Iteration
According to CollegeBoard, Iteration is a way to simplify code that would otherwise be repeated many times in succession. Using loops, we can finally implement complex algorithms and solutions to common problems that we weren’t able to before.
Iteration is repeating sequences to simplify code of advanced algorithms
Iteration accounts for 17.5%-22.5% of the APCSA AP Exam
4.1: WHILE LOOPS
Learning Objective: Represent Iterative Processes using a while loop
Question: What is a loop and what are some real life examples of this (Setting a Song to repeat on your music player)
You know the i variable that you use for while/for loops? It actually has a name, loop control variable
int i = 0; // initialize loop control variable
while (i < 10) // checks the loop control variable
{
System.out.println("Doing some code");
i++; // update the loop control variable
}
Doing some code
Doing some code
Doing some code
Doing some code
Doing some code
Doing some code
Doing some code
Doing some code
Doing some code
Doing some code
int i = 0;
while (i < 5)
{
System.out.println(i);
i++;
}
0
1
2
3
4
int i = 0;
while (i < 5) { System.out.println(i);
i++;
}
0
1
2
3
4
Infinite loop
An infinite loop is when a while loop always evaluates to true. avoid this when you can because it’s probably not good for your computer. if this happens by accident, I recommend copying all code in the block and deleting the block. After you delete the code block, close and reopen the tab that the code block was in.
What’s wrong with this code block? no stoppage
Do While loop
What will this code block output? Quite shrimple
// Quite shrimple
int i = 0;
do
{
System.out.print("Quite shrimple. ");
i++;
}
while (i < -5);
Quite shrimple.
In a do while loop, it will run the “do” once before it reaches the “while”, and at that point it will start to act like a while loop.
For loop
this is the standard structure of a for loop
for (initialization; Boolean expression; update)
{
System.out.println("Doing some code");
}
Initialization will run at the start of the loop, boolean expression will get checked with every loop, and update runs after every loop.
How many times will this code print “Doing some code?” 5
for (int num = 1; num <= 5; num++)
{
System.out.println("Doing some code");
}
Doing some code
Doing some code
Doing some code
Doing some code
Doing some code
In this code, it creates the variable num at the start of the loop, it checks if num is less than or equal to 5 after each loop, and it adds 1 to num after each loop.
Enhanced for loop
this is essentially a javascript for loop, as it will iterate through a list and run code in the loop to each variable inside the list
int[] list = {1, 4, 6, 2};
for (int j : list)
{
System.out.print(j);
System.out.print(" ");
}
1 4 6 2
Break and Continue
In java there are breaks, but there are also continues.
Break
Breaks, as you likely already know, end a loop. They tend to be used with an if statement
How many times will this code print “Big guy?” 5
int i = 0;
while (i < 10)
{
System.out.println("Big guy");
i++;
if (i == 5) {
break;
}
}
Big guy
Big guy
Big guy
Big guy
Big guy
Continue
Continue will skip code for an iteration, but will still keep the loop running
int i = 0;
while (i < 10)
{
if (i == 5) {
i++; // don't forget this, it creates an error similar to an infinite loop
System.out.println("");
continue;
}
System.out.println(i);
i++;
}
4.3: Developing Algorithms Using Strings
Learning Objective: For algorithms in the context of a particular specification that involves String objects:
Methods in Java that help to MANIPULATE STRINGS
- String.substring - Retrieves a particular portion of a String
- String.equals - Comparees the content of two strings
- String.length - Returns the length of a String
public class Compare {
public static void main(String[] args) {
String string1 = "Coding is cool!";
String string2 = "Coding is coding!";
int minLength = Math.min(string1.length(), string2.length());
for (int i = 0; i < minLength; i++) {
String subString1 = string1.substring(0, i + 1);
String subString2 = string2.substring(0, i + 1);
if (subString1.equals(subString2)) {
System.out.println("Common Prefix: " + subString2);
}
}
}
}
Compare.main(null)
Common Prefix: C
Common Prefix: Co
Common Prefix: Cod
Common Prefix: Codi
Common Prefix: Codin
Common Prefix: Coding
Common Prefix: Coding
Common Prefix: Coding i
Common Prefix: Coding is
Common Prefix: Coding is
Common Prefix: Coding is c
Common Prefix: Coding is co
Where are the 3 methods in the above Java Cell and how do they contribute to the program’s functionality?
- String.substring - Retrieves a particular portion of a String
- String.equals - Comparees the content of two strings
- String.length - Returns the length of a String
String word = "supercalifragilisticexpialidocious";
int count = 0;
for (int i = 0; i < word.length(); i++) {
char letter = word.charAt(i);
if (letter == 'a' || letter == 'e' || letter == 'i' || letter == 'o' || letter == 'u') {
count++;
}
}
System.out.println("The Number of vowels in \"" + word + "\" is " + count);
What does word.length() do and how do we use it above? Finds the length of the string
What Boolean Operator is used? True false is used in the found variable
public class Main {
public static void main(String[] args) {
String word = "Scooby Doo";
String sub = "Doo";
boolean found = false;
for (int i = 0; i <= word.length() - sub.length(); i++) {
String portion = word.substring(i, i + sub.length());
if (portion.equals(sub)) {
found = true;
}
}
if (found) {
System.out.println("We found the Smaller String!");
} else {
System.out.println("We did not find the Smaller String! \t Ruh Roh!");
}
}
}
Main.main(null)
We found the Smaller String!
String concatenation
String concatenation is when you want to add to strings together
String original = "String";
String reversed = "";
for (int i = 0; i < original.length(); i++)
{
String single = original.substring(i,i+1);
reversed = single + reversed;
}
System.out.println("Original String: " + original);
System.out.println("Reversed String: " + reversed);
Original String: String
Reversed String: gnirtS
4.4: Nested Iteration
Learning Objective: Represent nested iteration processes
Essential Knowledge:
- Nested iteration is when an iteration statement appears inside the body of another iteration statement
- The inner loop must complete all of its iterations before the outer loop can continue.
Before uncommenting the code, guess what the output will look like:
4 lines of 1 and 2
public class NestedLoops{
public static void main(String[] args){
for (int outer = 1; outer < 5; outer++){
for (int inner = 1; inner < 3; inner++){
System.out.print(inner + " ");
}
System.out.println();
}
}
}
NestedLoops.main(null)
1 2
1 2
1 2
1 2
What will the output of the code above be if we switch the loop headers (the stuff inside of the for loop)?
switch 2 and 1 After making a prediction actually switch the loop headers for yourself. What do you notice about the output compared to the output before the change? it goes 1 2 3 4 with space
4.5: Informal Code Analysis
Essential Knowledge:
- A statement exectution count indicates the number of times a statement is executed by the program
for (int outer = 0; outer < 3; outer++){
for (int inner = 0; inner < 4; inner++){
System.out.print(inner);
}
}
012301230123
In the code above, how many times will the inner loop execute when outer = 0? 3
In the code above, how many times will the inner loop execute when outer = 1? 2
In the code above, how many times will the inner loop execute when outer = 2? 1
In the code above, how many times will the inner loop execute in total? 12
for (int outer = 5; outer > 0; outer--){
for (int inner = 0; inner < outer; inner++){
System.out.print(inner);
}
System.out.println();
}
01234
0123
012
01
0
In the code above, how many times will the inner loop execute when outer = 5? 15
In the code above, how many times will the inner loop execute when outer = 4? 10
In the code above, how many times will the inner loop execute when outer = 3? 6
In the code above, how many times will the inner loop execute in total? 15
int k = 0;
while (k < 5){
int x = (int)(Math.random()*6) + 1;
while (x != 6){
//statement #1
x = (int)(Math.random()*6) + 1;
}
k++;
}
In the code above, how many times will the statement #1 execute? random
for (int k = 0; k < 135; k+=5){
System.out.print(k); // Statement #2
}
05101520253035404550556065707580859095100105110115120125130
In the code above, how many times will the statement #1 execute? 27
In the code above, how many times will the statement #2 execute? 27
Rewrite the code above to be more effecient based on execution count.
HACKS
These hacks will be due on Monday (October 16th) before class
Hacks
- Finish the popcorn hacks (0.2)
- Rewrite the for loop (0.25)
- Complete the Ceaser Cipher Code (0.45)
for (int k = 0; k < 40; k+=4){
System.out.println(k);
}
int w = 0;
while (w < 40) {
System.out.println(w);
w += 4;
}
int x = 0;
do {
System.out.println(x);
x += 4;
} while (x < 40);
0
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8
12
16
20
24
28
32
36
0
4
8
12
16
20
24
28
32
36
0
4
8
12
16
20
24
28
32
36
Rewrite the code above 3 different times
- Your code should be more efficient based execution count
- Your code should use 3 different types of loops that you learned above (Hint: You may need to use a list)
Ceaser Cipher Hacks
Try to write a cipher program that shifts each letter in a message 3 letters forward. Use any of the methods you learned today. Use it to decode the 3 messages we’ve given you!
public class CaesarCipher {
public static void main(String[] args) {
String[] letters = {
"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m",
"n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"
};
String message1 = "Kfzb gly!";
String message2 = "zlab zlab zlab";
String message3 = "prmbozxifcoxdfifpqfzbumfxifalzflrp";
System.out.println("Encoded Messages:");
String encodedMessage1 = caesarCipherEncrypt(message1, 3, letters);
System.out.println("Message 1: " + encodedMessage1);
String encodedMessage2 = caesarCipherEncrypt(message2, 3, letters);
System.out.println("Message 2: " + encodedMessage2);
String encodedMessage3 = caesarCipherEncrypt(message3, 3, letters);
System.out.println("Message 3: " + encodedMessage3);
System.out.println("Decoded Messages:");
String decodedMessage1 = caesarCipherDecrypt(encodedMessage1, 3, letters);
System.out.println("Message 1: " + decodedMessage1);
String decodedMessage2 = caesarCipherDecrypt(encodedMessage2, 3, letters);
System.out.println("Message 2: " + decodedMessage2);
String decodedMessage3 = caesarCipherDecrypt(encodedMessage3, 3, letters);
System.out.println("Message 3: " + decodedMessage3);
}
public static String caesarCipherEncrypt(String message, int shift, String[] letters) {
StringBuilder result = new StringBuilder();
for (char c : message.toCharArray()) {
if (Character.isLetter(c)) {
char base = Character.isLowerCase(c) ? 'a' : 'A';
int index = (c - base + shift) % 26;
if (index < 0) {
index += 26;
}
result.append((char)(base + index));
} else {
result.append(c);
}
}
return result.toString();
}
public static String caesarCipherDecrypt(String message, int shift, String[] letters) {
return caesarCipherEncrypt(message, 26 - shift, letters);
}
}
CaesarCipher.main(null)
Encoded Messages:
Message 1: Nice job!
Message 2: code code code
Message 3: supercalifragilisticexpialidocious
Decoded Messages:
Message 1: Kfzb gly!
Message 2: zlab zlab zlab
Message 3: prmbozxifcoxdfifpqfzbumfxifalzflrp
Basically the code is taking the letter, find the index of the letter in the alphabet, add 3 to the index, and then find the letter at the new index. Pretty cool.