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Java Forum / General / October 2006How to jump out of several nested control structures?
Hi, I am wondering how can I jump out of several layers of loops. break can only jump out of the inner most loop. for ( ...) //layer 1 { for (...) //layer 2 { for (...) //layer 3 { //code if (jump==true) //How can I jump out of all the layers at once? } } } > Hi, > > I am wondering how can I jump out of several layers of loops. break can > only jump out of the inner most loop. No, break can jump out of any labeled statement enclosing it. See http://java.sun.com/docs/books/jls/second_edition/html/statements.doc.html#6842 for the syntax and an example. Patricia > No, break can jump out of any labeled statement enclosing it. > [quoted text clipped - 4 lines] > > Patricia Wow. break is similar to GOTO, according to the following statements from the link you gave to me: "A break statement with label Identifier attempts to transfer control to the enclosing labeled statement (§14.7) that has the same Identifier as its label; this statement, which is called the break target, then immediately completes normally. In this case, the break target need not be a while, do, for, or switch statement. A break statement must refer to a label within the immediately enclosing method or initializer block. There are no non-local jumps." I almost have never seen people using break as GOTO feature. >> No, break can jump out of any labeled statement enclosing it. >> [quoted text clipped - 17 lines] > > I almost have never seen people using break as GOTO feature. It is very different from GOTO, because it can ONLY be used to break out of a statement enclosing the break. That said, I'm obviously aware of it, but I don't think I've used it so far in my own code. Patricia > It is very different from GOTO, because it can ONLY be used to break out > of a statement enclosing the break. "Break out of a statement enclosing the break". What does it mean? I am sorry. According to the cited paragraph, it doesnt' need for loop, while loop, etc. It seems the following code is legal: public class MyClass { ...//code public void myMethod() { ...//code if (wantToEnd) break here ...//more code here: System.out.println("I am going to exit"); return; } } Do you think the above feature is GOTO? > It is very different from GOTO, because it can ONLY be used to break out > of a statement enclosing the break. [quoted text clipped - 3 lines] > > Patricia Sorry. I see what you mean now. here: { ...//many layers inside if (jump==true) break here; //jump out of here scope ...//more code executed if not jumped } //reach here by jump or by normal execution >> No, break can jump out of any labeled statement enclosing it. >> >> See http://java.sun.com/docs/books/jls/second_edition/html/statements.doc.html#6842 >> for the syntax and an example. > > Wow. break is similar to GOTO, according to the following statements > from the link you gave to me: Yup, that's why I prefer the 'special exception' solution. Jumping to labels is spaghetti programming in the making, whereas a special exception just unwinds the stack.  Signaturesimon@jasmine.org.uk (Simon Brooke) http://www.jasmine.org.uk/~simon/ my other car is #<Subr-Car: #5d480> ;; This joke is not funny in emacs. > Yup, that's why I prefer the 'special exception' solution. Jumping to > labels is spaghetti programming in the making, whereas a special exception > just unwinds the stack. IMHO using exceptions for this is a case of abuse. From my point of view refactoring to another method and using "return" is much cleaner - and probably faster, too. Before I'd use an exception for a non exceptional condition I'd rather use "break" with label. However, so far I never felt the need for any of those. I typically use the "return" solution or have proper loop conditions - whatever seems more appropriate. Regards robert >Wow. break is similar to GOTO Nope. Break can only transfer control to an enclosing scope. It can only go to the end of a block. GOTO can jump anywhere. This is a humongous difference. That said, it can be abused. I've seen code that contained do { stuff; if (!something) break; more stuff; } while (false); As a way to avoid putting "more stuff" into a separate method because there were a lot of local variables that would have to be passed in and out. >There are no non-local jumps." This is the key phrase that makes break safe and goto unsafe. -- Mark Rafn dagon@dagon.net <http://www.dagon.net/> > That said, it can be abused. I've seen code that contained > do { [quoted text clipped - 5 lines] > As a way to avoid putting "more stuff" into a separate method because there > were a lot of local variables that would have to be passed in and out. Code like this can often be written more sensibly. But if the programmer thought it not worth the costs of splitting into a separate method, the technique doesn't seem completely unreasonable. (For the record I almost did it once that I remember. I then just rearranged the code into a clearer form.) BTW, you don't need the do while false;: block: { stuff; if (!something) { break block; /***/ } more stuff; } FWIW: I think all break statements should have labels (although that is unidiomatic) and continue should be outlawed (although I used it in an interview the other week). Oh, and nested loops tend to be an indicator that your method is getting a bit long (2D structures excepted). Tom Hawtin > Wow. break is similar to GOTO, GOTO is a *general* purpose jump. Most languages need *special* purpose jumps - like Java's break, continue, while, if and for. The difference is that GOTO's jump-anywhere nature makes it hard to analyse flow compared to the restricted and block-oriented alternatives. The C# developers apparently chose to let the programmer shoot themselves in the foot like that. But only within a method or switch statement, of course. > I am wondering how can I jump out of several layers of loops. break > can only jump out of the inner most loop. No, it can jump much further than that. Label your loops and specify a label in the break statement: gurka: for (...) { for (...) { break gurka; } } /gordon Signature[ don't email me support questions or followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e > I am wondering how can I jump out of several layers of loops. break can > only jump out of the inner most loop. Define your own exception class, wrap the whole lot in a try... catch, and throw your special exception when you want to jump out of the loop. Signaturesimon@jasmine.org.uk (Simon Brooke) http://www.jasmine.org.uk/~simon/ ;; Conservatives are not necessarily stupid, ;; but most stupid people are conservatives -- J S Mill > Hi, > [quoted text clipped - 14 lines] > > } Just use the return statement with no arguements...........suppose you have a loop say for(...){ for(...){ for(,.....){ //your code if(yourConditionSatisfied) return } } } or you dont want the function to end do something like this try{ for(...){ for(...){ for(...){ //your code if(yourConditionSatisfied) throw new yourException(); } } } }catch(YourException e){ //dummy } //continue your code here Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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