Filtering

boolean arrays¶

Arrays dont have to always contain numbers.

For instance, here is a array that holds boolean values.

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

If we index a array with another array that contains booleans, we can retrieve all the items where a true value is. 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)

output

> 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.

function	equivalent
numluau.equal	a = b
numluau.greater	a > b
numluau.greater_equal	a ≥ b
numluau.less	a < b
numluau.less_equal	a ≤ b

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.

function	equivalent
numluau.logical_and	a and b
numluau.logical_or	a or b
numluau.logical_not	not a
numluau.logical_xor	a ≠ b

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])