Filtering

Lets say you're analysing data and only want to operate on data if they pass a condition. Some values might be too large or too small, or you want to only observe values within an area.

This can be done using filtering.

boolean arrays¶

While most of our uses so far have used numbers, Arrays dont have to always contain numbers. For instance, here is a array that holds boolean values.

local a = numluau.array({true,false,false})

Here we are only selecting the values who's corresponding values are true. This is what we call masking.

local mask = numluau.array({true,false,false,true,false})
local a = numluau.array({1,2,3,4,5})

print(a[mask]) -- array([1 4])

Doing this on multidimensional arrays flatten the result into a 1d array.

local mask = numluau.array({
    {true,false,false,true,false},
    {false,false,true,true,false}
})
local a = numluau.array({
    {1,2,3,4,5},
    {6,7,8,9,10}
})

print(a[mask]) -- array([1 4 8 9])

If you want to preserve the shape we can use the numluau.where() function. This will create a new array with our same shape but replaces values where false values are, are replaced with a fill value.

local mask = numluau.array({
    {true,false,false,true,false},
    {false,false,true,true,false}
})
local a = numluau.array({
    {1,2,3,4,5},
    {6,7,8,9,10}
})

local fill_value = 0
local result = numluau.where(mask,a,fill_value)
print(result)

> array([
  [1 0 0 4 0]
  [0 0 8 9 0]
])

comparison¶

While we could use boolean masks for masking. Sometimes we want to be able to create these masks from conditions. This is where comparisons functions come in.

local a = numluau.array({1,2,3,4,5})
local b = numluau.array({5,4,3,2,1})

local mask = numluau.greater_equal(a,b)

print(a[mask]) -- array([3 4 5])

to work with multiple conditions we can use our logical functions. numluau.logical_and() and numluau.logical_or() act like luau's and and or operators.

local a = numluau.array({1,2,3,4,5})
local b = numluau.array({5,4,3,2,1})

local mask1 = numluau.greater_equal(a,b)
local mask2 = numluau.equal(a % 2, 0)

print(a[numluau.logical_and(mask1,mask2)]) -- array([4])