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C++: what did I do wrong?

  1. Jun 10, 2012 #1
    Since I'm doing this for a problem of sorts on a site anyone can join, I won't release my whole code, but here's what I tried.

    Code (Text):
    #include <iostream>

    enum month {jan,feb,mar,apr,jun,jul,aug,sep,oct,nov,dec};
    enum weekday {sun,mon,tue,wed,thur,fri,sat};

    int year;
    int numberofdays(month);
    void plusone7(weekday&);

    int main()
    {
        int number = 0;
        int suns = 0;
        //Some code
        std::cout << number << " days total." << std::endl;
        weekday dayofweek;
        for (int day = 1,dayofweek = mon;day<=number;day++,plusone7(dayofweek)) //"No matching function for call to plusone7"
        {
            if (dayofweek == sun)
            {
                suns++;
            }
        }
        std::cout << suns << " is the number of sundays" << std::endl;
    }

    //Defines a few functions

    void plusone7(weekday& added)
    {
        if (added == sat)
        {
            added = sun;
        }
        else
        {
            added++;
        }
    }
    But the prototype and the bit at the end would make there a matching function, I thought?
     
  2. jcsd
  3. Jun 10, 2012 #2

    jtbell

    User Avatar

    Staff: Mentor

    How do you know there's something wrong? Do you get a compiler error message? Does it compile, but not produce the output you expected?
     
  4. Jun 10, 2012 #3
    No, it doesn't compile, the error message is put in as a comment in the code.
     
  5. Jun 10, 2012 #4

    jtbell

    User Avatar

    Staff: Mentor

    Aha, I missed the error message before because it was out of view to the right. I had to scroll the text-box over, in order to see it.

    The prototype for plusone7() looks like it matches OK.

    Here's a possibly wild guess: it might be that you can't use a void function with the comma operator. The expression "day++,plusone7(dayofweek)" wants to return the value of plusone7(dayofweek), so the compiler may be looking for a non-void version of plusone7().

    Try moving plusone7(dayofweek) to the beginning of the loop body and see if that makes a difference.

    Do a Google search for "comma operator in C++" and you'll find some explanations of how the comma operator works.
     
  6. Jun 10, 2012 #5
    Huh.

    Code (Text):
        for (int day = 1,dayofweek = mon;day<=number;day++)
        {
            if (dayofweek == sun)
            {
                suns++;
            }
            plusone7(dayofweek);
        }
    doesn't help.
     
  7. Jun 10, 2012 #6
    Try defining the function plusone7 at the beginning of your script (immediately after calling int main{}, or at least before calling it during your loop). Some compilers are rather funny about these things.
     
  8. Jun 10, 2012 #7
    Also tried that, unfortunately didn't help. :(

    I did figure out what was wrong, though: http://www.cplusplus.com/forum/beginner/72969/
     
    Last edited: Jun 10, 2012
  9. Jun 10, 2012 #8

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Lesson to be learned here: Beware the comma operator my son.

    Especially beware thinking that a comma is the comma operator when it is in fact that comma is a separator.

    The comma operator can be oh-so-nice in collapsing multiple lines into one, particularly with for loops. My advice is to avoid that temptation toward that kind of cleverness. Don't do it unless the alternative is even harder to understand.
     
  10. Jun 11, 2012 #9

    Borek

    User Avatar

    Staff: Mentor

    Wow, a nice one. I think it has quality of a good homework or test problem for the C++ class.
     
  11. Jun 14, 2012 #10
    @Borek: It's for a problem from a very well-known site.
     
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