Loops and control structures

If

If statements are useful for executing different actions depending on certain conditions. It allows doing something like "if the given value is too big, warn the user; if not, execute some code" or "if the value is zero, send an error and stop; if not, do something useful" (or some less dramatic alternatives).

Lets see an example of a common case. As you may know, we cannot divide by Zero (it does not make any sense or is infinite).

-- if.lua
-- ===========

-- Tell Emp that we do not intend to solve anything
auto_solve = false 

a = 2
b = 1

if a == 0 then
    -- This will fail because we want to divide by a.
    error("value of 'a' must be greater than zero")
else
    -- Now that we know that a is not zero, lets divide
    print(b/a) --> 0.5
end

If structures can also have several conditions. In such cases, only the code after the first true condition will be executed. If none is true, the 'else' block will be executed. For instance:

-- if.lua
-- ===========

-- Tell Emp that we do not intend to solve anything
auto_solve = false 

a = 2
b = 1

if a == 0 then
    -- This will fail because we want to divide by a.
    error("value of 'a' must be greater than zero")
elseif a == 1 then
    -- Trivial result... result is b
    print(b) --> 1 or whatever is the value of b
elseif a > 99999999999999 then
    -- Lets assume a value of Zero
    print(0) --> 0
else
    -- Now that we know that a is not zero, lets divide
    print(b/a)
end

The conditions may be more complex than the ones shown above. For example if a ~= 1 and a ~= 2 then ...

While

The while is the first loop we will evaluate. This loop is used to run repetitive tasks until a certain condition is false.

-- while.lua
-- ===========

-- Tell Emp that we do not intend to solve anything
auto_solve = false 

a = 1
while a < 4 do
    print(a)
    -- increse the value of a... otherwise, 
    -- we fall in an into an infinite loop
    a = a + 1 
end

-- this prints
--[[
1
2
3
]]

The While loop is particularly useful for when you do not know how many iterations will it take for something to happen (or that is, exactly, what you want to find out). For instance

-- while.lua
-- ===========

-- Tell Emp that we do not intend to solve anything
auto_solve = false 

-- Pyramid Scams
-- ==============
--[[
    A friend of yours wants to recruit you for a "business". 
    The deal is that you need to pay $20 for entering, 
    and then recruit three new people to the business.
    Since they have to pay, you will receive your money 
    back, along with some profit.

    How many "generations" do we need for having
    100,000,000 people involved?

]]


sum = 0 -- The amount of people in the scam
generations = 0
population =100000000
while sum < population do
    -- Calculate the number of people in this generation
    this_generation = 3^generations

    -- add it to the the total
    sum = sum + this_generation

    -- Increase generation
    generations = generations + 1
end

print(generations) --> 18

As you can see, with a reasonable number of new people to recruit (i.e. 3), the pyramid will have included more than 100 million people in 18 generations! It actually takes 22 to grow larger than the world's population expected stable population of 9 billion. Try printing the this_generation variable to see how fast these things scale, and remember that most of the people are at the latest generations and only the few people at the first ones actually make money.

Repeat

For