ITEC 120 2008spring ibarland, jdymacek

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lect12c
while loops, continued
closing thoughts

lists and while

Processing a list using while and get() For example:
Suppose doggies is a list of Dog.

// A for-each loop to add all the ages of every Dog in doggies.
int agesSoFar = 0;
for ( Dog d : doggies ) {
  agesSoFar = agesSoFar +1 d.getAge();
  }

But it turns out, before Java 1.5, the Java language had while loops, but no for-each loop! How on earth did programmers add up the ages in a list-of-Dogs? Well, a while loop can suffice, but we need to get d to refer to a different Dog each time through the loop?

// A for-each loop to add all the ages of every Dog in doggies.
int agesSoFar = 0;
Dog d;
                
while (                  ) {
                  
  agesSoFar = agesSoFar + d.getAge();
                  
  }

• Loop control logic We need to have d refer (successively) to the first Dog in the list, then the second Dog in the list, then the third Dog in the list, etc.. We can do this by declaring our own counter variable, nextDogIndex, and using the LinkedList method get(int). Every time through the loop, we'll increment that by one:

  // A while-loop to add all the ages of every Dog in doggies. int agesSoFar = 0; int nextDogIndex = 0; Dog d; while ( nextDogIndex < doggies.size() ) { d = doggies.get( nextDogIndex ); agesSoFar = agesSoFar + d.getAge(); nextDogIndex = nextDogIndex +1; }

• Loop initialization We still need to declare variable d, and of course we also need to declare our loop counter nextDogIndex.
• Loop body remains the same (well, I included updating the loop-variables as part of the loop-control logic, above).

Searching, revisited

We saw earlier that we could use for-each loops to search for an element of a list; in fact we saw multiple versions. Here's a version which looks for a happy dog in a list:

• boolean foundSoFar = false; for ( Dog d : aDogList ) { foundSoFar = foundSoFar || /* code for: d is a happy dog */; } // Here,foundSoFar is true iff aDogList contains a dog we're looking for.

  The drawback of this version is that if aDogList has 1000 Dogs, this for-each loop looks at all 1000 of them, even if the third Dog we find had been happy.
• Re-write this search as a while-loop, in such a way that we stop as soon as we find a happy dog (and don't search through the remaining elements of the list).

A while loops is more appropriate, since we didn't want to process every single element of the list identically. We can avoid using return as part of our loop-control logic.

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©2008, Ian Barland, Radford University Last modified 2008.Jan.24 (Thu)

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