Thread: switch () {default: <something>; case ...

  1. #1
    {Jaxom,Imriel,Liam}'s Dad Kennedy's Avatar
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    switch () {default: <something>; case ...

    Consider the following:
    Code:
    #include <stdio.h>
    
    
    int main(void)
    {
            int i = 3;
    
            switch (i){
                    default:
                            i = -1;
                            break;
                    case 1:
                    case 2:
                    case 3:
                            i = 5;
                            break;
            }
    
            printf ("i = %i.\n", i);
            return 0;
    }
    Output is:
    i = 5.

    Is this a definition of the standard, or is this compiler specific (ie, gcc). This code example is one that I tried and it works as if the default was at the end, but I would think it would match the default FIRST and never test against 1,2,3.

    The code that coRnfused me was from the Linux kernel in the file kernel/posix-cpu-timers.c.

    Thanks in advance for your answers.

    Andy

  2. #2
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by Kennedy
    Is this a definition of the standard, or is this compiler specific (ie, gcc).
    Well...
    Quote Originally Posted by C99 Clause 6.8.4.2 Paragraph 5 (part)
    If a converted value matches that of the promoted controlling expression, control jumps to the statement following the matched case label. Otherwise, if there is a default label, control jumps to the labeled statement.
    Hence the behaviour follows that described by the standard.
    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

  3. #3
    {Jaxom,Imriel,Liam}'s Dad Kennedy's Avatar
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    Hmm, so then everyone that I polled here believed it to be a syntax error, however, it worked, based upon the standard. So, this leads me to believe that near everyone is taught that default must come at the end, which is erroneous.

    Yet again, I have found something I was taught to be fact as false.

    Bummer man.

    Thanks laserlight!

    Andy

  4. #4
    and the hat of int overfl Salem's Avatar
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    It's only stylistic convention that places it at the end.

    For extra fun, mis-spell it as defualt and see how long it takes people to spot it
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  5. #5
    and the hat of int overfl Salem's Avatar
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    > Hmm, so then everyone that I polled here believed it to be a syntax error
    Better not show them Duff's device then, they'd all explode!
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

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