Module 6: More Loopiness: while-loops, do-loops, for-each

Consider this simple problem:

• We call Math.random() repeatedly.
• Suppose we are interested in getting a number between 0 and 0.01. It might take several tries (calls to Math.random()).
• The first trial is trial 0, the second one is trial 1, etc.
         ⇒ We'd like the trial # when we get our desired result.

Here's one attempt:

6.1 Exercise: Add a println to the above program to print out each value of x generated, writing your code in MyRandomCount.java. Run the above program several different times (because the output is random). How many trials are needed to get a random number generated that's between 0 and 0.01?

6.2 Exercise: There is a problem with the above program. To see it, suppose we change the question to: how many trials are needed to get a random number generated that's between 0 and 0.95?

To address the problem, we need stop updating trial once we get the first "hit"

We can do this using a boolean so-called "flag" variable:

• Here, done is initially false.

• The original body of the for-loop is now in an if-statement that tests done.

• The code inside is executed only if done is false:

  

• This way, when we want to not execute the code inside, we change done to true.

  

  This way, the if-body does not get executed after that, even if the for-loop runs on.

6.3 Exercise: Write this up in MyRandomCount2.java. Again, add one println just inside the for-loop that prints the current values of i and done. Add another println inside the if so that each value of x generated is printed. Change 0.95 to 0.25. Then execute a few times to see how this is working.

Preventing further execution of the for-loop:

• There is a more elegant way to stop further execution of a loop.

• We can do this using a break statement:

  

• When the break executes, execution continues after the for-loop
         ⇒ It's a way to jump out of a for-loop.

6.4 Exercise: Try the above program in MyRandomCount3.java to see that it works. Change the problem back to the first one, of seeing how many trials are needed to generate the first number less than 0.01. Again, add a println to print each generated value.

The problem we have not yet solved: what if the for-loop upper-limit is not enough?

To solve this problem, we will use a while loop:

• Here, you can read the while statement as "as long as (x > 0.01)" ... (execute the loop body).

6.5 Exercise: Try the above program MyRandomCount4.java to see that it works. Again, add a println inside the loop at the end, to print each generated value.

Initialization for a while loop:

• One has to be careful when initializing the variables that are tested in a while-loop.

• Notice that the following would not work:

       int trial = 0;
       double x = 0;
       while (x > 0.01) {
           trial++;
           x = Math.random ();
       }
       System.out.println (trial);
       

6.6 Exercise: Why not? Explain the above issue in module6.pdf

6.7 Audio:

In the sequel, we'll look at some examples of both while-loops and the break statement.

Rules of thumb about which to use:

• When you do NOT know the upper limit, use a while-loop.

• When you do, prefer a for-loop (even though you can still use a while-loop).

• If you need to stop a loop after some condition is true, then use break.