Control Structures

2  Control – the order in which instructions are performed in a program.  Control structures are well defined ways of determining instruction flow.  Two main types  Selection  Repetition  Others later

3  Sometimes need to make a choice  If the amount of an Amazon.com purchase is less than 25 then add shipping  If a bank withdrawal would yield a negative balance then don’t allow the withdrawal  If a user enters a SSN that is not consist of exactly 9 digits then print an error message.

4  The single-branch (one way) if statement is the simplest selection. if( some_condition ) { then_clause } if( some_condition ) { then_clause }  some_condition: Any expression that has boolean type  then_clause: A statement sequence  the ‘{}’ is not needed if the then clause is a single statement but should be used

5 PROBLEM: Write a code fragment that prints the message "Overtime" if an employee has worked more than 40 hours in the past week.. double hoursWorked; // assume that hoursWorked // has been initialized.

6  The if-else statement is a two-way conditional. if( some_condition ) { then_clause } else { else_clause } if( some_condition ) { then_clause } else { else_clause }  some_condition: Any expression that has boolean type  then_clause: A statement sequence  else_clause: A statement sequence

7 PROBLEM: Write a code fragment that determines the total cost of an internet purchase. The company charges a flat fee of $5 shipping if the purchase amount is less than $30 otherwise they charge $5 plus 5% of the purchase for shipping. All values are in pennies. int totalCost, purchaseAmount; //assume purchaseAmount is initialized

8 PROBLEM: Write a code fragment that takes two ints (variables ‘a’ and ‘b’) and computes the largest of the two values as variable ‘max’. PROBLEM: Write a code fragment that takes three ints and computes the largest of the three values.

9  The if statement can be ‘multi-way’ if( condition1 ) { clause1 } else if( condition2 ) { clause2 } else if( condition3) { clause3 } else { default } if( condition1 ) { clause1 } else if( condition2 ) { clause2 } else if( condition3) { clause3 } else { default }

10 PROBLEM: Write a code fragment that determines the final cost for an internet purchase. If the purchase amount exceeds $100 the discount is 15%. If the amount is more than $75 up to $100 the discount is 10% else if the amount is more than $50 up to $75 the discount is 5% else the discount is 2.5%. int finalCost, purchaseAmount; //assume purchaseAmount is initialized

11  Sometime need to execute a code fragment more than once.  Loops can be classified as either ‘definite’ or ‘indefinite’  Definite – the number of repetitions is known by the programmer immediately prior to execution of the loop.  Indefinite – the number of repetitions is not known by the programmer.

12  Assume the following problems are to be solved with loops. Identify as either definite or indefinite.  Print the first ten letters of the alphabet.  Print the alphabet up to and including the letter “R”  Calculate the first integer power of 83 that is greater than one billion  Starting with distance d1 and d2 repeatedly divide them both in half until the resulting lengths are within 1 inch of each other.

13  While loop  Best used to solve indefinite looping problems while( condition ) { loopBody } while( condition ) { loopBody }

14 int sum, count; sum = 0; count = 1; while (count < 6 ) { sum = sum + count; count++; } System.out.println(sum);

15  Every loop has four parts  Initialization – establish the state prior to entering the loop  Primary work – the code that is to be repeatedly executed  Condition – a boolean criteria that controls when the loop stops  Make progress – code that moves the loop toward termination. int sum, count; sum = 0; count = 1; while (count < 6 ) { sum = sum + count; count++; } System.out.println(sum);

16  Original: sum ==  What changes are needed to cause the following postconditions?  sum == …+25  sum == …+25  sum == …+11 int sum, count; sum = 0; count = 1; while (count < 6 ) { sum = sum + count; count++; } System.out.println(sum);

17  Write code to print the circumference and area of circles with radius of 10, 20, …, 100.  Sam earns $100 per day with a daily raise of $100. Sue earns $0.01 per day with a salary that doubles every day. How many days pass before Sue’s total income exceeds Sam’s?

18 totalSam = 0; totalSue = 0; perDaySam = 10000; perDaySue = 1; dayCount = 0; while ( totalSue <= totalSam ) { dayCount++; totalSam = totalSam + perDaySam; totalSue = totalSue + perDaySue; perDaySam = perDaySam ; perDaySue = perDaySue * 2; }

DayperDaySueperDaySamtotalSuetotalSam

20  Write a code fragment to compute N!  Factorial definition  N! = 1*2*3*…*N  By convention we define 0! = 1  N must be non-negative int n = Keyboard.readInt(); int result = 1; while( n > 0) { result = result * n; n--; } int n = Keyboard.readInt(); int result = 1; while( n > 0) { result = result * n; n--; }

21  Write a code fragment to compute X raised to the Y power.  X can be any real value  Y must be a non-negative integer double x = Keyboard.readDouble(); int y = Keyboard.readInt(); double result = 1; while( y > 0) { result = result * x; y--; } double x = Keyboard.readDouble(); int y = Keyboard.readInt(); double result = 1; while( y > 0) { result = result * x; y--; }

22  An infinite loop occurs when termination is never reached. The loop never ends! int x = Keyboard.readInt(); int sum = 0; while( x != 0) { sum = sum + x; x = x - 2; } int x = Keyboard.readInt(); int sum = 0; while( x != 0) { sum = sum + x; x = x - 2; } int x = Keyboard.readInt(); while( x > 0) { x = Math.round( x/ 2.0); } int x = Keyboard.readInt(); while( x > 0) { x = Math.round( x/ 2.0); }

23  Infinite loops are not always ‘bad’! int n = Keyboard.readInt(); int result = 1; while(n != 0) { result = result * n; n--; } int n = Keyboard.readInt(); int result = 1; while(n != 0) { result = result * n; n--; } Computes n! but what about the logic of n=-1? int n = Keyboard.readInt(); int result = 1; while(n > 0) { result = result * n; n--; } int n = Keyboard.readInt(); int result = 1; while(n > 0) { result = result * n; n--; } Which is more logically correct?

24 int k = 1; while( k != 100 ) { // main work k++; } int k = 1; while( k != 100 ) { // main work k++; } int k = 100; while( k >= 0 ) { // main work k--; } int k = 100; while( k >= 0 ) { // main work k--; } int k = 0; while( k < 99 ) { // main work k++; } int k = 0; while( k < 99 ) { // main work k++; } int k = 1; while( k < 100 ) { // main work k++; } int k = 1; while( k < 100 ) { // main work k++; } Which of these executes the main work 100 times? int k = 99; while( k > 0 ) { // main work k--; } int k = 99; while( k > 0 ) { // main work k--; }

25  Premise: A loop is a Java statement  Premise: The body of a loop contains a sequence of Java statements  Conclusion: The body of a loop may contain a loop. int x = 1; while(x < 4) { int y = 1; while(y < 5) { System.out.print(x*y); System.out.print(“ “); y++; } System.out.println(); x++; } int x = 1; while(x < 4) { int y = 1; while(y < 5) { System.out.print(x*y); System.out.print(“ “); y++; } System.out.println(); x++; }

26  A repetition statement that executes the body 1 or more times. do { loopBody } while( condition );

27  Write a code fragment that reads input from the keyboard asking the user repeatedly for input until the input is valid. int minValue = 10, maxValue = 100; int result; do { result = Keyboard.readInt(); } while( ?? ); int minValue = 10, maxValue = 100; int result; do { result = Keyboard.readInt(); } while( ?? );

28  Best used for ‘definite’ looping  Typically uses a counter for( init ; condition ; make_progress) { main_work }

29  Print all whole numbers between 5 and 10 for( int n=1; n <= 5; n++) { System.out.println(n + 4); } for( int n=0; n < 5; n++) { System.out.println(n + 5); } for( int n=5; n <= 10; n++) { System.out.println(n); }

30  Print all odd numbers between 1 and 100 for( int n=1; n <= 100; n+=2) { System.out.println(n); }

31  Write code to print the first 5 powers of each integer value between 1 and N where N is a positive integer read from the keyboard. Print each values power on a single line int n = Keyboard.readInt(); for(int base=1; base<=n; base++) { for(int exp=1; exp<=5; exp++) { System.out.print(((int)Math.pow(base,exp)) + “ “); } System.out.println(); }

32  A specialized conditional  Use when there are many cases  Can be more efficient switch( expression ) { case CONSTANT1: statements1; break; case CONSTANT2: statements2; break; … case CONSTANTN: statements3; break; default: statementsDefault; }

33  Write a code fragment to determine the interest rate for a mortgage. The bank allows customers to take 3, 5, 7, 20, and 30 year mortgages with annual interest rates of 9%, 8.3%, 7.8%, 7.4% and 6.2% respectively. double interestRate; System.out.print(“Enter the number of years (3,5,7,20,30): “); int years = Keyboard.readInt(); switch( years ) { case 3: interestRate =.09; break; case 5: interestRate =.083; break; case 7: interestRate =.078; break; case 20: interestRate =.074; break; case 30: interestRate =.062; break; default: System.out.println(“invalid year selection”); }