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Home Iteration More Generalization
 
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More Generalization

As another example of generalization, imagine you wanted a program that would print a multiplication table of any size, not just the 6x6 table. You could add a parameter to printMultTable:

  public static void printMultTable (int high) {
    int i = 1;
    while (i <= high) {
      printMultiples (i);
      i = i + 1;
    }
  }

I replaced the value 6 with the parameter high. If I invoke printMultTable with the argument 7, I get

1   2   3   4   5   6
2   4   6   8   10   12
3   6   9   12   15   18
4   8   12   16   20   24
5   10   15   20   25   30
6   12   18   24   30   36
7   14   21   28   35   42

which is fine, except that I probably want the table to be square (same number of rows and columns), which means I have to add another parameter to printMultiples, to specify how many columns the table should have.

Just to be annoying, I will also call this parameter high, demonstrating that different methods can have parameters with the same name (just like local variables):

  public static void printMultiples (int n, int high) {
    int i = 1;
    while (i <= high) {
      System.out.print (n*i + "   ");
      i = i + 1;
    }
    newLine ();
  }

  public static void printMultTable (int high) {
    int i = 1;
    while (i <= high) {
      printMultiples (i, high);
      i = i + 1;
    }
  }

Notice that when I added a new parameter, I had to change the first line of the method (the interface or prototype), and I also had to change the place where the method is invoked in printMultTable. As expected, this program generates a square 7x7 table:

1   2   3   4   5   6   7
2   4   6   8   10   12   14
3   6   9   12   15   18   21
4   8   12   16   20   24   28
5   10   15   20   25   30   35
6   12   18   24   30   36   42
7   14   21   28   35   42   49

When you generalize a method appropriately, you often find that the resulting program has capabilities you did not intend. For example, you might notice that the multiplication table is symmetric, because ab = ba, so all the entries in the table appear twice. You could save ink by printing only half the table. To do that, you only have to change one line of printMultTable. Change

      printMultiples (i, high);

to

      printMultiples (i, i);

and you get

1
2   4
3   6   9
4   8   12   16
5   10   15   20   25
6   12   18   24   30   36
7   14   21   28   35   42   49

I'll leave it up to you to figure out how it works.
Home Iteration More Generalization

Last Update: 2011-01-24