Why does Lua have no continue statement

Question

I have been dealing a lot with Lua in the past few months  and I really like most of the features but I m still missing something among those    Why is there no continue  What workarounds are there for it

User · Answer

Again with the inverting  you could simply use the following code   for k v in pairs t  do   if not isstring k  then      -- do something to t k  when k is not a string end

User · Answer

The way that the language manages lexical scope creates issues with including both goto and continue  For example    local a 0 repeat      if f   then         a 1 --change outer a     end     local a f   -- inner a until a  0 -- test inner a   The declaration of local a inside the loop body masks the outer variable named a  and the scope of that local extends across the condition of the until statement so the condition is testing the innermost a   If continue existed  it would have to be restricted semantically to be only valid after all of the variables used in the condition have come into scope  This is a difficult condition to document to the user and enforce in the compiler  Various proposals around this issue have been discussed  including the simple answer of disallowing continue with the repeat     until style of loop  So far  none have had a sufficiently compelling use case to get them included in the language   The work around is generally to invert the condition that would cause a continue to be executed  and collect the rest of the loop body under that condition  So  the following loop  -- not valid Lua 5 1  or 5 2  for k v in pairs t  do   if isstring k  then continue end   -- do something to t k  when k is not a string end   could be written   -- valid Lua 5 1  or 5 2  for k v in pairs t  do   if not isstring k  then      -- do something to t k  when k is not a string   end end   It is clear enough  and usually not a burden unless you have a series of elaborate culls that control the loop operation

User · Answer

We can achieve it as below  it will skip even numbers   local len   5 for i   1  len do     repeat          if i 2    0 then break end         print   i      i          break     until true end   O P   i   1 i   3 i   5

User · Answer

The first part is answered in the FAQ as slain pointed out   As for a workaround  you can wrap the body of the loop in a function and return early from that  e g   -- Print the odd numbers from 1 to 99 for a   1  99 do    function       if a   2    0 then       return     end     print a    end    end   Or if you want both break and continue functionality  have the local function perform the test  e g   local a   1 while  function     if a  gt  99 then     return false  -- break   end   if a   2    0 then     return true  -- continue   end   print a    return true  -- continue end    do   a   a   1 end

User · Answer

We encountered this scenario many times and we simply use a flag to simulate continue  We try to avoid the use of goto statements as well   Example  The code intends to print the statements from i 1 to i 10 except i 3  In addition it also prints  loop start   loop end    if start   and  if end  to simulate other nested statements that exist in your code   size   10 for i 1  size do     print  loop start       if whatever then         print  if start           if  i    3  then             print  i is 3               --continue         end         print j          print  if end       end     print  loop end   end   is achieved by enclosing all remaining statements until the end scope of the loop with a test flag   size   10 for i 1  size do     print  loop start       local continue   false   -- initialize flag at the start of the loop     if whatever then         print  if start           if  i    3  then             print  i is 3               continue   true         end          if continue  false then          -- test flag             print j              print  if end           end     end      if  continue  false  then            -- test flag         print  loop end       end end   I m not saying that this is the best approach but it works perfectly to us

User · Answer

Why is there no continue   Because it s unnecessary    There s very few situations where a dev would need it   A  When you have a very simple loop  say a 1- or 2-liner  then you can just turn the loop condition around and it s still plenty readable   B  When you re writing simple procedural code  aka  how we wrote code in the last century   you should also be applying structured programming  aka  how we wrote better code in the last century   C  If you re writing object-oriented code  your loop body should consist of no more than one or two method calls unless it can be expressed in a one- or two-liner  in which case  see A   D  If you re writing functional code  just return a plain tail-call for the next iteration   The only case when you d want to use a continue keyword is if you want to code Lua like it s python  which it just isn t     What workarounds are there for it   Unless A  applies  in which case there s no need for any workarounds  you should be doing Structured  Object-Oriented or Functional programming  Those are the paradigms that Lua was built for  so you d be fighting against the language if you go out of your way to avoid their patterns       Some clarification      Lua is a very minimalistic language  It tries to have as few features as it can get away with  and a continue statement isn t an essential feature in that sense   I think this philosophy of minimalism is captured well by Roberto Ierusalimschy in this 2019 interview      add that and that and that  put that out  and in the end we understand the final conclusion will not satisfy most people and we will not put all the options everybody wants  so we don   t put anything  In the end  strict mode is a reasonable compromise       There seems to be a large ammount of programmers coming to Lua from other languages because whatever program they re trying to script for happens to use it  and many of them want don t seem to want to write anything other than their language of choice  which leads to many questions like  Why doesn t Lua have X feature    Matz described a similar situation with Ruby in a recent interview      The most popular question is   I   m from the language X community  can   t you introduce a feature from the language X to Ruby    or something like that  And my usual answer to these requests is     no  I wouldn   t do that   because we have different language design and different language development policies       There s a few ways to hack your way around this  some users have suggested using goto  which is a good enough aproximation in most cases  but gets very ugly very quickly and breaks completely with nested loops  Using gotos also puts you in danger of having a copy of SICP thrown at you whenever you show your code to anybody else

User · Answer

I ve never used Lua before  but I Googled it and came up with this  http   www luafaq org  Check question 1 26   This is a common complaint  The Lua authors felt that continue was only one of a number of possible new control flow mechanisms  the fact that it cannot work with the scope rules of repeat until was a secondary factor   In Lua 5 2  there is a goto statement which can be easily used to do the same job

User · Answer

Straight from the designer of Lua himself      Our main concern with  continue  is that there are several other control structures that  in our view  are more or less as important as  continue  and may even replace it   E g   break with labels  as in Java  or even a more generic goto    continue  does not seem more special than other control-structure mechanisms  except that it is present in more languages   Perl actually has two  continue  statements   next  and  redo   Both are useful

User · Answer

You can wrap loop body in additional repeat until true and then use do break end inside for effect of continue  Naturally  you ll need to set up additional flags if you also intend to really break out of loop as well   This will loop 5 times  printing 1  2  and 3 each time   for idx   1  5 do     repeat         print 1          print 2          print 3          do break end -- goes to next iteration of for         print 4          print 5      until true end   This construction even translates to literal one opcode JMP in Lua bytecode     luac -l continue lua   main  lt continue lua 0 0 gt   22 instructions  88 bytes at 0x23c9530  0  params  6 slots  0 upvalues  4 locals  6 constants  0 functions     1    1  LOADK       0 -1      1     2    1  LOADK       1 -2      3     3    1  LOADK       2 -1      1     4    1  FORPREP     0 16      to 21     5    3  GETGLOBAL   4 -3      print     6    3  LOADK       5 -1      1     7    3  CALL        4 2 1     8    4  GETGLOBAL   4 -3      print     9    4  LOADK       5 -4      2     10   4  CALL        4 2 1     11   5  GETGLOBAL   4 -3      print     12   5  LOADK       5 -2      3     13   5  CALL        4 2 1     14   6  JMP         6     to 21 -- Here it is  If you remove do break end from code  result will only differ by this single line      15   7  GETGLOBAL   4 -3      print     16   7  LOADK       5 -5      4     17   7  CALL        4 2 1     18   8  GETGLOBAL   4 -3      print     19   8  LOADK       5 -6      5     20   8  CALL        4 2 1     21   1  FORLOOP     0 -17     to 5     22   10     RETURN      0 1

User · Answer

In Lua 5 2 the best workaround is to use goto   -- prints odd numbers in   1 10   for i 1 10 do   if i   2    0 then goto continue end   print i      continue   end   This is supported in LuaJIT since version 2 0 1

User · Answer

Lua is lightweight scripting language which want to smaller as possible  For example  many unary operation such as pre post increment is not available  Instead of continue  you can use goto like  arr    1 2 3 45 6 7 8  for key val in ipairs arr  do   if val  gt  6 then      goto skip to next   end        perform some calculation     skip to next   end

[loops] Why does Lua have no "continue" statement?

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