Conditional statements

Tutorial: Conditional Statements
This is a tutorial will teach you how to make your script react to different situations.
Objective	To gain a basic understanding of programming in ComputerCraft.
Prerequisites	A basic understanding of Variables.

The If statement

Conditional statements are one of the fundamental building blocks for any coding language. They are just as they sound, if a condition is met then a block of code will run. The simplest form of conditional statement available in Lua is the "If" statement, which indicates that if one or more conditions are met, the following block of code will run. If statements take the following format

 local Variable = 1
 if Variable==1 then
   write("The variable is 1!\n")
 end
 if Variable==2 then
   write("The variable is 2!\n")
 end

This script will simply output the text "The variable is 1!". The "==" operator simply means "is equal to", so the line reads along the lines of "If Variable is equal to 1, then do this". "end" marks the end of the code block, and anything after it is run normally regardless of whether the conditions were met.

Brackets may optionally be used around conditions, although it is not necessary unless you want to control the order of a calculation.

Note that since lua disregards new lines, it would be equally acceptable to write the following

if Variable==1 then write("The variable is 1!\n") end

However, this method is generally less readable and not commonly used.

Lua determines whether an if statement passes by using the boolean values returned by the condition, so it would be possible (but unnecessary) to enter a boolean as the condition

if true then write("This will always write!\n") end
if false then write("This will never write!\n") end

Available conditions

There are a number of different conditions that can be used to make the conditional statement.

Is equal to

 if Variable == Variable2 then
   --Do stuff
 end

The "==" condition is the most basic operator, simply comparing two values to see if they are the same. If both values are the same, the condition is met and the if statement passes.

Is not equal to

 if Variable ~= Variable2 then
   --Do stuff
 end

The "~=" condition compares two values to see if they are the same, and if they are not the same, the condition is met and the if statement passes.

Greater than

 if Number > Number2 then
   --Do stuff
 end

The ">" condition compares two number values, and if the first is greater than the second, the condition is met and the if statement passes. The ">=" (Greater than or equal to) operator acts as both the "greater than" and the "equal to" conditions, meaning the condition is met if either the first number is equal to the second, or the first number is greater than the second.

Less than

 if Number < Number2 then
   --Do stuff
 end

The "<" condition compares two number values, and if the first is lower than the second, the condition is met and the if statement passes. The "<=" (Less than or equal to) operator acts as a combination of the "less than" and the "equal to" conditions.

And, Or, Not

Sometimes it is desirable to combine conditions. Although it is possible to use multiple if statements, or ifs inside ifs, it is a best practice to use "and" and "or" if possible.

And

local Num1,Num2,Num3 = 1,2,3

--Multiple if
if Num1 > Num2 then
  if Num2 < Num3 then
    write("Condition is true!")
  end
end

--Using And
if (Num1>Num2) and (Num2<Num3) then
  write("Condition is true!")
end

The "and" operator means that both conditions must match before the statement passes. If either side fails, the whole condition is false. Note that if the left side fails, the right side is ignored, so if you have a critical function it should be on the left of the "and".

Or

local Num1,Num2,Num3 = 1,2,3

--Multiple if
if Num1 > Num2 then
  write("Condition is true!")
end
if Num2 < Num3 then
  write("Condition is true!")
end

--Using Or
if (Num1>Num2) or (Num2<Num3) then
  write("Condition is true!")
end

The "or" operator means that if either condition matches the statement passes. This means that if either the left side, the right side, or both pass, the statement is true. Note that if the left side passes, the right side is ignored, so if you have a critical function it should be on the left of the "or".

Not

local Num1,Num2,Num3 = 1,2,3

if (Num1 < Num2) and not (Num1 > Num3) then
  write("Condition is true!")
end

While the "not" statement is rarely a necessity, it can be used to reverse a value from "true" to "false" or vice versa.

In some situations, it may be preferable not to use these statements as code will run before the second condition needs to be assessed, such as as follows

local Num1,Num2 = 1,2

--And should not be used here
if Num1>Num2 then
  local Num3 = 3
  
  if Num3>Num2 then write("Text!\n") end
end

The Else statement

It is common that you may want to run some code on the failure of the condition. If this is the case, it may be achieved through the use of two separate if statements, but it's simpler to use the "else" statement.

 local Num1 = 1
 local Num2 = 2
 
 --Using multiple if
 if Num1 > Num2 then
   write("Number 1 is greater!\n")
 end
 if Num1 <= Num2 then
   write("Number 1 is not greater!\n")
 end
 
 --Using else
 if Num1>Num2 then
   write("Number 1 is greater!\n")
 else
   write("Number 1 is not greater!\n")
 end

The ElseIf statement

Sometimes it may be desirable to run the "else" code only under certain conditions. This is where "elseif" becomes useful. It acts as if it is another if statement, but also functions as the "else" of the if it's in.

local Num1, Num2, Num3 = 1,2,3

--With elseif
if Num1>Num2 then
  write("First condition passed!\n")
elseif Num3>Num2 then
  write("First condition failed, second condition passed!\n")
end

The "elseif" statement can become even more useful if you chain together more "elseif"s and "else"s with it, but remember that the first if that passes will break the chain!

if Bool then
  --Stuff
elseif Var1 == Var2 then
  --Other stuff
elseif Table[1] then
  --More stuff
else
  --More again!
end