Lesson 2 - Loops, Strings, Math

After a (probably pretty long) first lesson, let's explore a couple more features of Java and see what you can do with everything that you learned.

Loops

Loops are used to execute a block of code enclosed in a set of braces ({}) multiple times. After all, computers are very good at doing the same thing over and over again! The number of times a block executes depends on a boolean expression.

`while` Loops

while loops execute as long as a boolean expression is true. It may not execute at all if the expression is false before entering the loop.

while (/*some boolean expression*/) {
    //Execute code here multiple times as long as the expression is true
}

Remember that the boolean expression should become false at some point during the execution of the code in the loop. Otherwise, an infinite loop will be created.

For example, the following will print the numbers from 1 to 10 (inclusive):

int counter = 1;
while (counter<=10) {
  System.out.println(counter);
  counter++;
}

do-while loops may also be written. The main difference between these and while loops is that the boolean expression must be true at the end of the loop, not at the beginning. The following prints the numbers from 1 to 10 (inclusive):

int counter = -1;
do {
  counter++;
  System.out.println(counter);
} while (counter<10);

`for` Loops

for loops provide a more controlled way for iterating through a loop multiple times. There are three components to the for loop: the initial statement, the exit condition (boolean expression), and the post-excution statement. The example

for (/*initial statement*/; /*exit condition*/; /*post-execution statement*/) {
    //Code to be executed each loop
}

is essentially equal to the following while loop:

//Initial statement
while (/*exit condition*/) {
    //Code to be executed each loop
    //Post-execution statement
}

The for loop is commonly used to execute a loop a certain number of times. Below, the loop prints the numbers from 0 to 9 (10 iterations):

for (int count = 0; i < 10; count++) {
    System.out.println(count);
}

Keep in mind that the variable count is declared within the loop, and it cannot accessed outside it.

Loops may also be placed inside each other. For example, executing

 for (int row = 1; row <=12; row++) {
    for (int col = 1; col <=12; col++) {
        System.out.print(row*col);
        System.out.print(" "); //adds a space
    }
    System.out.println();//line break
 }

will print a (poorly formatted) times table.

Note that all three parts of a for loop are optional. That is, a loop like

for (;;) {
    //something
}

will execute forever.

Questions

Determine the number of times the following for loops will execute.

for (int i = 4; i <=8; i += 2) { /*do something*/ }
for (int i = 10; i >= 0; i -= 3) { /*do something*/ }
for (int j = 2; j >= 4; j++) { /*do something*/ }

Answers

3
4
0

Exercises

Write a program that computes the sum of all odd integers from 1 to a number specified by the user
Write a program that computes $b^e$ (a number raised to the power of an exponent), where b and e are positive integers
Write a program that allows the user to enter a number, setSize. setSize numbers, num, follow and are inputted by the user. Print the average, largest, and smallest of the numbers.

Loop Control

There are two control statements that are used with loops:

`break`

The break statement will immediately exit a while or for loop. This can be useful if you're looking for a specific value, and you want the loop to end after you've found that value. For instance, in the below example, the loop will terminate immediately after the value key is found:

int key = 8;
for (int i = 0; i < 64; i++) {
  if (i == key) {
    System.out.println("Found");
    break;
  }
  System.out.println("Not found...");
}

As a result, Not found... will be printed out eight times at first, followed by Found. The loop will then terminate. The break statement will be more useful after you learn about arrays.

`continue`

The continue statement will immediately go to the next iteration of a for or while loop, bypassing all statements after it. This could be used when you only want to execute a loop for some values. Of course, an if-else statement can often accomplish the same thing, but break and continue statements can prevent too many nested blocks of code.

For instance, the following block of code

for (int i = 0; i < 4; i++) {
  System.out.println("Going... " + i);
  if (i >= 2) {
    continue;
  }
  System.out.println("Here");
}

will print

Going... 0
Here
Going... 1
Here
Going... 2
Going... 3

Characters

In addition to the primitive data types (boolean, int, double) we learned about before, chars represent one of the 256 ASCII characters. These include the numbers, standard symbols and punctuation on a keyboard, and both uppercase and lowercase letters. A char is assigned a value with the declaration

char allianceColor = 'R';

Note the use of single quotation marks rather than double quotation marks (which are used for Strings) and that a variable of type char may only contain a single character.

Escape characters are the first 32 ASCII characters, which serve special functions. The most commonly used is '\n', which creates a line break. In addition, quotation marks must be escaped. Thus, executing the following code

char quote1 = '\'';
char quote2 = '\"';
char newLine = '\n'
System.out.print(quote1);
System.out.print(newLine);
System.out.print(quote2);

will print

'
"

Note that strings can be thought of as a series of chars concatenated together. For instance, executing

String someText = "Kickoff, 2017\nAs we anticipated the new game, the livestream blared, \"STEAM POWERED! STEAM POWERED! STEAM POWERED!\"";
System.out.println(someText);

will print

Kickoff, 2017
As we anticipated the new game, the livestream blared, "STEAM POWERED! STEAM POWERED! STEAM POWERED!"

The value of characters may be compared with the equivalence (==) operator. For example, allianceColor=='R' is true. Note, however, that a character is not equal its int equivalent. That is, '1'==1 is false.

String Manipulation

The String class in Java contains many methods to manipulate strings. We will present an overview of the most useful ones, though you may read the full documentation here.

Concatenation

A string can be concatenated, or joined together, with other strings or primitive datatypes (and literals) with the + operator. For example, executing the following:

 String mascot = "Raider";
 int teamNum = 25;
 String teamName = mascot + " Robotix";
 System.out.println("The year is " + 2017);
 System.out.println("I am on Team " + teamNum + ", " + teamName);

will print

  The year is 2017
  I am on Team 25, Raider Robotix

You may also concatenate a string onto the end of another with the += operator. Executing

 String s = "Hello";
 s+=" World!";
 System.out.println(s);

will print Hello World!.

String Properties

You can check the length of a string (number of characters) with the string.length() method. Executing

 String s = "Hello";
 System.out.println(s+ " is " + s.length() + " characters long");
 System.out.print("World!".length());

will print

 Hello is 5 characters long
 6

You can also check if a string contains a specific character sequence with the string.contains(<character sequence>) method, which returns a boolean value. If String s = "Hello"; is declared, s.contains("ell") returns true, while s.contains("f") returns false.

Parsing a String

You may want to parse through a string sometimes, or check the sequence of its characters. The string.charAt(/*index*/) method will return the character currently at the index. There are two things you should know about the method:

The method will return a value of type char (character), rather than String.
The index is a value that is greater than or equal to 0 and less than string.length(). That is, the index of the first character in the string is 0, and the index of the last character in the string is string.length()-1. If String s = "World"; is declared, s.charAt(0) returns 'W', s.charAt(3) returns 'l', and s.charAt(4) returns 'd'.

A standard way to parse through all of the characters of a string is with the following for loop:

String str = "Hello World!";
for (int i = 0; i < str.length(); i++) {
    char currentChar = str.charAt(i);
    //Do something here
}

Substrings

Suppose you want to return a slice of a string, known as a substring, such as the first letter four letters of String s = "Raider Robotix";. An easy-to-understand way would be to use string parsing, similar to what we did above:

String s = "Raider Robotix";
String result = ""; // an empty string
for (int i = 0; i < 4 && i < s.length(); i++) {
    result += s.charAt(i);
}
System.out.println(result);

This would print out Raid. Note that the expression i < s.length() was still present in the exit condition in order to prevent errors if s was an input string that was less than four characters long.

However, a faster way to get result would be to use the substring(beginIndex, endIndex) method, in which beginIndex and endIndex are integers that represent the indicies that you want to search. Similar to the for loop above, beginIndex is inclusive, while endIndex is exclusive. Here are some examples:

s.substring(0, 4); returns Raid
s.substring(7, s.length()); returns Robotix
"hamburger".substring(4, 8); returns urge

Miscellaneous String Functions

Assume that String s = "Hello"; is declared in the following examples:

s.toLowerCase(); returns hello
s.toUpperCase(); returns HELLO
s.indexOf("l"); returns 2 (the first occurence of l in s), while s.lastIndexOf("l") returns 3
s.concat("world"); returns Helloworld

As mentioned before, string equality is checked with s.equals("Hello");, which would return true. On the other hand, after declaring String anotherString = "Hello";, the boolean expression s == anotherString || s == "Hello" would be false, as string are objects, not primitives.

Exercises

Write a program to reverse a string if it has an R in it.
Write a program to check if a alphanumeric string is a palindrome (reads the same backward as forward, such as racecar). Note that Racecar, 123racecar321, and RaCeCaR are also palindromes.
Write a program to replaces all commas from a string with semicolons
Given a sentence, write a program to print its words backwards, so that My name is Bob becomes Bob is name My.
Given two strings and an integer n, check if there exist a substring of length n in the first string that is also a substring of the second string

The `Math` Class

The Math class contains several methods that you may find helpful. It is imported by default, and documentation is found here.

For these examples, we will declare double a = 4.5, b = -2.5;

Math.abs(b): takes the absolute value of a number, returns 2.5 in the example
Math.ceil(a): takes the ceiling of a number (rounds it up if it is a decimal), returns 5 in the example
Math.floor(a): takes the floor of a number (rounds it down if it is a decimal), returns 4 in the example
Math.pow(a-0.5,-b+0.5): takes the first number to the power of the second number, returns 125.0 in the example
Math.sqrt(a-0.5): takes the square root of a number, returns 2.0 in this example
Math.PI: returns the value of pi (3.141592653589793)
Math.random(): returns a random double in the interval [0.0, 1.0). This can be used to generate a random integer within a particular range by clamping the number down, then casting it to an int. For example, if you want a random integer in the interval [lower, upper], you would declare int randInt = (int) (Math.random() * (upper-lower)) + lower;. Try it out!

Exercises

Write a program that solves quadratic equations of the form $0=ax^2+bx+c$ for $x$ , where $a$ , $b$ , and $c$ are coefficients given by the user
Write a program that calculates the average of a data set, given the numbers in the set and its length
Write a program to find the surface area and volume of a square pyramid, given its side length and height
Write a program to round a number to a specific number of decimal places

Assignment

Complete the Raider Robotix Printer and Binary Converter programs.

Please email or DM me via Slack the programs that you finish for feedback.

Lesson 2 - Loops, Strings, Math

Loops

while Loops

for Loops

Loop Control

break

continue

Characters

String Manipulation

Concatenation

String Properties

Parsing a String

Substrings

Miscellaneous String Functions

The Math Class

Assignment

`while` Loops

`for` Loops

`break`

`continue`

The `Math` Class