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The break statement is usually used inside loops or switch case statements. If one used it outside of these statements what kind of error would this constitute?
Would that be a syntactic or rather a semantic error?

Thanks for your time!
Posted 26-Oct-11 4:29am
Updated 26-Oct-11 5:36am
v2
Comments
Richard MacCutchan 26-Oct-11 11:23am
   
bc i m
Please don't use this childish txtspk it makes it very difficult to understand your questions.
Manfred R. Bihy 26-Oct-11 12:13pm
   
Corrected! :)
Sweety Khan 26-Oct-11 12:15pm
   
thanks :)
Sweety Khan 26-Oct-11 12:11pm
   
ok sorry for that
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Solution 2

I took me a while to check since I first had to find a grammar file for C#, but it is definitely a semantic error. The grammar specifies what are valid constructs in a language. If the written program follows the grammar then it is syntactically correct.

I'll only post the relevant parts of the grammar that will make it clear why a break anywhere in a C# program is syntactically correct, but is semantically wrong here in the sense of "There is nothing to break out of, so it does not make sense to break here.".

Excerpt of C# grammar file:
block:
	';'
	| '{'   statement_list?   '}';
 
statement_list:
	statement+ ;
 
statement:
    (declaration_statement) => declaration_statement
    | (identifier   ':') => labeled_statement
    | embedded_statement
    ;
 
embedded_statement:
    block
    | selection_statement   // if, switch
    | iteration_statement   // while, do, for, foreach
    | jump_statement        // break, continue, goto, return, throw
    | try_statement
    | checked_statement
    | unchecked_statement
    | lock_statement
    | using_statement
    | yield_statement
    | unsafe_statement
    | fixed_statement
    | expression_statement  // expression!
    ;
 
jump_statement:
	break_statement
	| continue_statement
	| goto_statement
	| return_statement
	| throw_statement ;
 
break_statement:
	'break'   ';' ;

The chain is:
block -> statement_list -> statment -> embedded_statement -> jump_statement -> break_statement -> "break"

From the grammar file we can infer that the break statement would be allowed anywhere inside a block. Thus programs that have a break statement that is not within a looping construct or a switch statement are still syntactically correct. The compiler has enough cleverness built in though to see that the break statement which is not enclosed in the aforementioned statements does not make any sense and throws a compile time error.

The error is a semantic error because the grammar allows for break statements to appear anywhere inside a block ({})

I hope I explained it well enough, if you still have questions leave me a comment.

[Update]
Forgot the link where to find said grammar file. On this site there is a download of a zip file: http://antlrcsharp.codeplex.com/[^]. In the folder UnitTest there is a file called cs.g which contains the complete grammar.
[/Update]

Regards,

—MRB
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v4
Comments
Nishant Sivakumar 26-Oct-11 11:36am
   
Excellent answer. 5!
Manfred R. Bihy 26-Oct-11 12:10pm
   
Thanks Nish!
Sweety Khan 26-Oct-11 12:19pm
   
i got it. thanks million times :)
Manfred R. Bihy 26-Oct-11 13:54pm
   
You're welcome!
Espen Harlinn 26-Oct-11 15:01pm
   
Well, 5'ed again :)
antlr is one of my favourite tools ...
Manfred R. Bihy 26-Oct-11 15:08pm
   
It's great! It is useful and has a geek factor too! :)
BillWoodruff 26-Oct-11 20:18pm
   
+5 Answers like these, imho, deserve to be "harvested" and put in a special forum for easy access, rather than submerging into the tidal flux of homework and plz-gimme.

Thanks for this articulate, well-researched, and documented complete reply.
Manfred R. Bihy 27-Oct-11 1:10am
   
Thank you Bill!
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Solution 1

It is a syntax error to have it outside of a breakable loop. It is not a semantic error because the logic is not incorrect, but simply makes no sence, i.e. syntax error.

Semantic errors are incorrect logic but correct flow. For example, I can have

bool flag = true;
bool otherFlag = false;
...
while(flag = otherFlag)
{
   ...
}

This 'could' be a semantic error. The logic of checking the flag is correct. The added effect of setting the flag to otherFlag could be the semantic error (maybe what the developer wanted though).

If however one had:
int flag = 0;
int otherFlag = 1;
...
while(flag = otherFlag)
{
   ...
}

You now have a syntax error, for it does not make sence to while on an integer. Just as it does not make sence to break from a non-existing loop.
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v3
Comments
Manfred R. Bihy 26-Oct-11 11:28am
   
You missed the point. If the grammar doesn't allow it then it's a syntax error. If the grammar would allow for it, but it just doesn't make any sense it's a semantic error.

Since the C# grammar allows for break to appear anywhere inside a block, it can only be a semantic error.

Cheers!
Collin Jasnoch 26-Oct-11 11:49am
   
Your definition is one way to look at it (breaking down the grammer file).

However, a semantic error by most definitions is a run time error also known as a logic error. You can not at run time break out of a non existing loop. That is why the compiler generates a syntax error. It may not just use the grammer file to determine syntax errors, but it is a syntax error non the less.
Manfred R. Bihy 26-Oct-11 12:23pm
   
No, sorry it is a compiler error! A syntax error would be if you spelled "break" as "bread". If the grammar cannot continue because it enounters something that cannot be recognized then it is a syntax error. You can even envision a C# language specification where not even a compile time eror would be thrown:
What if a break statement in a function/method would break out of a loop if the method was called from within a loop/switch construct. OK I admit that this is a quite horrific scenario, but if you define it that way it still is valid. It is impossible to tell by statical analysis if a break is valid as this could only be discovered at runtime.
I do recognize that there is some confusion about what exactly is a syntactic error. Most compilers even throw an syntax error even if it is formally a grammatic error.
Collin Jasnoch 26-Oct-11 12:58pm
   
OK, your scenerio does create a case where it would be a run-time error so I stand corrected.

Have a 5 :)
Manfred R. Bihy 26-Oct-11 13:11pm
   
Thanks! :)
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Solution 3

You need to make braced groups* for loop operations push a different context onto the context stack (just as the braced groups for namespaces, classes and methods each push a different type of context and have different valid operations inside them). The loop context follows the same rules as a normal code context, except that within a loop context (including child contexts) break and continue are syntactically valid.

* - including implicitly created groups that are just one statement, e.g.
foreach(var thing in list) if(thing.name == targetname) break;
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