Recursive example

This is a discussion on Recursive example within the C Programming forums, part of the General Programming Boards category; This was given as an example of recursivity. It prints 123456789987654321. Code: #include <stdio.h> void printnum ( int begin ) ...

  1. #1
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    Recursive example

    This was given as an example of recursivity. It prints 123456789987654321.

    Code:
    #include <stdio.h>
    
    void printnum ( int begin )
    {
        printf( "%d", begin );
        if ( begin < 9 )         /* The base case is when begin is no longer */
        {                           /* less than 9 */
            printnum ( begin + 1 );
        }
        /* display begin again after we've already printed everything from 1 to 9
         * and from 9 to begin + 1 */
        printf( "%d", begin );
    }
    
    int main() {
        printnum(1);
    }
    Now, I can see this code printing 1234567899, but whence does it count back down? I don't see where the code goes back and counts down.

    And, I know this has nothing to do with that, but it's easier than two threads. Is it possible to write a program that will open another progam? Such as, a program that will accept integers to select from a menu of programs, like notepad = 1, and then run that program's executable, and close itself?

  2. #2
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    Quote Originally Posted by CodeGeek20 View Post

    Now, I can see this code printing 1234567899, but whence does it count back down? I don't see where the code goes back and counts down.
    The easiest way to see what is going on is to run your program through a debugger.

    What's happening is the call to printnum(9) is called from printnum(8). So when it returns, it returns back to printnum(8), which continues to execute it's last line, before returning to printnum(7). This goes on untill printnum(1), which returns to main, and ends the program.

    And, I know this has nothing to do with that, but it's easier than two threads. Is it possible to write a program that will open another progam? Such as, a program that will accept integers to select from a menu of programs, like notepad = 1, and then run that program's executable, and close itself?
    Yes, but it'd be platform specific.
    It is too clear and so it is hard to see.
    A dunce once searched for fire with a lighted lantern.
    Had he known what fire was,
    He could have cooked his rice much sooner.

  3. #3
    Registered User NeonBlack's Avatar
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    Also, it's not very safe. If someone moves or renames notepad, or worse, replaces it with another executable, your program breaks.
    Why do you want to do that? If you want to make a task launcher, fine, but otherwise there is probably a better way.
    I copied it from the last program in which I passed a parameter, which would have been pre-1989 I guess. - esbo

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