1. Monday, 11/11/02, Slide #1 CS 106 Intro to Comp. Sci. 1 Monday, 11/11/02  Questions? HW 04 due today at 5.  Today – Lists and an introduction to searching  Looking ahead:  Friday: Lab 10  Next week: Review  Friday, Nov 22: Test #2  Reading: Chapter 10 (omit 10.7)  New files/handouts: ListFuncs.cpp

2. Monday, 11/11/02, Slide #2 Abstract Data Type List  By a list we mean a collection of data items,  All data is the same type  The data has a definite order (first, second,..., last)  Operations on list objects include  Declaring, initializing  Inputting, Outputting (Printing) the list  Inserting, deleting data items  Searching for a data item  Sorting the list

3. Monday, 11/11/02, Slide #3 Implementing ADT List with arrays  An array is a natural place to store the data items of a list. Objects needed include:  The name of the array (say, Items[ ])  The size of the array (say, MAXSIZE, a const)  The size of the list (say, Length <= MAXSIZE)  In HW #5 (after vacation), you’ll create a class to implement lists using arrays  For now, we’ll look at some list operations without using classes.  See file ListFuncs.cpp

4. Monday, 11/11/02, Slide #4 Initializing List Elements  If we know Length in advance, we can use a for-loop: for (int i = 0; i < Length; ++i) { Items[i] = 0; Items[i] = 0; //or cin >> Items[i]; //or cin >> Items[i]; //or some other way of //or some other way of //assigning values //assigning values} Index i starts at 0! Index i ends at Length-1!

5. Monday, 11/11/02, Slide #5 Initializing with while-loops  If Length is not known in advance, we can use a while-loop to initialize: int i = 0; int Val; while ((i > Val)) {Items[i] = Val; ++i;} Length = i; //now we know the length  WARNING: Watch out for off-by-one errors: executing the loop one too many or few times!

6. Monday, 11/11/02, Slide #6 Printing lists  Once a list has been initialized, we can print (or otherwise process) each element in order, using a for-loop: for (i = 0; i < Length; ++i) { cout << Items[i] << endl; cout << Items[i] << endl; // or do something else // or do something else // with Items[i] // with Items[i]}

7. Monday, 11/11/02, Slide #7 Linear Search Algorithm  Searching for an element (called the "key") in a list is a fundamental task, for which there are several different algorithms.  LINEAR SEARCH ALGORITHM:  We begin at one end of the list, and compare elements to the key, stopping when we either find the key, or reach the other end of the list.

8. Monday, 11/11/02, Slide #8 Linear Search Code; two alternatives int Index = 0; while (Index < Length && Key != List[Index]) {++Index; } //Set boolean flag for found/not found bool Found = (Index < Length); for (int Index = 0; Index < Length && Key != List[Index]; ++Index) {//do nothing!} bool Found = (Index < Length);

9. Monday, 11/11/02, Slide #9 Time Complexity of Linear Search  The time complexity of an algorithm is the approximate number of steps required to finish the job.  The number of times we compare the key to a list item gives us an idea of how long Linear Search can take.  At most n comparisons (where n is the length of the list);  On average, if key is in list, about n/2 comparisons.  Is there a better way???? (Yes! Later)