CS51 - Fall 2009 - Lecture 23

while loops
   - while(<condition>){
      // do the following
   }
   - as long as <condition> is true, continue in the loop

Take a look at our FallingLeaves demo
   - what pattern did we use to generate 10 leaves in Tree.java?
   - let's look at the construction of the while loop in Tree.java
      - What role does the treeCount variable play?
      - initialize a variable for counting
      - while loop
         - conditioned on the counter
         - inside the while loop we increment the count

      <initial_statement>
      while( <condition> ){
         // do the following
         <update_statement>
      }

For loops
   - don't add any functionality to the language
   - makes our life easier
   - makes code easier to understand for certain types of loops
   - for( <initial_statement>; <condition>; <update_statement>){
      // do the following
   }
   - When we first execute the for loop, execute <initial_statement>
      - often used to initialize a counter variable
      - only happens once
   - As long as <condition> is true, continue in the loop
   - At the end of each iteration in the loop, execute <update_statement>
      - often used to increment a counter variable
      - happens EVERY iteration

How can we change our Tree class to use a for loop?

Show Knitting demo again

Show Knitting code
- class: change this code to use for loops

Look at Recursive Fibonacci again
   private int Fib(int n){
      if( n <= 2 ){
         return 1;
      }else{
         return Fib(n-1) + Fib(n-2);
      }
   }
   - How efficient is Fibonacci?

Look at Fibonacci without recursion
   - Do Fib(10) on paper...
   - How did you do it?
   - Started at 1, 1, then wrote the numbers out
   - We'd like to write a function that does it this way
   - What do we need?
      - We need some way of keeping track of the intermediary numbers
   - How many numbers do we have?
      - n, if we include the final number
   - Arrays allow us to keep track of a sequence of things, using just one variable
   - Arrays are just like other variables:
      - They have a declaration
         - <type_of_thing>[] <variable_name>;
         - int[] a;
         - double[] b;
         - FramedRect[] c;
      - We can create new arrays - we must specify the length!
         - a = new int[10];
         - b = new FramedRect[1000];
      - Array variables are like any other variables, b's type is "FramedRect[]" for example
      - An array, contains a set of index variables
         - a[index] is just another variable!
         - They are indexed starting at 0, so 0 is the first element, 1 the second, etc
         - We can assign to individual elements of arrays
            - a[0] = 50;
            - b[30] = new FramedRect(...);
         - We can access elements in an array
            - int val = a[1] + a[2];
            - b[30].move(...);
   - Back to Fibinacci... How do arrays help us?

   private int Fib(int n){
      int[] fib = new int[n];

      fib[0] = 1;
      fib[1] = 1;

      for( int i = 2; i < n; i++ ){
         fib[i] = fib[i-1] + fib[i-2];
      }

      return fib[n-1];
   }

Show SlidesF09 demo: how can we do this? Hint: use arrays!

Look through SlidesF09 code