# Is there a way to create a comprehensive set of conditional combinations
**URL:** https://discourse.julialang.org/t/is-there-a-way-to-create-a-comprehensive-set-of-conditional-combinations/85368
**Category:** General Usage
**Tags:** question
**Created:** [August 5, 2022, 8:00pm UTC](https://discourse.julialang.org/t/is-there-a-way-to-create-a-comprehensive-set-of-conditional-combinations/85368 "2022-08-05T20:00:46Z")
**Posts on this page:** 12
**Page:** 1
### Author:  [@phantom](https://discourse.julialang.org/u/phantom)
#### Post date: [August 5, 2022, 8:00pm UTC](https://discourse.julialang.org/t/is-there-a-way-to-create-a-comprehensive-set-of-conditional-combinations/85368/1 "2022-08-05T20:00:46Z")
Hi I was just wondering if there was a way to generate a combinatorial collection subject to some condition. For example using the Combinatorics package If I can generate all combinations of some vector
```julia
collect(combinations(1:6,2))
15-element Vector{Vector{Int64}}:
[1, 2]
[1, 3]
[1, 4]
[1, 5]
[1, 6]
[2, 3]
[2, 4]
[2, 5]
[2, 6]
[3, 4]
[3, 5]
[3, 6]
[4, 5]
[4, 6]
[5, 6]
```
However what if I only want the combinations where a certain condition is met, e.g. the second number minus the first is greater than one or the sum of the selected set is equal to some value. Is there a way to input these constraints as a parameter into combinations?
---
### Author:  [@jbytecode](https://discourse.julialang.org/u/jbytecode)
#### Post date: [August 5, 2022, 8:03pm UTC](https://discourse.julialang.org/t/is-there-a-way-to-create-a-comprehensive-set-of-conditional-combinations/85368/2 "2022-08-05T20:03:23Z")
how about using something like
`filter(x -> (x[2] - x[1])>0 || sum(x) == value, collect(combinations(1:6,2)))`
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### Author:  [@Jeff\_Emanuel](https://discourse.julialang.org/u/Jeff_Emanuel)
#### Post date: [August 5, 2022, 8:04pm UTC](https://discourse.julialang.org/t/is-there-a-way-to-create-a-comprehensive-set-of-conditional-combinations/85368/3 "2022-08-05T20:04:22Z")
You should drop the `collect` to avoid allocating memory to store the discarded combos. If you must, add the `collect` outside `filter`.
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### Author:  [@jbytecode](https://discourse.julialang.org/u/jbytecode)
#### Post date: [August 5, 2022, 8:05pm UTC](https://discourse.julialang.org/t/is-there-a-way-to-create-a-comprehensive-set-of-conditional-combinations/85368/4 "2022-08-05T20:05:24Z")
> [@jbytecode](#):
>
> filter(x → (x[2] - x[1])\>0 || sum(x) == value, collect(combinations(1:6,2)))
oh sorry, just copy-pasted!
note: filter() does not work without using collect() in this case without using `collect`.
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### Author:  [@Jeff\_Emanuel](https://discourse.julialang.org/u/Jeff_Emanuel)
#### Post date: [August 5, 2022, 8:11pm UTC](https://discourse.julialang.org/t/is-there-a-way-to-create-a-comprehensive-set-of-conditional-combinations/85368/5 "2022-08-05T20:11:14Z")
It appears that you need `Iterators.filter` if the second argument is an iterator.
`Iterators.filter(x → (x[2] - x[1])>0 || sum(x) == value, combinations(1:6,2))`
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### Author:  [@jbytecode](https://discourse.julialang.org/u/jbytecode)
#### Post date: [August 5, 2022, 8:17pm UTC](https://discourse.julialang.org/t/is-there-a-way-to-create-a-comprehensive-set-of-conditional-combinations/85368/6 "2022-08-05T20:17:47Z")
yes this is a more memory friendly version just because it consumes memory when needed only.
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### Author:  [@phantom](https://discourse.julialang.org/u/phantom)
#### Post date: [August 5, 2022, 8:50pm UTC](https://discourse.julialang.org/t/is-there-a-way-to-create-a-comprehensive-set-of-conditional-combinations/85368/7 "2022-08-05T20:50:48Z")
> [@Jeff\_Emanuel](#):
>
> Iterators.filter(x → (x[2] - x[1])\>0 || sum(x) == value, combinations(1:6,2))
thanks guys! this is really helpful. When I try to run it with Iterators.filter without collect rather than the actual combinations I get this. `Iterators.filter(x -> (x[2] - x[1])>0 ||sum(x) == 6 ,combinations(1:6,2)) Base.Iterators.Filter{var"#15#16", Base.Generator{Combinatorics.Combinations, Combinatorics.var"#10#13"{Combinatorics.var"#reorder#11"{UnitRange{Int64}}}}}(var"#15#16"(), Base.Generator{Combinatorics.Combinations, Combinatorics.var"#10#13"{Combinatorics.var"#reorder#11"{UnitRange{Int64}}}}(Combinatorics.var"#10#13"{Combinatorics.var"#reorder#11"{UnitRange{Int64}}}(Combinatorics.var"#reorder#11"{UnitRange{Int64}}(1:6)), Combinatorics.Combinations(6, 2)))`
Is this normal?
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### Author:  [@jbytecode](https://discourse.julialang.org/u/jbytecode)
#### Post date: [August 5, 2022, 8:54pm UTC](https://discourse.julialang.org/t/is-there-a-way-to-create-a-comprehensive-set-of-conditional-combinations/85368/8 "2022-08-05T20:54:50Z")
> [@Jeff\_Emanuel](#):
>
> Iterators.filter(x → (x[2] - x[1])\>0 || sum(x) == value, combinations(1:6,2))
how about
`Iterators.filter(x -> (x[2] - x[1])>0 || sum(x) == value, combinations(1:6,2)) |> collect`
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### Author:  [@phantom](https://discourse.julialang.org/u/phantom)
#### Post date: [August 5, 2022, 9:05pm UTC](https://discourse.julialang.org/t/is-there-a-way-to-create-a-comprehensive-set-of-conditional-combinations/85368/9 "2022-08-05T21:05:18Z")
awesome thanks! quick question what is the piping` |>` operator doing and how does that differ from your initial code ?
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### Author:  [@jbytecode](https://discourse.julialang.org/u/jbytecode)
#### Post date: [August 5, 2022, 9:09pm UTC](https://discourse.julialang.org/t/is-there-a-way-to-create-a-comprehensive-set-of-conditional-combinations/85368/10 "2022-08-05T21:09:05Z")
Yes it only collects the result returned by the filtered one.
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### Author:  [@phantom](https://discourse.julialang.org/u/phantom)
#### Post date: [August 5, 2022, 9:10pm UTC](https://discourse.julialang.org/t/is-there-a-way-to-create-a-comprehensive-set-of-conditional-combinations/85368/11 "2022-08-05T21:10:09Z")
gotcha thanks again!
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### Author:  [@jbytecode](https://discourse.julialang.org/u/jbytecode)
#### Post date: [August 5, 2022, 9:11pm UTC](https://discourse.julialang.org/t/is-there-a-way-to-create-a-comprehensive-set-of-conditional-combinations/85368/12 "2022-08-05T21:11:09Z")
Your welcome!