Introduction to Computer Programming

Introduction to Computer Programming Counting Loops

Do something three times public class sayHello { public static void main() { System.out.println(“Let’s get started”); System.out.println(“Hello World”); System.out.println(“Hello World”); System.out.println(“Hello World”); System.out.println(“Goodbye”); } } You try – please code this

Why loops? • Computers offer several advantages over calculators. • If it is necessary, they can perform the same steps over and over again, simply by rerunning the program or calling a method many times. • But is this the only way to get a computer to perform the same action repeatedly? And is this the only reason for getting a computer to repeat itself?

Loops • We need the ability to perform the same set of instructions repeatedly so we don’t have to write them over and over again. • This is why Java includes several ways of using repetition in a program. • Each case where we repeat a set of statement is called a loop.

Counting Loops • The first type of loop is a counting loop. • Counting loops are repeated a specific number of times. • If you read the loop, you can easily figure out how many times its statements will be performed.

Counting Loops • We use a for loop to write counting loops • In Java, it looks like this: for ( count = start; count <= finish; count=count+1) { statements }

Counting Loops (continued) 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

Repeat 3 times another way 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”); } } You try – Print 2 lines 7 times

Print the counter 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 Number“ + count ); } System.out.println(“Goodbye”); }} You try – Try starting the counter at 10 and ending at < 14 – How many times did it print; what was in counter?

Call methods many times 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(“ *”); } } You try : make the program loop to call the diamond method 3 times.

Use For loop to repeat 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.

Repeat formulas • Remember this assignment: You are given these two formulas:z = 80/b+5y = 2z+b Write the code to create variables z, b and y. Write the code to calculate y and z when b is 6. Print: When b is 6, y is (and insert the value of y) and z is (and insert the value of z) Then change the value of b to 10. Print: When b is 10, y is (and insert the value of y) and z is (and insert the value of z)

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 = 5; • 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+4; • }System.out.println("the final value of z is " + z);

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

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.

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?

Accumulating a total • Variables don't remember what they were • You can create a variable to increase and run it each time the loop runs. double total = 0; for (int count = 1; count <= 3; count++) { total = total + count;} // add what you are counting to what you have System.out.println(total); at end of first loop, count = 1 and total = 1 +0 = 1 at end of second loop, count = 2 and total = 2 + 1 = 3 at end of third loop, count = 3 and total = 3 + 3 = 6 at end of entire loop, count is gone and total = 3

Total – you try • Add up the total profit as you go through the loop • You will need to create a variable to hold the total and set it to 0 first. • Add a statement to keep adding to the total bucket inside the loop

Outer loops • Repeat a loop to print 1 – 10 3 times • Outer loops need their own counter. • You can use the fullRunCount inside the inner loop double b, z, y; for (int fullRunCount = 1; fullRunCount <= 2; fullRunCount ++) { System.out.println("Starting run # " + fullRunCount; for (int count = 1; count <= 10; count++) { System.out.println(count); System.out.println("This is part of run # " + rullRunCount;) } }

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++) { print("*"); } println(); } and now you have ******* ******* *******

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++) { print("*"); } println(); } • and now you have this shape * ** ***

Example: Interest Program • 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

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.

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

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

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;

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++) { }

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;

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);

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;

// 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); } } }

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

Summary • 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 • 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.

Introduction to Computer Programming