for statement iterators were covered in the ForTutorial. These are some notes on writing your own custom iterators.
We can write our own iterators to be called by the for statement. The following Lua pseudo-code describes how the for statement works with iterators:
-- Equivalent to "for var1, キキキ, varn in explist do block end" do local iterator, state, var1 = explist local var2, ... , varn while true do var1, ..., varn = iterator(state, var1) if var1 == nil then break end -- for block code end end
The state and current key value are passed into the iterator. The iterator returns the new value of the key and any other values, e.g. the value generated by the iterator. If nil is returned then the for loop is terminated.
The following iterator will return a sequence of squared values. When we return no value (i.e. n>=state), Lua returns nil which terminates iteration. Notice the iterator is returning the next value in the sequence for a given iteration. We use the state to hold the number of iterations we wish to perform.
> function square(state,n) if n<state then n=n+1 return n,n*n end end
Here is the for statement calling the iterator:
> for i,n in square,5,0 do print(i,n) end 1 1 2 4 3 9 4 16 5 25
We could wrap the above example up (like pairs()) and provide a squares(nbvals) iterator constructor function. E.g.,
> function squares(nbvals) return square,nbvals,0 end -- iterator,state,initial value
Now we can call it like pairs():
> for i,n in squares(5) do print(i,n) end 1 1 2 4 3 9 4 16 5 25
The following iterator is similar to ipairs, but allows for multiple tables to be iterated over.
function ipairs(...) local t = {...} local tmp = {...} -- if nothing to iterate over just return a dummy iterator if #tmp==0 then return function() end, nil, nil end local function mult_ipairs_it(t, i) i = i+1 for j=1,#t do local val = t[j][i] if val == nil then return val end tmp[j] = val end return i, unpack(tmp) end return mult_ipairs_it, t, 0 end local t1 = {'a', 'b', 'c', 'd', 'e'} local t2 = {'A', 'B', 'C', 'D', 'E', 'F'} for k,v1,v2 in ipairs(t1, t2) do print(k,v1,v2) end
This iterator produces the prime divisors of its argument. It relies on the obvious fact that the smallest positive divisor, greater than one, of a nonzero integer must be a prime number.
primdiv = function (n) assert(n ~= 0) if n < 0 then n = -n end local f = function (s,v) -- s not used local p if n == 1 then return end while n%v > 0 and v*v < n do if v == 2 then v = 3 else v = v + 2 end end -- while if n%v == 0 then n = n/v; return v end if v*v > n then p = n n = 1; return p end end -- function return f,nil,2 end -- function for p in primdiv(84) do io.write(p," ") end --> 2 2 3 7
RiciLake says: I couldn't resist rewriting that:
function primdiv(n) assert(n ~= 0) if n < 0 then n = -n end local function f(_, v) if n > 1 then while n%v > 0 do v = v + (v == 2 and 1 or 2) if v*v > n then v = n end end -- while n = n / v return v end -- if end -- function return f,nil,2 end -- function primdiv for p in primdiv(84) do io.write(p," ") end --> 2 2 3 7
GavinWraith says: That is much nicer. I do regard, however, the use of function primdiv(n) as
tantamount to methadone for first-order language junkies.
function ipairs(t) local function ipairs_it(t, i) i = i+1 local v = t[i] if v ~= nil then return i,v else return nil end end return ipairs_it, t, 0 end
function ripairs(t) local max = 1 while t[max] ~= nil do max = max + 1 end local function ripairs_it(t, i) i = i-1 local v = t[i] if v ~= nil then return i,v else return nil end end return ripairs_it, t, max end
-- from the end backwards function ripairs(t) local function ripairs_it(t,i) i=i-1 local v=t[i] if v==nil then return v end return i,v end return ripairs_it, t, #t+1 end
-- traversing the whole 'array' function ripairs(t) idx={} for k,v in pairs(t) do if type(k)=="number" then idx[#idx+1]=k end end table.sort(idx) local function ripairs_it(t,_) if #idx==0 then return nil end k=idx[#idx] idx[#idx]=nil return k,t[k] end return ripairs_it, t, nil end t1 = {'a', 'b', nil, 'd', 'e', nil} for k,v in ripairs(t1) do print(k,v) end --> 5 e --> 4 d --> 2 b --> 1 a