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Loops
 What if you wanted a block of code to execute more
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than once?
You could, of course, copy and paste the code over, and
over, and over, and … but what if the code needed to be
repeated for an unknown amount of times (like based
on an unknown data set)?
You could use loops.
A loop is a block of code which repeats until a certain
condition is met.
There are two types of loops that you will be working
with in this class.
 Here is a snippet of code which contains a while loop:
boolean keepGoing = true;
int i = 0;
while (keepGoing) {
System.out.println(“I am inside the loop and i is ” + i);
i++; //increase i by 1
if (i == 6) {
keepGoing = false;
}
}
while (keepGoing) {
System.out.println(“I am inside the loop and i is ” + i);
i++; //increase i by 1
if (i == 6) {
keepGoing = false;
}
}
 That looks like an if statement!
 It works in the same way, although an if statement will
only run once if the condition is met.
while (keepGoing) {
System.out.println(“I am inside the loop and i is ” + i);
i++; //increase i by 1
if (i == 6) {
keepGoing = false;
}
}
 The variable i, in this case, is something referred to as a counter.
It is a variable which is changed somehow in the loop, and is
used to stop the loop eventually.
 Without the i in this loop, the loop would run forever (infinite
loop).
 i++ means i = i + 1;
 This is why a language is called C++ (it was like C + 1)
 i– means i = i - 1;
while (keepGoing) {
System.out.println(“I am inside the loop and i is ” + i);
i++; //increase i by 1
if (i == 6) {
keepGoing = false;
}
}
 Once i is equal to 6, the keepGoing variable is set to false. This will end
the loop.
 The output for this loop is:
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I am inside the loop and i is 0
I am inside the loop and i is 1
I am inside the loop and i is 2
I am inside the loop and i is 3
I am inside the loop and i is 4
I am inside the loop and i is 5
 The loop will run 6 times.
 for loops are another type of loop that is used in
computer science. Most languages have their own
version of a for loop.
 In Java, the syntax will seem unfamiliar to you.
 A for loop looks like:
for (<variable declaration>;<condition>;<counter update) {
Block of Code
}
 Unlike a while loop, the counter is updated
implicitly in the for statement. You do not need
to keep track of it in the block of code.
for (int i=0;i<6;i++) {
System.out.println(“I am inside the loop and i is” + i);
}
 This will output to the screen:
 I am inside the loop and i is 0
 I am inside the loop and i is 1
 I am inside the loop and i is 2
 I am inside the loop and i is 3
 I am inside the loop and i is 4
 I am inside the loop and i is 5
 This does exactly the same thing as the previous while loop!
 The ! in math means factorial. A factorial is the
product of the original number and all of the numbers
prior to it, except for 0.
 You have been tasked to write a program that prints
out the factorial of a number entered from input.
 You can do this using either of the two loops types!
import java.util.Scanner;
class FactorialWhile {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
System.out.print("Please enter the number to be factorial-ed: ");
int originalNumber = myScanner.nextInt();
int counter=originalNumber;
int product=1; //do not set to 0
System.out.println("The original product is " + product);
while (counter > 0) {
System.out.print("counter is " + counter + " and ");
product = product * counter;
System.out.println("the new product is " + product);
counter--;
}
System.out.println(originalNumber + "! is " + product);
}
}
int originalNumber = myScanner.nextInt();
int counter=originalNumber;
 This may seem redundant. Why would I have two
variables that are essentially the same thing?
 Answer: I edit the original number in the while loop, so I
would not be able to output it in a println statement at
the end of the program with the correct value.
 System.out.println(originalNumber + “!”); - would
always print 0 for originalNumber if I did not save the
original value elsewhere.
int product=1; //do not set to 0
 What would have happened if I set it to 0?
 Let’s say the person wanted to get 3! (which is 6).
 Product = product * counter is the line that calculates
the product as the loop executes.
 If the value of product is originally 0, you will get:
 product = 0 * 3 (which is 0)
 product = 0 * 2 (which is 0)
 product = 0 * 1 (which is 0)
 The final answer would be 0…which is bad.
 If you set product equal to 1 in the beginning, you get:
 product = 1 * 3 (which is 3)
 product = 3 * 2 (which is 6)
 product = 6 * 1 (which is 6)
 The final answer is 6 – Yay!
 Since 1 is the multiplicative identity, set product
originally to 1.
 What if you were doing addition?!
 Set it to the additive identity, which is 0.
Please enter the number to be factorial-ed: 6
The original product is 1
counter is 6 and the new product is 6
counter is 5 and the new product is 30
counter is 4 and the new product is 120
counter is 3 and the new product is 360
counter is 2 and the new product is 720
counter is 1 and the new product is 720
6! is 720
import java.util.Scanner;
class FactorialFor {
public static void main(String[] args) {
Scanner myScanner = new Scanner(System.in);
System.out.print("Please enter a number to be factorial-ed: ");
int originalNumber = myScanner.nextInt();
int product = 1;
System.out.println("The original product is " + product);
for (int i=originalNumber;i>0;i--) {
System.out.print("i is " + i + " and " );
product = product*i;
System.out.println("the new product is " + product);
}
System.out.println(originalNumber + "! is " + product);
}
}
for (int i=originalNumber;i>0;i--) {
System.out.print("i is " + i + " and " );
product = product*i;
System.out.println("the new product is " + product);
}
 The loop is a lot cleaner, but perhaps harder to understand.
 int i=originalNumber sets i equal to the original number
value.
 i>0 is the condition for the loop to continue. Once i is less
than or equal to 0, the loop stops.
 i-- is what is done to i in the loop. Here, we start at the
original number and decrement it until it is 0.
Please enter a number to be factorial-ed: 6
The original product is 1
i is 6 and the new product is 6
i is 5 and the new product is 30
i is 4 and the new product is 120
i is 3 and the new product is 360
i is 2 and the new product is 720
i is 1 and the new product is 720
6! is 720
 This is almost exactly the same as the while loop!
 For the most part, it’s a matter of preference.
 *I* will be telling you which to use for this course.
Choosing on your own is a skill to be developed in the
AP Computer Science course.
 However, there are some key differences between the
two which requires one or the other.
 for is typically used for iterating through arrays (huh)
or when working with numerical values.
 while is used when working with more complex data
types, such as Vector or ArrayList (AP Comp Sci
things)
 Do not worry about this!
class TestingForLoops {
public static void main(String[] args) {
boolean blah=true;
int i=0;
for (;blah;) {
i++;
System.out.println("In loop and i is " + i);
if (i==10) {
blah = false;
}
}
}
}