COMP 110: Introduction to Programming Tyler Johnson Feb 11, 2009 MWF 11:00AM-12:15PM Sitterson 014

COMP 110: Spring Announcements Lab 3 due tomorrow by midnight Program 1 has been graded

COMP 110: Spring Questions?

COMP 110: Spring Today in COMP 110 Programming with Loops Programming Demo Using the Debugger

COMP 110: Spring Review The while statement can be used to construct a loop in Java while(Boolean_Expression) { Statement_1 Statement_2 … Statement_N } As long as Boolean_Expression is true, the statements in the loop body are executed

COMP 110: Spring Review Do-While Syntax do { Statement_1 Statement_2 … Statement_N } while (Boolean_Expression); //note the semicolon! Execute Statements_1…N If Boolean_Expression is true, repeat

COMP 110: Spring Review For Loop Syntax for(Initializing_Action; Boolean_Expression; Update_Action) { Statement_1 Statement_2 … Statement_N } Initializing_Action is used to initialize a counter variable Boolean_Expression is the stopping condition for the loop Update_Action is used to update a counter variable after the loop body is executed

COMP 110: Spring Review Choosing a loop Do-While When you want the body to be executed at least once Useful for checking user input For More convenient/readable when the number of iterations is known beforehand, e.g. stored in some counter variable While Safest choice, can be used to create any kind of loop When it might be necessary for the loop to iterate zero times

COMP 110: Spring The Break Statement Causes the immediately enclosing switch statement or loop to end The remainder of the loop (or switch) is not executed int i = 0; while (i < 5) { if(i == 3) break; i++; } //value of i is 3

COMP 110: Spring The Continue Statement Causes the current loop iteration to end immediately Execution resumes at the loop condition for (int i = 0; i < 5, i++) { if(i == 3) continue; System.out.println(i + ", "); } 0, 1, 2, 4, Output

COMP 110: Spring Break & Continue Avoid using break and continue inside loops Can make your programs difficult to understand!

COMP 110: Spring Nested Loops Its possible, and sometimes necessary to place loops inside other loops This is called nesting Just like with nested if-statements

COMP 110: Spring Nested Loops Example int sum = 0; int i = 0; while(i < 10) { for(int j = 0; j < 20; j++) sum = sum + j; //executes 10 * 20 times i++; //executes 10 times, i = [0, 9] }

COMP 110: Spring Commas in For Statements We can perform multiple initializations or updates using the comma operator int n, c; int product; for (n = 1, c = 1; n <= 5; c = c + 2, n++) { product = product * c * n; } System.out.println(product);

COMP 110: Spring Designing Loops Creating a loop involves designing three things Initialization of Variables Loop Body Stopping Condition

COMP 110: Spring Designing a Loop Body How to determine what statements should be inside the body of a loop? Example Calculate the sum of numbers entered by the user

COMP 110: Spring Designing a Loop Body Output instructions to the user Initialize variables Prompt user for input Read a number into variable next sum = sum + next; Prompt user for input Read a number into variable next sum = sum + next; Prompt user for input Read a number into variable next sum = sum + next;... Output sum Repeated statements become your loop body Statements that are only done once are not part of your loop body

COMP 110: Spring Pseudocode with Loop Body Output instructions to the user Initialize variables Do the following for the appropriate number of times: { Prompt user for input Read a number into variable next sum = sum + next; } Output sum Initializing statements How do we end the loop? How many iterations?

COMP 110: Spring Initializing Statements Variables used in your loop need to be initialized (set to a value) before the loop next Read a number into variable next We read a new value for next before using it during each iteration of the loop so we do not need to initialize it sum sum = sum + next; sum is on the right side of an assignment statement. sum MUST have a valid value before the loop starts.

COMP 110: Spring Initialize Sum What should sum be initialized to? Consider the first iteration After executing the first iteration, the expected value of sum should be sum == next (the first input value) The assignment statement is sum = sum + next; Therefore, initial value of sum is sum = 0;

COMP 110: Spring Loop with Initializations Output instructions to the user sum = 0; Do the following for the appropriate number of times: { Prompt user for input Read a number into variable next sum = sum + next; } Output sum Initializing statements

COMP 110: Spring Ending Loops How to choose a stopping condition for your loop? Count-controlled loops When you know number of loop iterations for(count = 0; count < iterations; count++) User-controlled loops Ask-before-iterating Sentinel value Boolean The value of a boolean indicates whether the loop should stop

COMP 110: Spring Count-Controlled Loops A program to average exam scores input by the user We know the size of the class is "numStudents" double next, sum = 0, average; int i; for(i = 0; i < numStudents; i++) { //# iterations is fixed next = keyboard.nextDouble(); sum = sum+next; } average = sum/numStudents;

COMP 110: Spring Off-by-One errors Loop repeats one too many or one too few times How many times do these loops iterate? for (count = 1; count < 10; count++); Loop for count = [1,9] (9 iterations) for (count = 0; count <= 10; count++); Loop for count = [0,10] (11 iterations) for (count = 1; count <= 10; count++); Loop for count = [1,10] (10 iterations)

COMP 110: Spring Ask-Before-Iterating Sometimes it is useful to have the user decide when a loop should end String answer; do { //perform some computations System.out.print("Continue? yes/no"); answer = keyboard.next(); } while (answer.equalsIgnoreCase("yes"));

COMP 110: Spring Sentinel Value Signal end of input System.out.print("Enter a negative number to end the loop"); int next = keyboard.nextInt(); int sum = 0; while (next >= 0) { sum = sum + next; System.out.print("Enter a number: "); next = keyboard.nextInt(); }

COMP 110: Spring Booleans int next, sum = 0; boolean moreNumbers = true; Scanner keyboard = new Scanner(System.in); while(moreNumbers ) { next = keyboard.nextInt(); if (next < 0) moreNumbers = false; //this will be the last iteration else sum = sum + next; } System.out.print("The sum is " + sum);

COMP 110: Spring Exercise What does the following display? int product = 1; int max = 20; for(int i = 0; i <= max; i++) product = product * i; System.out.println("The product is " + product); The product is 0 Output

COMP 110: Spring Example Problem Find the lowest and highest ages in the class

COMP 110: Spring Loop Body Get age of student 1 Update min/max ages Get age of student 2 Update min/max ages... Get age of student 37 Update min/max ages End loop Output min and max ages

COMP 110: Spring Min/Max Ages int min = 2000; // initialize to large value int max = 0; // initialize to small value for (int count = 1; count <= 37; count++) { //Ask student #count for age if (age > max) max = age; if (age < min) min = age; } //Output min and max ages

COMP 110: Spring Min/Max Ages if (age > max) max = age; if (age < min) min = age; Agesmaxmin …

COMP 110: Spring Debugging What to do if theres something wrong with your program, but youre not sure what? Trace the variables Watch the variables change as the program executes

COMP 110: Spring Tracing Variables Two ways to trace variables Manually Using print statements Automatically Using the debugger

COMP 110: Spring Manual Tracing Insert print statements to output the values of variables System.out.print("Enter a negative number to end the loop"); int next = keyboard.nextInt(); int sum = 0; while (next >= 0) { System.out.println(next); sum = sum + next; System.out.println(sum); System.out.print("Enter a number: "); next = keyboard.nextInt(); }

COMP 110: Spring Using a DEBUG flag Use a DEBUG flag to disable the tracing of variables when no longer needed final boolean DEBUG = true; //trace variables? … if(DEBUG) { //print the values of variables }

COMP 110: Spring Automatic Tracing Use a debugger jGRASP provides an integrated debugger Allows you to step through your program to see how variables change with each line of code

COMP 110: Spring Programming Demo Hand-shaking problem Given a group of n people, everyone shakes hands with everyone else How many handshakes?

COMP 110: Spring Programming Demo Example, n = 4 Person 1 shakes hands with Person 2 Person 1 shakes hands with Person 3 Person 1 shakes hands with Person 4 Person 2 shakes hands with Person 3 Person 2 shakes hands with Person 4 Person 3 shakes hands with Person 4 6 handshakes

COMP 110: Spring Programming Demo Example, n = 4 Person 1 shakes hands with Person 2 shakes hands with Person 3 shakes hands with Person 4 Person 2 shakes hands with Person 3 shakes hands with Person 4 Person 3 shakes hands with Person 4 6 handshakes

COMP 110: Spring Handshake Problem XXXXX 2XXXX 3XXX 4XX 5X 6 n = 6 Outer Loop Inner Loop

COMP 110: Spring Programming Demo Implementation using nested loops

COMP 110: Spring Tutorial Using the jGRASP Debugger

COMP 110: Spring Friday Recitation Bring Laptops (fully charged) Textbook Questions about Program 2