C and other programming languages recognize the widespread need to leave a loop early. C provides the break statement for this purpose. A break statement is a variant of goto. It is an unconditional branch that jumps out of its containing for , while , or switch , instead of to an explicit label. Sometimes we would like to break out of more than one nested loop.
Suppose we have inherited some code that prints the contents of a 3-by-3 array of integers:. Now suppose we have a new requirement that if any of the array elements are negative, we must immediately stop printing anything.
We want to break out of both loops. It is possible to use a flag to avoid a goto , like this:. But a more elegant way to handle this situation is to use a goto that exits both loops:.
This is more concise and easier to understand than the preceding version of the code. It says what it means. It expresses the idea with less conceptual overhead. C programs must detect and handle errors. When an error occurs, it is important to bail out while still cleaning up any dangling resources. The code should close any open files and free any allocated memory.
Suppose your task is to write a C program that reads lines of text from a file. Each line is supposed to contain three floating point numbers.
And no matter what, your program should always clean up by freeing memory and closing the input file. One way to implement this program is shown here. It is a correct implementation that meets all the requirements. There is something unsavory about this code. An option for how to not only perform common cleanup after exiting, but to guarantee that the cleanup will always be done is to create an "in-scope" object, which is cleaned up by IDL's automatic garbage collection.
Basically, the idea is to put any cleanup code in a temporary object's ::Cleanup method. When exiting the routine, the object falls out of scope, and once the object falls out of scope, IDL will automatically destroy, or "garbage collect," the object. When doing so, the Cleanup method automatically gets called. I wrote an IDL Data Point blog post last year describing the in-scope object, which can be found here.
Over time, I've learned one thing about rules of programming - they are meant to be broken, but when you break them, do so carefully. Occasionally, using a goto statement really is the easiest thing to do, such as if you want to quickly jump over code or perform a multi-way split. Sometimes you want to simply trade off the "don't use goto" rule of thumb with convenience, and doing so can be a matter of personal preference.
It's true that using goto often results in fewer lines of code to write compared to other types of logic that would provide the equivalent functionality. The bottom line is that although goto can almost always be replaced by other forms of logic, it still exists today and is legitimate IDL syntax.
There are quite a few hazards to it, but when used sparingly and with care, it can be a useful tool. Gerry: that's a good one. I hadn't thought of that. Actually, with the goto he used to goto a subroutine of some sort , raptors should attack him. There are more implications, and you are right to mention it. Show 2 more comments. Did you google the issue? The founder of the anti-goto movement is Edsger Dijskstra with his legendary "Goto Considered Harmful" To get you started you can goto ha ha!
John Smith John Smith Ironically, he actually tolerated far more GOTO than programmers today. Ole V. Nikolai Fetissov Nikolai Fetissov Apparently the author of the question is unaware why "goto" is available in C, too. And I still don't get what you are so upset about. Hey, thanks, I didn't know there was russian mob around here : — Nikolai Fetissov.
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Linked See more linked questions. Related Hot Network Questions. Active 4 months ago. Viewed 25k times. Is using this statement ever worthwhile? Improve this question. Eagle-Eye 3 3 bronze badges. Casebash Casebash 7, 4 4 gold badges 39 39 silver badges 62 62 bronze badges. Can't you see a similarity? And what do you mean by "no mark"? Label is such a mark.
SK-logic, depends on how far away you will allow goto's to come from. State machines do not require goto to implement. It is just the most rational way of implementing them in the imperative languages, since it is the closest thing to the very semantics of the state transition.
And its destination can be quite far from the source, it won't hinder the readability. My favourite example of such a code is D. Knuth's implementation of the Adventure game. Show 2 more comments. Active Oldest Votes. This has been discussed several times on Stack Overflow, and Chris Gillum summarized the possible uses of goto : Cleanly exiting a function Often in a function, you may allocate resources and need to exit in multiple places.
Exiting nested loops If you're in a nested loop and need to break out of all loops, a goto can make this much cleaner and simpler than break statements and if-checks. Low-level performance improvements This is only valid in perf-critical code, but goto statements execute very quickly and can give you a boost when moving through a function. Improve this answer. Community Bot 1. The first problem is solved very neatly by finally blocks in modern languages, and the second is solved by labelled break s.
If you're stuck with C however, goto is pretty much the only way of solving these problems elegantly. Very good points. I like how Java allows breaking out of multiple levels of loops — Casebash. Chinmay - finally blocks only apply to 1 modern languages that have them and b you can tolerate the overhead exception handling does have an overhead. That is, using finally is only valid under those conditions. There are very rare, yet valid situations where a goto is the way to go.
Okay people Oh that's right just the keyword. MatthewWhited: Scoping. If you enter scope at arbitrary point it is not clear to human what constructors should be called the problem exists in C as well.
Show 12 more comments. In C, for example, I believe there are three basic scenarios where a goto is appropriate. Breaking out of a nested loop. This would be unnecessary if the language had a labeled break statement. Bailing out of a stretch of code typically a function body in case of an error or other unexpected event. This would be unnecessary if the language had exceptions. Implementing an explicit finite state machine. In this case and, I think, only in this case a goto corresponds directly to a concept in the problem domain, transitioning from one state to a specified other state, where the current state is represented by which block of code is currently executing.
Keith Thompson Keith Thompson 6, 2 2 gold badges 27 27 silver badges 34 34 bronze badges. This is actually the best answer so far — rather surprising, for a question that is one and a half years old.
In switch-statement state machine, every write to the control value is really a goto. SSSM's are often good structures to use, since they separate out the SM state from the execution construct, but they don't really eliminate "gotos". Thank you for this. The "structured program theorem" amuses me, since it tends to clearly demonstrate the point that using structured programming statements to emulate a goto is much less readable than just using a goto! Show 1 more comment. Timwi Timwi 4, 27 27 silver badges 37 37 bronze badges.
Your point is that C 's goto is not evil? If so, that is the case for every routinely used modern language with a goto construct, not only C Add a comment. Chinmay Kanchi Chinmay Kanchi 6, 2 2 gold badges 37 37 silver badges 50 50 bronze badges. When your loops are nested several levels deep, your problem is that you failed to architect your code to avoid the multinested loops. Procedure calls are NOT the enemy. Read the "Lambda: The Ultimate
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