CSC 180
Notes for Tutorial #4

Monday 7 October  
Wednesday 9 October  
============ Notes for your tutorial =========================================

 SOME SMALL PROGRAMMING EXAMPLES
================================

 - Some suggested problems to solve:

     I  o Write a program that asks the user to enter a postitive 
          integer and then prints the sum of the digits in the number.

          The user interface might looks somthing like:
             This program sums the digits in an integer.
             Enter a positive integer:  1729 <Enter>
             The sum of digits is 19

         <To develop the algorithm and program, ask them what approach
          they can take to get the digits.  Should they start with the
          leading digit or the trailing digit?>

          <(Try writing the exit condition as n >= 0 first, and then
           figure our why it needs to be n >0 ) >

          A sample solution follows.

    II  o Note that our solution to I does not explicitly check whether 
          or not the user entered a positive integer.

          What happens if the user enters an negative or 0 input?

          <Trace through the program to see that the original program
          "works" correctly, though an error message might be nice
          if invalid input were entered.>

   III  o Modify the program for I so that it repeatedly processes positive
          integers until the user enters a number <= 0.

          <They haven't seen nested loops yet - so introduce the terms
          nested loop, inner loop and outer loop.>

          <Trace through the loops a few times, so they can see how 
          everything works.>

          <Point out that the initialization of the inner loop needs to
          be repeated.>


   IV
          Rewrite the program for I so that instead of adding the digits
          in the number, it generate the number that has the same digits,
          but in reverse order, as illustrated by this sample run:

            This program reverses the digits in an integer.
            Enter a positive integer:  1729 <Enter>
            The reversed number is 9271

          <Try to get some of the more inexperienced programmers to give
          ideas on how to develop the algorithm.  The idea of having a 
          partial solution that you update will be new to them.>

          <To this end, perhaps start with a few examples, like reversing:
              7
              42
              624
          and ask them how they can do it.>


   V      A times-table is often used to teach people simple multiplication 
          results.

          For example, the 4 by 4 times table is:

             1x1=1   2x1=2   3x1=3   4x1=4
             1x2=2   2x2=4   3x2=6   4x2=8
             1x3=6   2x3=6   3x3=9   4x3=12
 
          Write a program that generates the n by n times table for
          an input value of n.  Assume that n has only one digit,
          and format the output so that each example lines up.
        
          <Try to develop the inner loop first, and then the outer loop.
          Use a format to line things up.>

  VI      The times-table has redundant information.  e.g.  2x1=2 and 1x2=2.
          How would you modify the program from V so that only new
          times are printed?

          <Consider changing the initialization of the inner loop
             to j = i   or the exit condition on the inner loop to be j <=i 
             but not both! >

  VII     Write a program to display the following diagram on the screen.
          The number of rows in the figure should be a #define constant
          which has the value 8 for this sample run:

           *
           **
           ***
           ****
           *****
           ******
           *******
           ********

           <They haven't really used strings, so don't buildup a char[]
            to print.  Print each character at a time. >

           Exercise for them:  Modify the program above so that it
            generates a different triangle.  In this triangle, each line
            contains two more points than the previous line does,
            and the point of the triangle faces upwards, as follows:
                       *
                      ***
                     *****
                    *******

            If you have time, talk to them about how the original program
            will have to change.


Here are sample solutions to the problems:
===========================================

 Problem I 


main()
{
  int n, dsum;

    printf("This program sums the digits in an integer.\n");
    printf("Enter a positive integer: ");
    scanf("%d", &n);
    dsum = 0;
    while (n > 0) {
        dsum = dsum + ( n%10 );
        n = n / 10;
    }
    printf("The sum of the digits is %d\n", dsum);
}


 Problem IV
main()
{
  int n, reversedn;

    printf("This program reverses the digits in an integer.\n");
    printf("Enter a positive integer: ");
    scanf("%d", &n);
    reversedn = 0;
    while (n > 0) {
        reversedn = reversedn * 10 + ( n%10 );
        n = n / 10;
    }
    printf("The reversed number is %d\n", reversedn);
}


 Problem V


main()
{
  int i, j, n;

    printf("This program prints a times-table.\n");
    printf("Enter a positive integer: ");
    scanf("%d", &n);
  
    printf("Here is the %dx%d times-table.\n", n, n );
    for ( i = 1; i <=n ; i++ ){
        for ( j = 1; j <=n ; j++ ){
            printf("%dx%d=%-4d", i, j, i*j );  /* note format to line up. */
        }
        printf("\n");  /* so next line starts on a new line */
    }
}


Program VII


main()
{

#define n 8

    int i, j;

    for (i = 1; i <= n; i++ ){
        for (j = 1; j <= i; j++ ){
            printf("*");
        }
        printf("\n");
    }
}