agrim mittal (Everything is a file)

Getting to know Lua (Part 2)

21 Jan 2018

This post is in continuation of previous post Getting to know Lua (Part 1)

Session 2: Diving into Expressions

Relational Operators

<   >   <=  >=  ==  ~=

Comparing values with different types? Careful!!

"9" == 9        -- false

Moreover,

4 < 20          -- true
"4" < "20"      -- false (alphabetical order)

To avoid inconsistent results, Lua raises an error when you mix strings and numbers in an order comparison, such as 4 < "20".

Logical Operators

The family consists of and, or and not.

print(4 and 2)               -- true
print(false and 1)           -- false
print(false or true)         -- true
print(nil and 15)            -- false
print(not nil)               -- true
print(not 4)                 -- false

Table Constructors

mylist = {"item1", "item2", "item3", "item4"}
print(mylist[4])         -- "item4"  (the first element has always index 1, not 0)

Assignment

a = 210                         -- a gets 210
a, b = 4, 3                     -- a gets 4 and b 3
x, y = y, x                     -- swap x and y
a, b, c = 0                     -- a get 0 and b and c gets nil
a, b, c = 0, 0, 0               -- a, b, c get 0 each

function f()
    return 0, 1
end

a, b = f()                       -- a gets 0 and b 1

Local variables and blocks

a = 10                          -- global variable
local b = 20                    -- local variable

Local variable have scope limited to the block where they are declared.

b = 2              -- global variable

function a()
    local i = 1
    print(i)       -- local to scope of a()
end

print(i)           -- nil (as not available outside function block)

Control Structures

if, else for conditionals

where, for, repeat for iteration

All control structures have an explicit terminator: end terminates the if, for and while structures; and until terminates the repeat structure.

if x > 0 then
    return x
else
    return -1
end

local i = 0
while i < 10 do
    print(i)
    i = i + 1
end

-- print the first non-empty line
repeat
    line = os.read()
until line ~= ""
print(line)

for i=1,10 do 
    print(i)
end

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