Can I use break to exit multiple nested for loops

Question

Is it possible to use the break function to exit several nested for loops   If so  how would you go about doing this  Can you also control how many loops the break exits

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goto can be very helpful for breaking nested loops   for  i   0  i  lt  1000  i          for  j   0  j  lt  1000  j              for  k   0  k  lt  1000  k                   for  l   0  l  lt  1000  l                                          if  condition                      goto break me here                                                        break me here     Statements to be executed after code breaks at if condition

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Just to add an explicit answer using lambdas     for  int i   0  i  lt  n1    i          amp           for  int j   0  j  lt  n2    j            for  int k   0  k  lt  n3    k              return     yay we re breaking out of 2 loops here                                  Of course this pattern has a certain limitations and obviously C  11 only but I think it s quite useful

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break will exit only the innermost loop containing it   You can use goto to break out of any number of loops   Of course goto is often Considered Harmful      is it proper to use the break function         Using break and goto can make it more difficult to reason about the correctness of a program   See here for a discussion on this   Dijkstra was not insane

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How about this   for unsigned int i 0  i  lt  50  i          for unsigned int j 0  j  lt  50  j                  for unsigned int k 0  k  lt  50  k                            Some statement             if  condition                                j 50                  k 50

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The break statement terminates the execution of the nearest enclosing do  for  switch  or while statement in which it appears  Control passes to the statement that follows the terminated statement    from msdn

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A code example using goto and a label to break out of a nested loop   for        for          goto theEnd  theEnd

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One nice way to break out of several nested loops is to refactor your code into a function   void foo         for unsigned int i 0  i  lt  50  i                  for unsigned int j 0  j  lt  50  j                          for unsigned int k 0  k  lt  50  k                                     If condition is true                 return

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I do think a goto is valid in this circumstance   To simulate a break continue  you d want   Break  for                 for                       Code here           if  condition                goto theEnd                    theEnd    Continue  for                for                       Code here           if  condition                i                goto multiCont                      multiCont

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Although this answear was already presented  i think a good approach is to do the following   for unsigned int z   0  z  lt  z max  z          bool gotoMainLoop   false      for unsigned int y   0  y  lt  y max  amp  amp   gotoMainLoop  y                  for unsigned int x   0  x  lt  x max  amp  amp   gotoMainLoop  x                                          do your stuff                           if condition                              gotoMainLoop   true

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No  don t spoil it with a break  This is the last remaining stronghold for the use of goto

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I know this is old post   But I would suggest a bit logical and simpler answer   for unsigned int i 0  i  lt  50  i                  for unsigned int j 0  j  lt  conditionj  j                          for unsigned int k 0  k lt  conditionk   k                                     If condition is true                  j  conditionj                 break

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I know this is an old thread but I feel this really needs saying and don t have anywhere else to say it  For everybody here  use goto  I just used it  Like almost everything  goto is not 100  either xor  quot bad quot  or  quot good quot   There are at least two uses where I d say that if you use a goto for them - and don t use it for anything else - you should not only be 100  okay  but your program will be even more readable than without it  as it makes your intention that much clearer  there are ways to avoid it  but I ve found all of them to be much clunkier    Breaking out of nested loops  and Error handling  i e  to jump to a cleanup routine at the end of a function in order to return a failure code and deallocate memory     Instead of just dogmatically accepting rules like  quot so-so is  evil  quot   understand why that sentiment is claimed  and follow the  quot why quot   not the letter of the sentiment  Not knowing this got me in a lot of trouble  too  to the point I d say calling things dogmatically  quot evil quot  can be more harmful than the thing itself  At worst  you just get bad code - and then you know you weren t using it right so long as you heard to be wary  but if you are wracking yourself trying to satisfy the dogmatism  I d say that s worse  Why  quot goto quot  is called  quot evil quot  is because you should never use it to replace ordinary ifs  fors  and whiles  And why that  Try it  try using  quot goto quot  instead of ordinary control logic statements  all the time  then try writing the same code again with the control logic  and tell me which one looks nicer and more understandable  and which one looks more like a mess  There you go   Bonus  try and add a new feature now to the goto-only code   That s why it s  quot evil quot   with suitable scope qualification around the  quot evil quot   Using it to short-circuit the shortcomings of C s  quot break quot  command is not a problematic usage  so long as you make it clear from the code what your goto is supposed to accomplish  e g  using a label like  quot nestedBreak quot  or something   Breaking out of a nested loop is very natural   Or to put it more simply  Use goto to break out of the loop  I d say that s even preferable  Don t use goto to create the loop  That s  quot evil quot    And how do you know if you re being dogmatic  If following an  quot xyz is evil quot  rule leads your code to be less understandable because you re contorting yourself trying to get around it  such as by adding extra conditionals on each loop  or some flag variable  or some other trick like that   then you re quite likely being dogmatic  There s no substitute for learning good thinking habits  moreso than good coding habits  The former are prior to the latter and the latter will often follow once the former are adopted  The problem is  however  that far too often I find  the latter are not explicated enough  Too many simply say  quot this is bad quot  and  quot this needs more thought quot  without saying what to think  what to think about  and why  And that s a big shame   FWIW  in C    the need to break out of nested loops still exists  but the need for error codes does not  in that case  always use exceptions to handle error codes  never return them unless it s going to be so frequent that the exception throw and catch will be causing a performance problem  e g  in a tight loop in a high demand server code  perhaps  some may say that  exceptions  should be  used rarely  but that s another part of ill-thought-out dogmatism  no  at least in my experience after bucking that dogma I find they make things much clearer - just don t abuse them to do something other than error handling  like using them as control flow  effectively the same as with  quot goto quot   If you use them all and only for error handling  that s what they re there for

User · Answer

I m not sure if it s worth it  but you can emulate Java s named loops with a few simple macros    define LOOP NAME name        if    maybe unused   constexpr bool  namedloop InvalidBreakOrContinue   false                      maybe unused   CAT  namedloop break  name   break              maybe unused   CAT  namedloop continue  name   continue                else   define BREAK name  goto CAT  namedloop break  name   define CONTINUE name  goto CAT  namedloop continue  name    define CAT x y  CAT  x y   define CAT  x y  x  y   Example usage    include  lt iostream gt   int main            Prints         0 0        0 1        0 2        1 0        1 1      for  int i   0  i  lt  3  i    LOOP NAME foo                for  int j   0  j  lt  3  j                          std  cout  lt  lt  i  lt  lt       lt  lt  j  lt  lt    n               if  i    1  amp  amp  j    1                  BREAK foo                       Another example    include  lt iostream gt   int main            Prints          0        1        0        1        0        1      int count   3      do LOOP NAME foo                for  int j   0  j  lt  3  j                          std  cout  lt  lt       lt  lt  j  lt  lt    n               if  j    1                  CONTINUE foo                       while count--  gt  1

User · Answer

You can use try   catch   try       for int i 0  i lt 10    i            for int j 0  j lt 10    j                if i j    42                  throw 0     this is something like  break 2                    catch int e        just do nothing    just continue with other code   If you have to break out of several loops at once  it is often an exception anyways

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Another approach to breaking out of a nested loop is to factor out both loops into a separate function  and return from that function when you want to exit   Of course  this brings up the other argument of whether you should ever explicitly return from a function anywhere other than at the end

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AFAIK  C   doesn t support naming loops  like Java and other languages do   You can use a goto  or create a flag value that you use   At the end of each loop check the flag value   If it is set to true  then you can break out of that iteration

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Other languages such as PHP accept a parameter for break  i e  break 2   to specify the amount of nested loop levels you want to break out of  C   however doesn t  You will have to work it out by using a boolean that you set to false prior to the loop  set to true in the loop if you want to break  plus a conditional break after the nested loop  checking if the boolean was set to true and break if yes

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Break any number of loops by just one bool variable see below     bool check   true   for  unsigned int i   0  i  lt  50  i          for  unsigned int j   0  j  lt  50  j                  for  unsigned int k   0  k  lt  50  k                            Some statement             if  condition                                check   false                  break                                  if   check                        break                      if   check                break            In this code we break  all the loops

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while  i lt n        bool shouldBreakOuter   false      for  int j i   1  j lt n    j          if  someCondition              shouldBreakOuter   true                     if  shouldBreakOuter    true        break

User · Answer

Breaking out of a for-loop is a little strange to me  since the semantics of a for-loop typically indicate that it will execute a specified number of times  However  it s not bad in all cases  if you re searching for something in a collection and want to break after you find it  it s useful  Breaking out of nested loops  however  isn t possible in C    it is in other languages through the use of a labeled break  You can use a label and a goto  but that might give you heartburn at night    Seems like the best option though

[c++] Can I use break to exit multiple nested 'for' loops?

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