Control Structure Tutorial

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while

The conditional looping statement while has the form:

while exp do block end

For example, a simple loop:

> i = 3
> while i>0 do
>>  print(i)
>>  i = i-1
>> end
3
2
1

We can exit the control of a while statement using the break keyword. Note, in Lua the break keyword must be the last statement in a block--i.e. the keyword end must follow--otherwise you will get compilation errors.

> a,b = 0,1
> while true do               -- infinite loop
>>  io.write(b, ", ")
>>  a,b = b,a+b
>>  if a>500 then break end   -- exit the loop if the condition is true
>> end
1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, >

repeat

The conditional looping statement repeat has the form:

repeat block until exp

For example, a simple loop:

> i = 3
> repeat
>>  print(i)
>>  i = i-1
>> until i==0
3
2
1

Like the while statement we can exit a repeat loop using a break statement:

> i = 1
> repeat
>>  print(i)
>>  i = i+1
>>  if i>3 then break end
>> until cows_come_home
1
2
3

for

The iterating statement for has two forms. The first is for numerical iteration, e.g.,

> for count = 1,3 do print(count) end  -- numerical iteration
1
2
3

The second is for sequential iteration, e.g. to print the contents of a table. Here, for is passed an iterator function, which here is pairs(), whose purpose it is to supply the values of each iteration:

> for key,value in pairs({10, math.pi, "banana"}) do print(key, value) end
1       10
2       3.1415926535898
3       banana

There is more detail on all the forms of for in the ForTutorial.

if ... then ... else ... end

The statement if has the form:

if exp then block { elseif exp then block } [ else block ] end

For example, if ... then ... end

> if 10>2 then print("bigger") end
bigger

if ... then ... else ... end

> if 1>10 then print("bigger") else print("smaller") end
smaller

if ... then ... elseif ... else ... end

> number = 3
> if number < 1 then
>>   value = "smaller than one"
>> elseif number==1 then
>>   value = "one"
>> elseif number==2 then
>>   value = "two"
>> elseif number==3 then
>>   value = "three"
>> else
>>   value = "bigger than three"
>> end
> print(value)
three