, 4:01pm UTC](https://discourse.julialang.org/t/reversing-order-of-prod-itr/51613/6 "2020-12-10T16:01:00Z")
Well you can `reverse(collect(mydict))` if efficiency is not a concern and if reverse unspecified order looks better than unspecified order
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [December 10, 2020, 7:59pm UTC](https://discourse.julialang.org/t/reversing-order-of-prod-itr/51613/7 "2020-12-10T19:59:42Z")
> [@jlbosse](#):
>
> because my iterator (a dict) happens to be ordered the way I want it.
You realize that you can’t rely on this? The ordering of a `Dict` may change unpredictably as you add elements, or if you update Julia to a new version. Don’t use `Dict` if you want a specific iteration order; use something else like the [`OrderedDict` type in the OrderedCollections.jl package](https://github.com/JuliaCollections/OrderedCollections.jl).
> [@jlbosse](#):
>
> ```julia
> str = ""
> for i in [1,2,3]
> str = str * "$i"
> end
>
> ```
Don’t accumulate strings this way — every iteration of your loop allocates and copies into a new string. See this discussion: [Accumulating strings dynamically - #3 by stevengj](https://discourse.julialang.org/t/accumulating-strings-dynamically/51209/3)
---
### Author:  [@jlbosse](https://discourse.julialang.org/u/jlbosse)
#### Post date: [December 11, 2020, 1:23pm UTC](https://discourse.julialang.org/t/reversing-order-of-prod-itr/51613/8 "2020-12-11T13:23:27Z")
Thanks for pointing out the thread about string accumulation! About the order: As @FPGro said the reverse, unspecified order looks better than the unspecified order, but the exact order is no hard constraint. Just a “nice to have”