Problem with goto

This is a discussion on Problem with goto within the C++ Programming forums, part of the General Programming Boards category; Code: teste: cout << "Nath, digite ate que numero deseja pesquisar por numeros primos.\n"; if(!(cin >> MAX)){ cout << "Erro! ...

  1. #1
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    Problem with goto

    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);
    }
    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)
    Why?

    Thanks.

  2. #2
    C++ Witch laserlight's Avatar
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    Have you considered using a loop?
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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    Why use a loop? My code is supposed to work on this way..

  4. #4
    The superhaterodyne twomers's Avatar
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    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?

  5. #5
    Reverse Engineer maxorator's Avatar
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    Questions and Suggestions:
    -- Use tabs (or at least spaces)
    -- I think that return(0) in the IF statement is useless
    -- MAX is an int?
    "The Internet treats censorship as damage and routes around it." - John Gilmore

  6. #6
    C++ Witch laserlight's Avatar
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    Why use a loop?
    Structured programming.

    My code is supposed to work on this way..
    Write it as a loop first and post it. If you cannot even do that, you have no business talking about gotos.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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    Yes the return is useless, i forgot to get rid of it.
    and yes, MAX is an int

  8. #8
    Reverse Engineer maxorator's Avatar
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    Quote Originally Posted by Tropicalia
    Why use a loop? My code is supposed to work on this way..
    Your code is more like assembly...

    Goto is quite "deprecated" feature in C++
    "The Internet treats censorship as damage and routes around it." - John Gilmore

  9. #9
    C++ Witch laserlight's Avatar
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    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.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  10. #10
    The superhaterodyne twomers's Avatar
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    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";
    }

  11. #11
    (?<!re)tired Mario F.'s Avatar
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    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.
    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

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    twomers. your code show the cout message in the while two times if I input one string.

  13. #13
    C++ Witch laserlight's Avatar
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    Anyways, there is an error because if(!(cin >> MAX)) will return false if the user input is valid.
    That looks expected. It looks like that is intended to print an error message if the user input is invalid.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  14. #14
    (?<!re)tired Mario F.'s Avatar
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    Yes... just noticed that. my bad.
    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  15. #15
    Reverse Engineer maxorator's Avatar
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    Quote Originally Posted by Mario F.
    Anyways, there is an error because if(!(cin >> MAX)) will return false if the user input is valid.
    I thought of saying something about that IF statement, but I was afraid I'm wrong.
    "The Internet treats censorship as damage and routes around it." - John Gilmore

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