Introduction to Computer Programming Counting Loops 2

1. Introduction to Computer ProgrammingCounting Loops 2 Review Stepwise Refinement Scope Loop Inside Loop Fence Post

2. Counting Loops • We use a for loop to write loops that run a specific number of times: • In Java, it looks like this: for ( count = start; count <= finish; count=count+1) { statements }

3. Counting Loops (syntax reminder) for (count = start; count <= finish; count++) statement final value of the counter initial value of the counter variable used to count times through the loop

4. Loop Example reminder public class sayHello { public static void main() { System.out.println(“Let’s get started”); for ( int count = 0; // runs the first time count < 3; // test that runs each time count = count + 1) // runs all but first { System.out.println(“Hello World”); } System.out.println(“Goodbye”); } }

5. Some uses • Repeat statements • Repeat methods • Repeat formulas with new values • Accumulate

6. Repeating method calls public class printShapes { public static void main() { System.out.println(“Get started”); diamond(); diamond(); diamond(); System.out.println(“All Done”); } public static void diamond() { System.out.println(“ *”); System.out.println(“ * *”); System.out.println(“ * *”); System.out.println(“ * *”); System.out.println(“ *”); } }

7. Use For loop to repeat method public class printShapes { public static void main() { System.out.println(“Get started”); for ( int count = 1; count <= 3; count = count + 1) { diamond(); } System.out.println(“All Done”); } public static void diamond() { System.out.println(“ *”); System.out.println(“ * *”); System.out.println(“ * *”); System.out.println(“ * *”); System.out.println(“ *”); } } You try: make it print 30 of these diamonds; make it print the word “ Break “ between each diamond.

8. Repeat formulas • Remember this assignment: You are given these two formulas:z = 80/b+5y = 2z+b What is the result when b is 10? What is the result when b is 20? What is the result when b is 30? Use a loop:

9. Loops: Reset variables using new values • Instead of rewriting the variable setting, loop back through the setting code. • b = b+4 does not change the z = statement, but the z = statement runs right after the b = statement in the loop, so the new b value is used. • double b, z, y; • b = 10; • for (int count = 1; count <= 2; count++) • { • z = 80/b+5; y = 2*z+b; • System.out.println(“the value of z when b is " + b + " is " + z); • b = b+10; • }System.out.println("the final value of z is " + z);

10. Repeat a formula - you try • Open your quiz and copy out your profit = price – cost. See how it repeats • Put your profit calculation and printing into a loop that runs 2 times, and increase the price by 5 inside the loop. It will look something like: Cost = 2; Price = 5.5; for (int count = 1; count <=2; count++) { totalValue=price-cost; System.out.println(totalValue); price=price+5; } • See how it reruns the formula with the new values. • Add 1 to the cost also and rerun it to see what happens. • Add a line to print the count so you can see its value as it loops. • Does it work if you put price == 5.5 inside the loop? – try it • Does it work if you change the order of these lines – try it

11. Loops: Scope of Variables • Variables created in loops disappear when the loop ends • You can create the counter before the loop if you want to keep it for the entire program. • double b, y; • b = 5; • int count; • for (count = 1; count <= 2; count++) • { • double z = 80/b+5; y = 2*z+b; • System.out.println(“the value of z when b is " + b + " is " + z); • b = b+4; • }System.out.println("the final value of z is " + z); // can't do this because z was created inside loop.

12. Scope – you try More changes to our profit calculator: • Add code after the loop ends (after the }) to print totalValue and price. Do you see the first price or last price? • Try to print count then also after the loop and see that it wont compile – Why not? • Create count (int count;) before the loop starts instead, and then try to print count after the loop – why does that work now? • What happens when you create totalValue inside the loop instead of before the loop?

13. Design– Stepwise refinement • Approach problem by writing algorithm at high level and refining details down to code level • Flow chart

14. Example: Interest Program - problem • Example - Write a program that calculates the interest that the Canarsie Indians would have accumulated if they had put the $24 that they had received for Manhattan Island in the bank at 5% interest. Input - none; all the values are fixed Output - Year and Principle Other Information - Principle is initially 24 Interest = Interest Rate * Principle New Principle = Old Principle + Interest

15. Example: Interest Program • Our initial algorithm is: 1. Set the principle to 24 2. For every year since 1625, add 5% interest to the principle and print out the principle.

16. Refining The Interest Algorithm 1. Set the principle to 24 2. For every year since 1625, add 5% interest to the principle and print out the principle. 2.1 FOR Year goes from 1625 TO Present: 2.1.1 Add 5% interest to the principle 2.1.2 Print the current principle

17. Refining The Interest Algorithm 1. Set the principle to 24 2.1 FOR Year goes from 1625 TO Present: 2.1.1 Add 5% interest to the principle 2.1.2 Print the current principle 2.1.1.1 Calculate 5% Interest 2.1.1.2 Add the interest to the principle

18. Refining The Interest Algorithm 1. Set the principle to 24 2.1 FOR Year goes from 1625 TO Present: 2.1.1.1 Calculate 5% Interest 2.1.1.2 Add the interest to the principle 2.1.2 Print the current principle principle = 24;

19. Refining The Interest Algorithm principle = 24; 2.1 FOR Year goes from 1625 TO Present: 2.1.1.1 Calculate 5% Interest 2.1.1.2 Add the interest to the principle 2.1.2 Print the current principle for (year = 1625; year < present; year++) { }

20. Refining The Interest Algorithm principle = 24; for (year = 1625; year < present; year++) { 2.1.1.1 Calculate 5% Interest 2.1.1.2 Add the interest to the principle 2.1.2 Print the current principle } interest = rate * principle; principle = principle + interest;

21. Refining The Interest Algorithm principle = 24; for (year = 1625; year < present; year++) { interest = rate * principle; principle = principle + interest; 2.1.2 Print the current principle } System.out.println("year = " + year + "\tprinciple = “ + principle);

22. The Interest Program public class Interest { // Calculate the interest that the Canarsie // Indians could have accrued if they had // deposited the $24 in an bank account at // 5% interest. public static void main(String[] args) { final int present = 2005; int year; final double rate = 0.05; double interest, principle; // Set the initial principle at $24 principle = 24;

23. // For every year since 1625, add 5% interest // to the principle and print out // the principle for (year = 1625; year < present; year++) { interest = rate * principle; principle = principle + interest; System.out.println("year = " + year + "\tprinciple = " + principle); } } }

24. Output from the Compound Interest Program • What will our output look like? • year = 1625 principle = 25.2 • year = 1626 principle = 26.46 • year = 1627 principle = 27.783 • year = 1628 principle = 29.172150000000002 • … … … … … • year = 2001 principle = 2.3365602874289446E9 • year = 2002 principle = 2.4533883018003917E9 • year = 2003 principle = 2.5760577168904114E9 • year = 2004 principle = 2.704860602734932E9

25. Outer loops • Repeat a loop to print 1 – 10 3 times • Outer loops need their own counter. • The inner loop can use both its own counter and the outer loop’s counter for (int fullRunCount = 1; fullRunCount <= 3; fullRunCount ++) { System.out.println("Starting run # " + fullRunCount); for (int count = 1; count <= 10; count++) { System.out.println(count + “part of run “ + fullRunCount); } }

26. You Try • Print 1 to 30 by 5’s, and then repeat that 3 times: 5 10 15 20 25 30 5 10 15 20 25 30 5 10 15 20 25 30 • Extra1: Each time you repeat, drop the first number 5 10 15 20 25 30 10 15 20 25 30 15 20 25 30

27. You Try Answer for (int fullRunCount = 1; fullRunCount <= 3; fullRunCount ++) { for (int count = 5 ; count <= 30; count= count + 5) { System.out.print(count + " "); } } To make the Extra1 work, for (int count = 5 * fullRunCount)

28. Outer loop shapes • First I have a loop that prints 7 stars in a row using a loop. When the loop is done, go to the next line. • Now add an outer loop to print the 7 stars 3 times. (remember to use a different counter) for (int fullRunCount = 1; fullRunCount <=3; fullRunCount++) { for (int count = 1; count <=7; count++) { System.out.print("*"); } System.out.println(); } and now you have ******* ******* *******

29. Outer loop shapes • You can use the outer control counter as a variable inside the inner loop. • Now print as many stars as the counter in the outer loop for (int fullRunCount = 1; fullRunCount <=3; fullRunCount++) { for (int count = 1; count <=fullRunCount; count++) { System.out.print("*"); } System.out.println(); } • and now you have this shape * ** ***

30. Outer loop shapes • You can have many inner loops inside an outer loop • Now print as many stars as the counter in the outer loop for (int fullRunCount = 1; fullRunCount <=3; fullRunCount++) { for (int count = 1; count <=3-fullRunCount; count++) { System.out.print(" "); } for (int count = 1; count <=fullRunCount; count++) { System.out.print("*"); } System.out.println(); } and now you have this shape * 2 spaces 1 star ** 1 space 2 stars *** 0 spaces 3 stars

31. Remove the magic number 3 int basewidth = 3; for (int fullRunCount = 1; fullRunCount <=basewidth; fullRunCount++) { for (int count = 1; count <=basewidth-fullRunCount; count++) { System.out.print(" "); } for (int count = 1; count <=fullRunCount; count++) { System.out.print("*"); } System.out.println(); }

32. You try: make a diamond • Make a method to print the top half of a diamond that is 5 wide at its base and 1 wide at the top. * 2 spaces, 1 star *** 1 space, 3 stars ***** 0 space, 5 stars • Hint: if outer loop is 1,2,3 • Make a method to print the bottom half. • Extra: print the entire diamond 10 times • Extra: make the size of the diamond base a variable

33. You try: make a diamond • Make a method to print the top half of a diamond that is 5 wide at its base and 1 wide at the top. * 2 spaces, 1 star *** 1 space, 3 stars ***** 0 space, 5 stars • Hint: if outer loop is 1,2,3 • Fill out this chart:

34. You try: make a diamond- chart • Make a method to print the top half of a diamond that is 5 wide at its base and 1 wide at the top. * 2 spaces, 1 star *** 1 space, 3 stars ***** 0 space, 5 stars • Hint: if outer loop is 1,2,3 • Fill out this chart:

35. You try Diamond Top Answer • int basewidth = 5; • for (int fullRunCount = basewidth/2+1; fullRunCount >=0; fullRunCount--) • { • for (int count = 1; count <= fullRunCount; count++) • { • System.out.print(" "); • } • for (int count = 1; count <= basewidth -2*fullRunCount; count++) • { • System.out.print("*"); • } • System.out.println(); • }

36. You try Diamond Bottom • int basewidth = 5; • for (int fullRunCount = 1; fullRunCount < basewidth/2+1; fullRunCount++) • { • for (int count = 1; count <= fullRunCount; count++) • { • System.out.print(" "); • } • for (int count = 1; count <= basewidth -2*fullRunCount; count++) • { • System.out.print("*"); • } • System.out.println(); • }

37. Important Loop Facts • loops repeat the code in braces • Still goes line by line – (you should now know which line the system will read next) • variables persist with their value until they are destroyed – they don't reset at the beginning of a loop (unless a statement you write specifically sets it to 0). • variables created inside a loop are destroyed at the end of a loop • Loops can be inside other loops • You can accumulate a total by adding a new value to the accumulated total held in another variable. (total = total + whatever); - remember to start total at 0.

38. Summary • Stepwise Refinement – Approach as algorithm stepping down in detail until you get to code • Scope – variables die when the block they are created inside ends. • Can create before block and use inside block • Loop Inside Loop – Inside loop can use outside loops counter cleverly for control