For Statement

3.3.5 – For Statement

The for statement has two forms: one numerical and one generic.

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

stat ::= for Name ‘=’ exp ‘,’ exp [‘,’ exp] do block 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
  var = var - step
  while true do
    var = var + step
    if (step >= 0 and var > limit) or (step < 0 and var < limit) then
      break
    end
    local v = var
    block
  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.
• You can use break and goto to exit a for loop.
• The loop variable v is local to the loop body. If you need its value after the loop, assign it to another variable before exiting the loop.

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 nil. The generic for loop has the following syntax:

stat ::= for namelist in explist do block end
namelist ::= Name {‘,’ Name}

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
  while true do
    local var_1, ···, var_n = f(s, var)
    if var_1 == nil then break end
    var = var_1
    block
  end
end

Note the following:

• explist is evaluated only once. Its results are an iterator function, a state, and an initial value for 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.