# Subset array conditional on boolean

**URL:** <https://discourse.julialang.org/t/subset-array-conditional-on-boolean/14671>\
**Category:** New to Julia\
**Created:** [September 7, 2018, 5:06pm UTC](https://discourse.julialang.org/t/subset-array-conditional-on-boolean/14671 "2018-09-07T17:06:59Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![cosmia](https://avatars.discourse-cdn.com/v4/letter/c/9de0a6/32.png) [@cosmia](https://discourse.julialang.org/u/cosmia)\
**Post date:** [September 7, 2018, 5:06pm UTC](https://discourse.julialang.org/t/subset-array-conditional-on-boolean/14671/1 "2018-09-07T17:06:59Z")

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Hi, I am trying to subset a 2d array conditional on a boolean.

I can do it using filter and with 1d arrays like:

```julia
X = randn(100);
filter(a->a>=0,X)

```

However, suppose I have something like:

```julia
X = randn(100);
Y = 3 .* randn(100) .+ 2;
S = X .>= 0; 
data = [X Y S];

```

And I want to create two new arrays called `data0` and `data1` where the first contains X, Y, S when S == 0, and the latter when S == 1. `filter` doesn’t seem to work. What is the best way to do it?

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**Author:** ![JonasIsensee](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jonasisensee/32/4704_2.png) [@JonasIsensee](https://discourse.julialang.org/u/JonasIsensee)\
**Post date:** [September 7, 2018, 6:18pm UTC](https://discourse.julialang.org/t/subset-array-conditional-on-boolean/14671/2 "2018-09-07T18:18:35Z")

</div>

Hi,  
I’m not sure why you would want to save `S` into your new data matrix, since it will be all `1` or `0`  
but one thing you could do is the following:

```julia
X = randn(100);
Y = 3 .* randn(100) .+ 2;
S = X .>= 0; 
X_greater_0 = X[S]
X_smaller_0 = X[.!S]

data0 = [X_greater_0 Y[ S] ones(100) ]   
data1 = [X_smaller_0 Y[.!S] zeros(100)]

```

The relevant entry in the docs is [Logical Indexing](https://docs.julialang.org/en/v1/manual/arrays/#Logical-indexing-1).

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<div class="post-metadata">

**Author:** ![cosmia](https://avatars.discourse-cdn.com/v4/letter/c/9de0a6/32.png) [@cosmia](https://discourse.julialang.org/u/cosmia)\
**Post date:** [September 7, 2018, 7:46pm UTC](https://discourse.julialang.org/t/subset-array-conditional-on-boolean/14671/3 "2018-09-07T19:46:27Z")

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Thanks Jnaslsensee. I still have some questions though. I implemented your code as

```julia
X = randn(100);
Y = 3 .* randn(100) .+ 2;
S = X .>= 0; 

data0 = [X[ S] Y[S] ]   
data1 = [X[ .!S] Y[.!S] ]

```

I don’t really need S to be in the data matrix, I just put it there. However, I tried doing something like `data0 = data[S]` and it didn’t work. What if I have an arbitrarily large number of 1d arrays that I want to subset according to one boolean array, is there any better way to do this rather than `data0 = [A[S] (…) Z[S]]?

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<div class="post-metadata">

**Author:** ![JonasIsensee](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jonasisensee/32/4704_2.png) [@JonasIsensee](https://discourse.julialang.org/u/JonasIsensee)\
**Post date:** [September 7, 2018, 7:54pm UTC](https://discourse.julialang.org/t/subset-array-conditional-on-boolean/14671/4 "2018-09-07T19:54:05Z")

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You could also do the following:

```julia
data = [X Y]
data0 = data[S , :]
data1 = data[.!S, :]

```

This first copies your `X` and `Y` into a `data` matrix.  
Then you use the logical indexing ( `S` ) for the first index and `:` for the second index, meaning _take all_.

This introduces a little more copying of data but as long as your code is not performance critical, it shouldn’t be a problem.