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Actually, if i dont input a number, the code is entering 3 times in the IF before ask for another input. (and it dont show the first "cout" message)Code:teste:
cout << "Nath, digite ate que numero deseja pesquisar por numeros primos.\n";
if(!(cin >> MAX)){
cout << "Erro! Apenas numeros sao validos! (0123456789)\n";
cin.clear();
cin.ignore();
goto teste;
return(0);
}
Why?
Thanks.
Have you considered using a loop?
Why use a loop? My code is supposed to work on this way..
I can't remember, I don't use goto much, but I think
teste:
should be
teste:;
I could be absolutely wrong of course.
The thing people say about goto is that you shouldn't really use it unless there isn't really a viable alternative. Here, as laserlight said, a loop would suffice, so why not just use one?
Questions and Suggestions:
-- Use tabs (or at least spaces)
-- I think that return(0) in the IF statement is useless
-- MAX is an int?
Structured programming.Quote:
Why use a loop?
Write it as a loop first and post it. If you cannot even do that, you have no business talking about gotos.Quote:
My code is supposed to work on this way..
Yes the return is useless, i forgot to get rid of it.
and yes, MAX is an int
Your code is more like assembly...Quote:
Originally Posted by Tropicalia
Goto is quite "deprecated" feature in C++
Incidentally, I do not really see anything wrong about the code (other than the use of an unnecessary goto). I decided to test, and the programs runs as I would expect. Rather strange that you had a problem.
Code:cout << "Nath, digite ate que numero deseja pesquisar por numeros primos.\n";
while ( !(cin >> MAX) )
{
cout << "Erro! Apenas numeros sao validos! (0123456789)\n";
cin.clear();
cin.ignore();
cout << "Nath, digite ate que numero deseja pesquisar por numeros primos.\n";
}
Because this use of goto is bad. There's a maybe a couple of situations where goto makes sense. Other than that it's best if you quickly learn how to use loops.
Anyways, there is an error because if(!(cin >> MAX)) will return false if the user input is valid. That is why the got statement is never read. The if statement returns false.
twomers. your code show the cout message in the while two times if I input one string.
That looks expected. It looks like that is intended to print an error message if the user input is invalid.Quote:
Anyways, there is an error because if(!(cin >> MAX)) will return false if the user input is valid.
Yes... just noticed that. my bad.
I thought of saying something about that IF statement, but I was afraid I'm wrong.Quote:
Originally Posted by Mario F.
>> twomers. your code show the cout message in the while two times if I input one string.
Get rid of one then. I don't understand the language, so I didn't know what it said. So I didn't know if it was valid in the loop.
In twomers code, if the input has 1 char, it gives one error message, if the input has 2 chars it gives two error messages, and there you go...
Code:cout << "Input a number please.\n";
while ( !(cin >> MAX) )
{
cout << "Error! Input numbers please! (0123456789)\n";
cin.clear();
cin.ignore();
cout << "Input a number please.\n";
}
twomers' code looks fine. I suggest that you post the smallest and simplest compilable program that demonstrates what you want to do.Quote:
twomers. your code show the cout message in the while two times if I input one string.
Twomers code still uses that bad IF statement.Quote:
Originally Posted by laserlight
Wow, fastest thread ever! About one post per minute!!
Mario (or laserlight, or anyone really), where would be a viable situation to use goto. I've read that major loop nesting is a good one. I can't think of any others really.
Oh, I see what you mean. The solution is to use cin.ignore(1000, '\n') instead of just cin.ignore(). Instead of 1000, you could use std::numeric_limits<std::streamsize>::max()Quote:
In twomers code, if the input has 1 char, it gives one error message, if the input has 2 chars it gives two error messages, and there you go...
I have seen some good uses of goto, but I don't remember where I saw them.
http://faq.cprogramming.com/cgi-bin/...&id=1043284392Code:#include <limits>
using namespace std;
int main( void )
{
int MAX;
while ( (cout << "Input a number please.\n") && !(cin >> MAX) )
{
cout << "Error! Input numbers please! (0123456789)\n";
cin.clear();
cin.ignore(std::numeric_limits < int >::max(), '\n');
}
}
Well, now it works =]
Thanks for all you guys :P
I looked at this thread for 10 seconds, pressed refresh and 2 new posts!Quote:
Originally Posted by twomers
(48-23)/25 wow! 1 post per minute
> Mario (or laserlight, or anyone really), where would be a viable situation to use goto. I've read that major loop nesting is a good one. I can't think of any others really.
I can't think two situations of the top of my head.
One is when you want to break completely out of nested loops. a break statement only breaks from the innermost loop. Of course an alternative was to keep a flag and test against it in each loop. But a goto can be important especially on situation where you want to extract every bit of performance out of your loops.
Another is less common... but far more important. When generating code from a grammar through a parser. If you design a script language that generates code in the fly, you will find the use of goto commands quiet useful in your C++ script parser.
should beCode:return(0)
But this may depend on your environment, I am just stating what is classed as standardCode:return 0
the only place i use goto is to write programs on my calculator and the PIC microprocessor (asm). You should listen to those who urged you to use a loop instead of goto. It is not recommended to use goto since its execution causes an unconditional jump ignoring any type of nesting limitations. goto really does not have a place in structured or object oriented programming.
@cdkiller:
I use goto in graphic programming, for example:
But I do see your point, goto is good at plotting coordinates like above.Code:gotoxy(5,1);cout << "PLOT THIS STRING HERE";
But I have never used it in any sort of "other" C++ programming.
that is a function that just happens to have a name that starts with "goto"... Not a goto statement. There's a world of difference.
Goto has its uses. And yes, it has a place on structured programming, object oriented programming, procedural programming and all the x programmings out there.
Mario. I guess in a way i see your point. in my first programming course i was a big fan of goto in my projects but the teacher did not like us using goto. i always thought there must have some reason why the creators of C++ put a goto function so why can't i get credit for using it. up to now i am not exactly clear as to why most books and other resources direct people away from goto, but i think what they really mean to say is to not use goto for control structures. but like swgh said, goto has other uses.
anyway hopefully somebody could tell us why we are schooled to shy away from goto and #define. speaking of that, i hate loud cars that just pass by and make so much noise for no good reason. hopefully someday soon i will complete my directed emp shockwave generator so i could teach those punks a lesson without blacking out the entire neighbourhood :D
With goto, it makes compiler optimizations very difficult; in fact often it will require a global disabling of compiler optimizing to make it actually work correctly. In general it makes the compiler's job a lot harder and there are often bugs in how goto is implemented which can leak memory or crash the program. In fact because many compilers will actually silently disable optimizations for code which contains goto statements, it's often far less efficient.Quote:
Originally Posted by cdkiller
As to #define, it does not respect any scope rules; it's essentially a "find and replace" that happens in preprocessing. There are other methods to solve these problems that do obey scope rules.
In general:
If the define is for a constant, use const int instead.
If it's for aliasing a typename, use typedef instead.
Not really...Anyone remember this thread?Quote:
Originally Posted by twomers
Back on topic though:
Maybe it was included because C had goto? Sometimes it might be more elegant to use goto in a case such as this (arguably, of course):Quote:
Originally Posted by cdkiller
However, in C++, I don't know if I've ever seen a case where goto was a good solution (consider in this case that if RAII is used, object destructors can perform cleanup)Code:void Foo(void)
{
//...
if(!Bar1())
goto Cleanup;
if (!Bar2())
goto Cleanup;
Cleanup:
//deallocation, etc.
}
As I said before on this thread there's at least three justifications for goto.
1. Exiting deeply nested loops.
2. Perfomance critical situations.
3. Parsers and code generators.
I disagree here. Many compilers have a very hard time with goto because it often seriously impairs their ability to optimize code. Often the compiler will choose not to optimize the code at all in the presence of goto statements.Quote:
Originally Posted by Mario F.
It all has to be evaluated Cat.
Not all situations impair the compiler from performing its optimizations. And certainly on those that do, some can also be dealt with by changing the way we code (and not necessarily removing the goto statement) and yet others by tweaking with the compiler options.
And then we have those situations that simply cannot be dealt with in the presence of a goto statement. For those, I give credit to your words.
Compilers are not that dumb in the face of a goto. It all depends on how it is used. Certainly a performance conscious developer, coding a high-performance algorithm will have the necessary knowledge to evaluate if a goto will improve his code or not. More often than not, it will indeed.
I don't know where you get this idea that goto statements seriously impair optimizations. I'm sure there are cases where they do. But I'm sure there are cases... where they don't.