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
, andstep
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
, andvar
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.
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