Indexing and slicing

numluau allows you to index arrays in many ways.

indexing¶

Indexing works similarly to how you'd index tables. Arrays are 1-indexed, meaning arrays start from 1.

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¶

Sometimes you dont want to get a single item, but instead a collection of items. 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])

When we create a slice, it doesn't create a whole new copy. This means if we mutate the slice, it will affect the original.

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]
])