For statement

The numeric for loop repeats a block of code while a control variable runs through an arithmetic progression. It has the following syntax:

statement ::= for name `=´ exp `,´ exp [`,´ exp] do block end

for i=1,1000 do
t[i] = t[i] + 1
end

The block is repeated for name starting at the value of the first exp, until it passes the second exp by steps of the third exp. More precisely, a for statement like

for v = e1, e2, e3 do block end

is equivalent to the code:

do 
        local var, limit, step = tonumber(e1), tonumber(e2), tonumber(e3) 
        if not (var and limit and step) then error() end
        while (step > 0 and var <= limit) or (step <= 0 and var >= limit)
 do 
          local v = var
          block        
          var = var + step
       end 
      end

Note the following:

• All three control expressions are evaluated only once, before the loop starts. They must all result in numbers.
• var, limit, and step are invisible variables. The names shown here are for explanatory purposes only.
• If the third expression (the step) is absent, then a step of 1 is used.

The generic for statement works over functions, called iterators. On each iteration, the iterator function is called to produce a new value, stopping when this new value is null. The generic for loop has the following syntax:

stat ::= for namelist in explist do block 

end namelist ::= Name {`,´ Name}

The pairs function is a build in iterator function which returns all of the key/value pairs in a Lua table.

for key,value in pairs(t) do
block
end

A for statement like

for var_1, ···, var_n in explist do block end

is equivalent to the code:

do
local f, s, var = explist

local var_1, ···, var_n = f(s, var)
var = var_1
if var == null then break end
block
end

end

Note the following:

• explist is evaluated only once. Its results are aniteratorfunction, astate, and an initial valuefor the first iterator variable.
• f, s, and var are invisible variables. The names are here for explanatory purposes only.
• You can use break to exit a for loop.
• The loop variables var_i are local to the loop; you cannot use their values after the for ends. If you need these values, then assign them to other variables before breaking or exiting the loop.