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Lecture 05
Loops!
Why Loops?
•
Allow for better user interaction
•
Make code much cleaner
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Save time copying/pasting
Types of loops
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We’ll be using 3 different kinds of loops in this class:
•
While
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Do…while
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For
While Loop
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while loops execute statements repeatedly while a
condition is true
while (loop-continuation-condition) {
statement(s);
}
While Loop
Infinite Loop
public class WhileLoopExample {
public static void main(String[] args) {
while (true){
System.out.println("Hello!");
}
}
}
Counter-controlled
Loop
public class WhileLoopExample {
public static void main(String[] args) {
int count = 0;
while (count < 10){
System.out.println("Hello " + (count+1) + " times!");
count++;
}
}
}
Increment counter
Off by one errors
Loop Design Strategies
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Step 1: Figure out which parts need to repeat
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Step 2: Put those statements inside a loop block
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Step 3: Figure out the loop-continuation condition and
statements for controlling the loop (increment, etc)
Guess the Secret Word
- with Loop
import java.util.Scanner;
public class GuessSecretWord_Loop {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
String secretWord = "SECRET";
String userGuess;
System.out.println("Try to guess my secret word");
userGuess = input.nextLine();
while (!userGuess.equals(secretWord)){
System.out.println("NOPE. Try again!");
userGuess = input.nextLine();
}
System.out.println("Great job!");
}
}
example of
sentinel-controlled
loop
Do-While Loop
•
Same as while loop - but checks loop-continuation
condition at the end of the loop
do {
statement(s);
} while (loop-continuation-condition);
Do-While Loop
Guess the Secret Word
- with Do While Loop
import java.util.Scanner;
public class GuessSecretWord_DoWhileLoop {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
String secretWord = "SECRET";
String userGuess;
do {
System.out.println("Try to guess my secret word");
userGuess = input.nextLine();
} while (!userGuess.equals(secretWord));
System.out.println("Great job!");
}
}
While vs. Do-while?
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Use a Do-while if the statements in the loop should
execute at least once
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One might be more convenient (less code), or make a
little better experience for your users.
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I see and use while loops much more often than dowhile loops, though they definitely have their place
For Loop
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Simplify counter-controlled loops (helps prevent errors)
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Very useful when the number of times to run is known
beforehand
for (initial-action; loop-continuation-condition; action-after-each-iteration)
statement(s);
}
For Loops
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Often looks something like this:
for (int i = initialValue; i < endValue; i++) {
statement(s);
}
Simple For Loop
Example
public class ForLoopExample {
public static void main(String[] args) {
for (int i = 0; i < 10; i++){
System.out.println(i);
}
}
}
Which Loop?
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Guess a number until you’re right
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Calculating the perimeter of a rectangle from user input
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Adding all the digits from 0-9
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Game loop
Which Loop?
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The fact is, you can use any of them. But one might make
more sense than the others, given your circumstances
Nested Loops
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Every time the outer loop is repeated, the inner loop
starts over
Nested Loops
public class NestedLoopExample {
public static void main(String[] args) {
// Outer loop of "rows"
for (int i = 0; i < 5; i++){
// Inner loop of "columns"
for (int j = 0; j < 5; j++){
// Print *
System.out.print("*");
}
// Go to next "row"
System.out.println("");
}
}
}
When to use nested
loops
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Structured data (Arrays, tables, Maps, etc)
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working with pixel array (graphics programming)
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I use these all the time
Break and Continue
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Keywords to control flow in
loops
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break = immediately get out of
entire loop
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continue = get out of current
iteration, keep looping
Break and Continue
public class BreakAndContinueExample {
public static void main(String[] args) {
String message = "Hey, this thing is a string";
for (int i = 0; i < message.length(); i++){
char currentChar = message.charAt(i);
if (currentChar == ' '){
break;
} else {
System.out.print(currentChar);
}
}
}
}
Loopify Fizz Buzz lite
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Let’s go back to Fizz Buzz and have it run from 1 to 100
instead of asking for a number
Loopify Fizz Buzz lite
import java.util.Scanner;
public class FizzBuzz {
public static void main(String[] args) {
for (int i = 1; i < 101; i++){
if (i % 3 == 0 && i % 5 != 0){
System.out.println("Fizz");
} else if (i % 5 == 0 && i % 3 != 0){
System.out.println("Buzz");
} else if (i % 3 == 0 && i % 5 == 0){
System.out.println("FizzBuzz");
} else {
System.out.println(i);
}
}
}
}
Find ✨perfect✨ numbers
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a number is considered perfect if it is equal to the sum of
all it’s positive divisors, except itself.
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so 6 is a perfect number since 6 = 3 + 2 + 1
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These are very rare, only 4 exist below 10,000
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Let’s find them using loops!
Find ✨perfect✨ numbers
public class PerfectNumberFinder {
public static void main(String[] args) {
// Loop through all the numbers
for (int i = 1; i < 10000; i++){
// for each number, start a new sum
int sum = 0;
// for each number, check all the numbers smaller than that to see if it's a
divisor
for (int j = 1; j < i; j++){
if (i % j == 0){
sum += j;
}
}
// if the sum is equal to the number, you found a perfect one!
if (sum == i){
System.out.println("Found a perfect number: " + i + "!");
}
}
}
}
Find the next leap years
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Write a program that will ask the user for a year to
start with and how many leap years they would like to
find.
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Using a loop, display the year if it’s a leap year for the
amount of leap years requested.
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Tip: A leap year happens every 4 years, but not every 100
years, then again every 400 years.
Find the next leap years
import java.util.*;
public class FindNextLeapYear {
public static void main(String[] args) {
int numberOfLeapYearsToFind;
int userYear;
Scanner input = new Scanner(System.in);
System.out.print("Enter the year you'd like to start from: ");
userYear = input.nextInt();
System.out.print("Enter how many leap years you'd like to find: ");
numberOfLeapYearsToFind = input.nextInt();
int foundLeapYears = 0;
while (foundLeapYears < numberOfLeapYearsToFind){
if (((userYear % 4 == 0) && (userYear % 100 != 0)) || userYear % 400 == 0){
foundLeapYears++;
System.out.println(userYear);
}
userYear++;
}
}
}