COMP 110 Introduction to Programming Tyler Johnson Feb

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

Announcements Lab 3 due tomorrow by midnight Program 1 has been graded 2 COMP 110: Spring 2009

Questions? 3 COMP 110: Spring 2009

Today in COMP 110 Programming with Loops Programming Demo Using the Debugger 4 COMP 110: Spring 2009

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 5 COMP 110: Spring 2009

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 6 COMP 110: Spring 2009

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 7 COMP 110: Spring 2009

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 8 COMP 110: Spring 2009

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 9 COMP 110: Spring 2009

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; Output 0, 1, 2, 4, System. out. println(i + ", "); } 10 COMP 110: Spring 2009

Break & Continue Avoid using break and continue inside loops Can make your programs difficult to understand! 11 COMP 110: Spring 2009

Nested Loops It’s possible, and sometimes necessary to place loops inside other loops This is called nesting Just like with nested if-statements 12 COMP 110: Spring 2009

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] } 13 COMP 110: Spring 2009

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); 14 COMP 110: Spring 2009

Designing Loops Creating a loop involves designing three things Initialization of Variables Loop Body Stopping Condition 15 COMP 110: Spring 2009

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 16 COMP 110: Spring 2009

Designing a Loop Body Output instructions to the user Initialize variables Prompt user for input Read a number into variable next Repeated statements become your loop body sum = sum + next; Prompt user for input Read a number into variable next sum = sum + next; . . . Output sum 17 Statements that are only done once are not part of your loop body COMP 110: Spring 2009

Pseudocode with Loop Body Initializing statements 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 18 How do we end the loop? How many iterations? COMP 110: Spring 2009

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 + next; sum is on the right side of an assignment statement. sum MUST have a valid value before the loop starts. 19 COMP 110: Spring 2009

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; 20 COMP 110: Spring 2009

Loop with Initializations Initializing statements 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 21 COMP 110: Spring 2009

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 22 COMP 110: Spring 2009

Count-Controlled Loops A program to average exam scores input by the user We know the size of the class is "num. Students" double next, sum = 0, average; int i; for(i = 0; i < num. Students; i++) { //# iterations is fixed next = keyboard. next. Double(); sum = sum+next; } average = sum/num. Students; 23 COMP 110: Spring 2009

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) 24 COMP 110: Spring 2009

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. equals. Ignore. Case("yes")); 25 COMP 110: Spring 2009

Sentinel Value Signal end of input System. out. print("Enter a negative number to end the loop"); int next = keyboard. next. Int(); int sum = 0; while (next >= 0) { sum = sum + next; System. out. print("Enter a number: "); next = keyboard. next. Int(); } 26 COMP 110: Spring 2009

Booleans int next, sum = 0; boolean more. Numbers = true; Scanner keyboard = new Scanner(System. in); while(more. Numbers ) { next = keyboard. next. Int(); } if (next < 0) more. Numbers = false; //this will be the last iteration else sum = sum + next; System. out. print("The sum is " + sum); 27 COMP 110: Spring 2009

Exercise What does the following display? int product = 1; int max = 20; Output The product is 0 for(int i = 0; i <= max; i++) product = product * i; System. out. println("The product is " + product); 28 COMP 110: Spring 2009

Example Problem Find the lowest and highest ages in the class 29 COMP 110: Spring 2009

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 30 COMP 110: Spring 2009

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 31 COMP 110: Spring 2009

Min/Max Ages max min 0 2000 20 23 23 20 18 23 18 25 25 18 12 25 12 94 94 12 36 94 12 if (age > max) max = age; if (age < min) min = age; … 32 COMP 110: Spring 2009

Debugging What to do if there’s something wrong with your program, but you’re not sure what? Trace the variables Watch the variables change as the program executes 33 COMP 110: Spring 2009

Tracing Variables Two ways to trace variables Manually Using print statements Automatically Using the debugger 34 COMP 110: Spring 2009

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. next. Int(); 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. next. Int(); } 35 COMP 110: Spring 2009

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 } 36 COMP 110: Spring 2009

Automatic Tracing Use a debugger j. GRASP provides an integrated debugger Allows you to “step” through your program to see how variables change with each line of code 37 COMP 110: Spring 2009

Programming Demo Hand-shaking problem Given a group of n people, everyone shakes hands with everyone else How many handshakes? 38 COMP 110: Spring 2009

Programming Demo Example, n = 4 Person Person 1 1 1 2 2 3 shakes shakes hands hands with with Person Person 2 3 4 4 6 handshakes 39 COMP 110: Spring 2009

Programming Demo Example, n = 4 Person 1 shakes hands with Person 2 shakes hands with Person 3 shakes hands with Person 4 6 handshakes 40 COMP 110: Spring 2009

Handshake Problem Inner Loop n=6 1 1 2 Outer Loop 3 4 5 2 3 4 5 6 X X X X 6 41 COMP 110: Spring 2009

Programming Demo Implementation using nested loops 42 COMP 110: Spring 2009

Tutorial Using the j. GRASP Debugger 43 COMP 110: Spring 2009

Friday Recitation Bring Laptops (fully charged) Textbook Questions about Program 2 44 COMP 110: Spring 2009