index.md

Concepts

Math expressions

Many times in a program you'll need to modify an existing variable and to do this the express looks like x = x + 1. This expression evaulates everything on the right hand first, then assigns the value to the variable on the left hand side of the equals. There is a shorter version of this that only works when your expression uses an existing variable and one other value. The expression looks like x += 1, this is the same as x = x + 1. It's important to note that if you need a more complex expression like x = x * 2 + 1 then the shorthand no longer works.

example

x = x - 2;    // subtract by 2
x -= 2        // subtract by 2
x = x * 2;    // multiply by 2
x *= 2;       // multiply by 2
x = x / 2;    // divide by 2
x /= 2;       // divide by 2

There is one other shorthand to be aware of, if you want to increment/decrement a variable by 1, you can use x++ for adding one and x-- for subtracting 1.

Loops

Loops are used to repeat a set of actions until some condition is met. There are two types of loops we will be learning about today.

For Loops

The first uses a counter and has the syntax for(INITIALIZER; EVALUATION; INCREMENTER).

class Main {
  public static void main(String[] args) {
      var myList = List.of("a", "b");
      for (int i = 0; i < 10; i++) {
          System.out.println(myList.get(i));
      }
  }
}

The second gets items directly from the list and assigns it to a new variable. The syntax looks like for(String NEWV_VARIABLE_NAME: LIST_VARIABLE_NAME.

import java.util.*;

class Main {
  public static void main(String[] args) {
    var myList = List.of("a", "b");
    
    for(String item: myList) {
      System.out.println(item);
    }
  }
}

While Loops

While loops are simpler and only evaulate one expression. If it's true, then the loop continues. If it's false, then the loop ends.

Example: syntax while BOOLEAN {}

int a = 0;
while(a < 10) {
    a++;
} 

Error handling (try/catch)

Some functions can cause errors, and in Java these are called exceptions. We can detect when an error happens with a try/catch block. In Java when a function has an error, you would say the function threw an exception.

Take for example, parsing a number from a string

Integer.parseInt("5");     // returns 5  with no error
Integer.parseInt("five");  // throws

We can use a try/catch block to handle the error above

try {
    Integer.parseInt("five");
} catch(Exception ex) {
    System.out.println("detected the following exception")
    System.out.println(ex.getMessage())
}

Project: Ad lib continued

Continue to work on the ad lib project from day 3. If you finish, go back and add error detection for the number input by the user. If they input anything other than a number, tell them that the input is invalid and prompt them to input the number again.