Lesson 8:

Loops - Repeatedly execute a sequence of statements

1. The do loop repeatedly executes the statements as long as the boolean expression is true.
   
   do
      ... statements ...
   while ( boolean expression) ;
   
2. The while loop repeatedly executes the statement (or statements if enclosed in { } ) as long as the boolean expression is true. Similar to the do loop except that the boolean expression is tested before the statements are executed (and may cause the loop to terminate immediately).
   
   while ( boolean expression)
       statement 
   
3. The for loop performs an initial statement before the loop is started.  The test expression is evaluated on each pass through the loop and the loop exits if it is false.  If the test expression is true, the loop body is performed, after which the iteration expression is executed.
   
   for ( initial statement ; test expression ; iteration expression)
       statement 
   

Arrays  - An indexed collection of data elements, all of the same type

1. To declare  an array we specify the type of the elements in the array followed by [] and the name of the array.
   
   type [] arrayName;
   
2. To create an array we can use the new command with the type and size of the array or we can specify initial values enclosed in { }.
   
   arrayName  = new type[ size ];
   
   or
   
   arrayName  = { value of element 0, value of element 1, ...  }
   
3. To use an array element we give the array name followed by the index enclosed in [ ].
   
   arrayName [ index ]
   

Try this:

1. Take a look at the file Loop1.java from the web page.  Try it out.  Be sure you try entering both positive and negative values for the loop termination.
2. Change the code so that the applet counts down from the entered value down to 0.
3. Change the code so that it behaves the same way but uses a for loop rather than a while loop.
4. Here is an interesting mathematical function: suppose you have a value x.  If x is even, the function returns x/2.  If x is odd, the function returns 3x+1.  If we iteratively apply this function to numbers larger than 1, mathematicians believe the sequence of values eventually returns to 1.  Write a little program (you can use Loop1.java as a starting point) in which the user enters a starting number and then we loop and print out the values as long as x is not equal to 1.
5. Take a look at the file Array1.java from the web page.  Try it out.  Be sure to try search for values that are in the array and values that are not. 
6. Change the code so the the applet prints ALL the locations that the target value is found.  You can print to the console if you wish.
7. Change the code so that you add a second array with indices from 0 to 9.  Use that second array to count the frequency of the values in the original array.  For example,  the value 3 appears twice in the original array so your array at index 3 would have the value 2. Print the frequencies after they are all computed to the console.