Array indexing with multiple conditions

New to Julia
1

I have a two-dimensional array and I want to select based on the values in certain columns. For example, let’s think of data to be an 1000x14 Array{Float64,2}. I can do

data[data[:,3].>-9.0,:]

but when I do

data[data[:,3].>-9.0 & data[:,3].<25.0,:]

I get an error that says

**ERROR:** MethodError: no method matching &(::Float64, ::Array{Float64,1})
Closest candidates are:
&(::Any, ::Any, ::Any, ::Any...) at operators.jl:538
Stacktrace:
[1] top-level scope at **REPL[96]:1**

Any suggestions on how I can manage to do this without making use of DataFrames?

Thanks!

1 Like
2

There are two things here:

  • You need to .& just like the other operators to make it work element-wise, too
  • You need to use parentheses because &'s precedence is different from &&.
data[(data[:,3] .> -9.0) .& (data[:,3] .< 25.0),:]

Alternatively, you could combine them into a single chained comparison in this particular case:

data[-9.0 .< data[:,3] .< 25.0,:]
8 Likes
3

This is great. Thanks so much for the help!

4

And since this can a bit visually dense, you can drop some characters without sacrificing performance or clarity:

data[-9 .< data[:,3] .< 25, :]

You don’t need the comparison values to be actual floats.

For performance reasons you could also consider using views.

5

For what it’s worth, I’d expect views to slightly speed up the data[:,3] operation and the generation of the logical index but be slower for the final logical indexing expression. YMMV depending upon array types and sizes and downstream uses.

1 Like