How to break out of a loop from inside a switch

Question

I m writing some code that looks like this   while true        switch msg- gt state        case MSGTYPE                  break             more stuff         case DONE          break       HERE  I want to break out of the loop itself             Is there any direct way to do that   I know I can use a flag  and break from the loop by putting a conditional break just after the switch  I just want to know if C   has some construct for this already

User · Answer

You can use goto   while              switch               case               goto exit loop          exit loop

User · Answer

There s no C   construct for breaking out of the loop in this case   Either use a flag to interrupt the loop or  if appropriate  extract your code into a function and use return

User · Answer

You could put your switch into a separate function like this   bool myswitchfunction         switch msg- gt state        case MSGTYPE                  break             more stuff         case DONE          return false       HERE  I want to break out of the loop itself             return true     while myswitchfunction

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The break keyword in C   only terminates the most-nested enclosing iteration or switch statement  Thus  you couldn t break out of the while  true  loop directly within the switch statement   however you could use the following code  which I think is an excellent pattern for this type of problem   for    msg- gt state    DONE  msg   next message          switch  msg- gt state        case MSGTYPE                        break                       If you needed to do something when msg- gt state equals DONE  such as run a cleanup routine   then place that code immediately after the for loop   i e  if you currently have   while  true        switch  msg- gt state        case MSGTYPE                         break                  case DONE          do cleanup            break             if  msg- gt state    DONE          break       msg   next message        Then use instead   for    msg- gt state    DONE  msg   next message          switch  msg- gt state        case MSGTYPE                        break                      assert msg- gt state    DONE   do cleanup

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Why not just fix the condition in your while loop  causing the problem to disappear   while msg- gt state    DONE        switch msg- gt state        case MSGTYPE                  break             more stuff         case DONE             We can t get here  but for completeness we list it          break       HERE  I want to break out of the loop itself

User · Answer

You could potentially use goto  but I would prefer to set a flag that stops the loop  Then break out of the switch

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while MyCondition    switch msg- gt state    case MSGTYPE              break         more stuff     case DONE     MyCondition false     just add this code and you will be out of loop      break       HERE  you want to break out of the loop itself

User · Answer

I got same problem and solved using a flag   bool flag   false  while true        switch msg- gt state        case MSGTYPE                  break             more stuff         case DONE          flag   true       HERE  I want to break out of the loop itself             if flag  break

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AFAIK there is no  double break  or similar construct in C    The closest would be a goto - which  while it has a bad connotation to its name  exists in the language for a reason - as long as it s used carefully and sparingly  it s a viable option

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It amazes me how simple this is considering the depth of explanations    Here s all you need     bool imLoopin   true   while imLoopin         switch msg- gt state             case MSGTYPE                      break                  more stuff              case DONE              imLoopin   false              break              LOL    Really   That s all you need   One extra variable

User · Answer

The simplest way to do it is to put a simple IF before you do the SWITCH   and that IF test your condition for exiting the loop            as simple as it can be

User · Answer

If I remember C   syntax well  you can add a label to break statements  just like for goto  So what you want would be easily written   while true        switch msg- gt state        case MSGTYPE                 break             more stuff         case DONE          break outofloop       HERE  I want to break out of the loop itself            outofloop     rest of your code here

User · Answer

Even if you don t like goto  do not use an exception to exit a loop  The following sample shows how ugly it could be   try     while               switch                case              throw 777     I m afraid of goto              catch   int         I would use goto as in this answer  In this case goto will make code more clear then any other option  I hope that this question will be helpful   But I think that using goto is the only option here because of the string while true   You should consider refactoring of your loop  I d suppose the following solution   bool end loop   false  while    end loop         switch  msg- gt state         case MSGTYPE                  break             more stuff         case DONE          end loop   true  break            Or even the following   while   msg- gt state    DONE         switch  msg- gt state         case MSGTYPE                  break             more stuff

User · Answer

An alternate solution is to use the keyword continue in combination with break  i e   for            switch msg- gt state        case MSGTYPE             code         continue     continue with loop     case DONE          break            break     Use the continue statement to finish each case label where you want the loop to continue and use the break statement to finish case labels that should terminate the loop  Of course this solution only works if there is no additional code to execute after the switch statement

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I think   while msg- gt state    mExit         switch msg- gt state               case MSGTYPE                  break        case DONE                                   msg- gt state  mExit        break          if  msg- gt state   mExit       msg- gt state  DONE

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A neatish way to do this would be to put this into a function   int yourfunc          while true             switch msg- gt state            case MSGTYPE                      break                 more stuff             case DONE              return                        Optionally  but  bad practices    as already suggested you could use a goto  or throw an exception inside the switch

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while true          switch x             case 1               break             break     case 2        some code here    break    default      some code here

User · Answer

No  C   does not have a construct for this  given that the keyword  break  is already reserved for exiting the switch block  Alternatively a do  while   with an exit flag could suffice   do        switch option           case 1      break                      case n      break          default  flag   false  break          while flag

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Premise  The following code should be considered bad form  regardless of language or desired functionality   while  true         Supporting Arguments  The while  true   loop is poor form because it    Breaks the implied contract of a while loop    The while loop declaration should explicitly state the only exit condition   Implies that it loops forever    Code within the loop must be read to understand the terminating clause  Loops that repeat forever prevent the user from terminating the program from within the program   Is inefficient    There are multiple loop termination conditions  including checking for  true    Is prone to bugs    Cannot easily determine where to put code that will always execute for each iteration   Leads to unnecessarily complex code  Automatic source code analysis    To find bugs  program complexity analysis  security checks  or automatically derive any other source code behaviour without code execution  specifying the initial breaking condition s  allows algorithms to determine useful invariants  thereby improving automatic source code analysis metrics   Infinite loops    If everyone always uses while true  for loops that are not infinite  we lose the ability to concisely communicate when loops actually have no terminating condition   Arguably  this has already happened  so the point is moot      Alternative to  Go To   The following code is better form   while  isValidState         execute       bool isValidState       return msg- gt state    DONE      Advantages  No flag  No goto  No exception  Easy to change  Easy to read  Easy to fix  Additionally the code    Isolates the knowledge of the loop s workload from the loop itself  Allows someone maintaining the code to easily extend the functionality  Allows multiple terminating conditions to be assigned in one place  Separates the terminating clause from the code to execute  Is safer for Nuclear Power plants   -    The second point is important  Without knowing how the code works  if someone asked me to make the main loop let other threads  or processes  have some CPU time  two solutions come to mind   Option  1  Readily insert the pause   while  isValidState         execute      sleep        Option  2  Override execute   void execute       super- gt execute      sleep        This code is simpler  thus easier to read  than a loop with an embedded switch  The isValidState method should only determine if the loop should continue  The workhorse of the method should be abstracted into the execute method  which allows subclasses to override the default behaviour  a difficult task using an embedded switch and goto    Python Example  Contrast the following answer  to a Python question  that was posted on StackOverflow    Loop forever  Ask the user to input their choice  If the user s input is  restart   continue looping forever  Otherwise  stop looping forever  End    Code  while True       choice   raw input  What do you want          if choice     restart           continue     else          break  print  Break      Versus    Initialize the user s choice  Loop while the user s choice is the word  restart   Ask the user to input their choice  End    Code  choice    restart    while choice     restart        choice   raw input  What do you want      print  Break     Here  while True results in misleading and overly complex code

[c++] How to break out of a loop from inside a switch?

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