ITEC 120 2007fall ibarland, jdymacek

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lect11b
for-each
looping over a list

review

Declare a list of Doubles; add three numbers to it; retrieve the last number in the list; check whether Math.PI is in the list.

Lists

We can process each element of a list, no matter how long the list is! Java provides a new type of statement, the “for-each loop”:

for ( Double d : dubs ) {  // "for each d in dubs, ..."
  System.out.println( "Currently looking at: " + d.toString() );
  }

Now, we already know that printing to the console is something which we can't build on with other methods, since it doesn't actually return any answer. So we can use loops to actually compute something:

/* Add up elements of a list, with a for-each loop. */
double totalSoFar = 0.0;   // The running total -- an "accumulator" variable.
for ( Double d : dubs ) {  // "for each d in dubs, ..."
  totalSoFar = totalSoFar + d.doubleValue();
  }
// For-each is only appropriate when we want to handle every element exactly the same.

// To-do:
// - write a function average, which computes 
//   the average of a (non-empty!) list of numbers.
// - Augment the above code so you also accumulate the sum-of-squares
//    (or, the absolute values)
// - Augment the above code so you also accumulate a String, with each
//    number separated by a space, with angle-brackets on each side:
//      "< 5.4 -1.2 4.2 0.0 5.4 >"

Lists aren't at all specific to Doubles; they can hold any sort of object at all. Note that technically, the list holds a reference to objects. Let's draw pictures on the board:

// ======================================

Dog d1 = new Dog( "lassie", 15 )
Dog d2 = new Dog( "fido", 3 )
Dog d3 = new Dog( "phydeaux", 5 )

java.util.LinkedList<Dog> pound = new java.util.LinkedList<Dog>();

pound.add( d1 )
pound.add( d2 )
pound.add( d3 )
pound.toString()
pound.contains( d1 )
pound.contains( new Dog( "fido", 3 ) )   /* .equals, or == ?  See docs. */
pound.contains( "fido" )
pound.remove( new Dog( "fido", 3 ) )     /* .equals, or == ?  See docs. */
pound.toString()
pound.remove( d1 )
pound.toString()

a Blah is not a List-of-Blah

A number is not a list of numbers; a Dog is not a LinkedList<Dog>. Think of the list as a bag. The bag might even be empty. It is a common confusion for people to declare a field to be (say) a Treasure, when really what they want to keep track of is a LinkedList<Treasure>. Or, to ask a LinkedList<Dog> to speak(), even though that's a behavior of things in the bag, not a behavior of the bag itself. (Use a for-each loop to work with each object inside the bag.)

Which of the following method calls make sense?

1. pound.getSound()
2. d1.get(1)
3. pound.get(1)
4. d1.get(1)
5. Dog d = pound.get(1) followed by d.getSound()
6. (pound.get(1)).getSound()

Looping over lists of objects

to do:

• Write a method which computes the total age of all dogs in a list-of-dogs.
• Write a method cacophany which returns one string containing the sound of each dog in the list, all strung together. (For instance, for a list of four dogs, it might return the single string “woof arf woof rrrruff”.)

Things to ask yourself, when writing a loop:

• What is a sample test case list, and what will my answer be?
• What information do I need, from each item in the list?
• What information will I accumulate? When I'm halfway through the loop, what will that piece of information be?
• How do I update my accumulated-information, upon seeing one more item? This will be the task you do over-and-over, in the body of the loop!

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©2007, Ian Barland, Radford University Last modified 2007.Nov.01 (Thu)

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