Tables in Lua

13 October 2021 - 4 mins read time
Tags: Lua

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Tables in Lua

Tables in Lua container type data-structure
They can be employed to represent arrays, sets, records, among other type of data-types.
We can access the entries of a table using index or key.
Even, Lua modules/ library can be represented as a Lua table.
Tables in Lua are dynamically allocated objects

Under the hood, Lua never makes hidden copies of new tables, like other languages like python 😡

{} is the table constructor in Lua.

For example

Creating empty table

a={}

Adding entries of table “on the fly”

a["x"]=10
k="y"
a[k]=20

Getting the values from table

print(a["x"]) -- we will get 10
a["y"] -- we will get 20

We can also use numbers as key

a[4]="hello world"

A table is always anonymous. There is no fixed relationship between a variable that holds a table and table itself.

a={}
a["x"]=1.0
a["y"]=2.0
a["z"]=3.0
b = a -- both b and a point to same data, no separate table.
a=nil --> nullify a, do not worry b is still there
b=nil --> now b is gone, we cannot have table entries now :()

When there is no references to a table/ object, then garbage collector comes and clean the space occupied by that object. In this way, as a user we do not have to worry about the memory leak.

We can use a.name instead of a["name"] to access the element from table which has key name. Therefore, in above example a.x will given us 1, and a.z with give us 3.0.

We can also use floating point as key 🐺.

Table constructor

We have already seen the empty table constructor, which is given by a={}. We then added entries to the table using a["x"]=10, or we can use a.x=10.

There is another way to construct the table.

extlibs={"LAPACK95", "BLAS95", "FACE", "PENF"}

the we can access the entries using:

extlibs[1] --> "LAPACK95"
extlibs[4] --> "PENF"

We can also use key-value construct.

a={x=10, y=10, z=20}

then we have

a["x"] --> 10
a.x --> 10

Removing fields of table

a={x=10, y=20, z=30, name="point1"}
a.name --> "point1"
a.name = nil --> a.name is gone

Insert an element

t={}
table.insert(t, 1, 10) --> insert 10 at t[1]
table.insert(t,20) --> insert 20 at t[2]
table.insert(t, "x", 30) --> t["x"] = 30

Removing an element

t={1,2,3,4}
table.remove(t) --> remove the last element
table.remove(t,1) --> remove first element

Table traversal

We can traverse all key-value pairs in a table with the pairs iterator.

t={x=10, y=20, z=30}
for k, v in pairs(t) do
  print(k,v)
end

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