# How to filter a tuple by the first element

**URL:** <https://discourse.julialang.org/t/how-to-filter-a-tuple-by-the-first-element/78053>\
**Category:** New to Julia\
**Created:** [March 17, 2022, 9:57pm UTC](https://discourse.julialang.org/t/how-to-filter-a-tuple-by-the-first-element/78053 "2022-03-17T21:57:57Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![lsaravia](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lsaravia/32/33532_2.png) [@lsaravia](https://discourse.julialang.org/u/lsaravia)\
**Post date:** [March 17, 2022, 9:57pm UTC](https://discourse.julialang.org/t/how-to-filter-a-tuple-by-the-first-element/78053/1 "2022-03-17T21:57:57Z")

</div>

I made a tuple with vectors and I want to filter and plot it by the first vector

```julia
function gen_data()
    a = collect(0:.1:200)
    b = rand(length(a))
    c = [rand()*i*0.001 for i in 1:length(a)]
    return a,b,c
end

d = gen_data()

typeof(d)
using Plots
plot(d[1],d[2])
plot!(d[1],d[3])

```

This is fine but if try to filter it

```julia
d1 = filter( i -> i[1]>100, d )

```

but the result is empty

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<div class="post-metadata">

**Author:** ![simonschoelly](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/simonschoelly/32/21606_2.png) [@simonschoelly](https://discourse.julialang.org/u/simonschoelly)\
**Post date:** [March 17, 2022, 10:10pm UTC](https://discourse.julialang.org/t/how-to-filter-a-tuple-by-the-first-element/78053/2 "2022-03-17T22:10:34Z")

</div>

I am not sure what you are trying to accomplish here, but it is possible, that you are making a mistake and intended to do something else.

When you are filtering `d`, you are iterating over `(d[1], d[2], d[3])`, and then checking if `d[1][1] > 100`, `d[2][1] > 100` and `d[3][1] > 100`, neither of which is true, because `d[1][1]` is `0.0`, `d[2][1]` is a number in the interval `[0, 1)` and `d[3][1]` is a number in the interval `[0, 0.001)`.

---

<div class="post-metadata">

**Author:** ![lsaravia](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lsaravia/32/33532_2.png) [@lsaravia](https://discourse.julialang.org/u/lsaravia)\
**Post date:** [March 17, 2022, 10:44pm UTC](https://discourse.julialang.org/t/how-to-filter-a-tuple-by-the-first-element/78053/3 "2022-03-17T22:44:24Z")

</div>

Imagine d[1] is time and d[2], d[3] are some kind of model, then I whish to plot the values for d[2] d[3] with time \> 100. So I would like to iterate only over d[1]. Maybe this should be done with a DataFrame.

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<div class="post-metadata">

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [March 17, 2022, 11:38pm UTC](https://discourse.julialang.org/t/how-to-filter-a-tuple-by-the-first-element/78053/4 "2022-03-17T23:38:10Z")

</div>

First you need to obtain an array marking the elements that you want, and then use that array to index the arrays (you have three independent arrays, and the fact that they are in a tuple does not help much):

```julia
julia> data = (rand(5),rand(5));

julia> inds = data[1] .> 0.5 # bool array
5-element BitVector:
 0
 1
 1
 1
 1

julia> data[2][inds]
4-element Vector{Float64}:
 0.3156756681271574
 0.3046881119816406
 0.6945063237653067
 0.8492254807551606

julia> plot(data[1][inds],data[2][inds])

```

A least general way, if the `data[1]` is always increasing, is to use something like:

```julia
julia> data = (sort(rand(5)),rand(5));

julia> data[1]
5-element Vector{Float64}:
 0.03852598530448814
 0.18616226772164635
 0.25586337027799577
 0.3970926196952723
 0.8350821471218625

julia> i = findfirst(x -> x > 0.2, data[1])
3

julia> plot(data[1][i:end],data[2][i:end])

```

(or if it is just plotting what you want, adjust the `xlims` of the plot)

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<div class="post-metadata">

**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:** [March 18, 2022, 6:10am UTC](https://discourse.julialang.org/t/how-to-filter-a-tuple-by-the-first-element/78053/5 "2022-03-18T06:10:54Z")

</div>

It can be cleaner and more convenient to use a table such as a `StructArray`:

```julia
julia> function gen_data()
           a = collect(0:.1:200)
           b = rand(length(a))
           c = [rand()*i*0.001 for i in 1:length(a)]
           return StructArray(;a,b,c)
       end

julia> d = gen_data()

julia> plot(d.a, d.b)

julia> d1 = filter(i -> i.a > 100, d)

julia> plot(d1.a, d1.b)

```