Indexing and slicing

numluau allows you to index arrays in many ways.

indexing¶

Arrays are intended to work similarly to how you'd use tables as arrays in luau. The indexes of a Array start at 1 and go up.

So the first element is at index 1, the second is at index 2 etc.

This is standard for Arrays in languages like julia or R but differs from python's numpy arrays which start at 0.

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

print(a[2]) -- 2

local a = numluau.array({
    {1,2,3,4},
    {2,4,6,8}
})

print(a[2][3]) -- 6

We can use a negative index to index from the back of the array instead of the front

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

print(a[-1]) -- 5
print(a[-2]) -- 4

Arrays can also be indexed using strings, allowing you to index is other ways.

If we wanted to index a 2d array, we could use regular table syntax. Or we can use the index col, row to also get that item.

local a = numluau.array({
    {1,2,3,4},
    {2,4,6,8}
})

print(a[2][3])    -- 6
print(a["2, 3"])  -- 6

slicing¶

Now lets say we're working with data but only want a subset of that data. Either because the data is large, or you're data contains multiple channels like left/right channels for audio or red/blue/green/alpha for images.

We can use a special index to get a subsection of our data from our array. This is what we call a slice.

1d arrays¶

Using the index first:last:step, we can retrieve a portion of the array with only some items.

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

-- get items 1-3
print(a["1:3"]) -- array([1 2 3])

-- get every second item
print(a["1:5:2"]) -- array([1 3 5])

A slice doesn't generate a copy of the array, but instead it points to the same data but steps through it differently.
This is what we call a view.

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

b[1] = 125

print(a) -- array([125 2 3 4 5])

You can also leave a parameter empty and it will assume the value for you.

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

print(a["1::2"])
print(a["::2"])
print(a[":3"])

> array([1 3 5])
> array([1 3 5])
> array([1 2 3])

2d+ arrays¶

We can also apply this notion to higher dimensional arrays

local a = numluau.array({
    {1,2,3,4},
    {5,6,7,8}
})
-- gets the entire 2nd row
print(a["2"])

-- gets every item on the 2nd column
print(a[":,2"])

> array([5 6 7 8])
> array([2 6])

We can expand this to multiple dimensions by combining first:last:step and col, row.

local a = numluau.array({
    {1,2,3,4},
    {5,6,7,8},
    {9,10,11,12},
    {13,14,15,16},
})

print(a["1:2, 1:2"])
print(a["1:2, 1:3:2"])

print(a["::2"])
print(a[":,::2"])

> array([ 
    [1 2] 
    [5 6] 
])
> array([ 
    [1 3] 
    [5 7] 
])
> array([
  [1 2 3 4]
  [9 10 11 12]
])
> array([
  [1 3]
  [5 7]
  [9 11]
  [13 15]
])