University of Washington, CSE 142

Lab 4: Return, if/else, and Scanner

lab document created by Marty Stepp, Stuart Reges and Whitaker Brand

Basic lab instructions

Mouse over highlighted words if you're not sure what they mean!
Talk to your classmates for help.
You may want to bring your textbook to future labs to look up syntax and examples.
Stuck? Confused? Have a question? Ask a TA for help, or look at the book or past lecture slides.
Complete as much of the lab as you can within the allotted time. You don't need to keep working on these exercises after you leave.
Feel free to complete problems in any order.
Make sure you've signed in on the sign-in sheet before you leave!

Today's lab

Goals for today:

use return values to send data between methods
gain a better understanding of the Math class
use Scanner to create interactive programs that read user input
use if, else if and else to have different branches of execution
Where you see this icon, you can click it to check the problem in Practice-It!

Exercise : Check Canvas scores

The scores you receive on labs are available for you to view in Canvas. You should be able to view your scores here. If that link doesn't work, you can check your scores by:

Logging into Canvas
Clicking CSE 190 under Courses (on the left)
Clicking Grades in the left sidebar to view your individual scores.

You earn 1 point for each lab that you attend/show up on time for. Receiving credit for CSE 190 requires attending 8-10 labs. You should see scores for lab01 and lab02.

If you believe there is a problem with one of your scores, please email Price Ludwig at shtiuif@cs.washington.edu.

Returning Values

A return value is information that a method gives back to the code that called it. Every method can only return one value: when a method returns, it stops executing (and we resume where we left off before the method was called). For the code that called the method to use the returned value, the returned value must be stored in a variable or used immediately.

public static type methodName(parameters) {      // calling methodName returns expression
    ...
    return expression;
}

type variableName = methodName(parameters);     // variableName stores return value

Exercise : Returns

What does the following output?

public static void main(String[] args) {
    int x = 5;
    x = mystery();
    System.out.println(x);
}

public static int mystery() {
    return 8;
}

Output:

Exercise : Returns

What does the following output?

public static void main(String[] args) {
    int x = 5;
    mystery();
    System.out.println(x);
}

public static int mystery() {
    return 8;
}

Output:

[an error occurred while processing this directive]

Math expression syntax

Method	Return	Example
`Math.abs`	absolute value	`Math.abs(-308)` returns `308`
`Math.ceil`	ceiling (rounds upward)	`Math.ceil(2.13)` returns `3.0`
`Math.floor`	floor (rounds downward)	`Math.floor(2.93)` returns `2.0`
`Math.max`	max of two values	`Math.max(45, 207)` returns `207`
`Math.min`	min of two values	`Math.min(3.8, 2.75)` returns `2.75`
`Math.pow`	power	`Math.pow(3, 4)` returns `81.0`
`Math.round`	round to nearest integer	`Math.round(2.718)` returns `3`
`Math.sqrt`	square root	`Math.sqrt(81)` returns `9.0`

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User input and `Scanner`

Method name	Description
`nextInt()`	reads the next token, returns it as an `int`, if possible
`nextDouble()`	reads the next token, returns it as `double`, if possible
`next()`	reads the next token, returns it as a `String`
`nextLine()`	reads an entire line, returns it as a `String`

Example:

import java.util.*;                       // so you can use Scanner
          
          ...

Scanner console = new Scanner(System.in); // "System.in" = scanner reads from the console
System.out.print("How old are you? ");    // prompt
int age = console.nextInt();              // reads from the console
System.out.println("You typed " + age);

Scanner method examples

Suppose we have a Scanner trying to read over the following:

    4.2 abc 4

The following methods would read this as:

Method	What happened?	What's going on?
`nextInt()`	java.util.InputMismatchException	tried to read next token 4.2, couldn't process it as an int.
`nextDouble()`	returns 4.2 as a double.	tried to read next token, succeeded because it could be read as a double.
`next()`	returned "4.2" as a String.	read the next word as a String.
`nextLine()`	returns "4.2 abc 4" as a String.	read the whole next line as a String.

⚠ CAUTION: do not mix `.next()` and `.nextLine()`! ⚠

Mixing .next() and .nextLine() will give unexpected results. Make sure that each Scanner you create only reads words (using .next()) OR entire lines (.nextLine())!

Exercise : Working with Scanners!

Copy and paste the following code into jGrasp.

import java.util.*;  // needed to use Scanners

public class Introduction {
   public static void main(String[] args) {
      Scanner input = new Scanner(System.in);
      System.out.print("What's your age? ");     // prompt, note the print() (not a println())
      int age = input.nextInt(); 
      System.out.println("You are " + age);
   }
}

Try running the program and entering in your age (or any number). What happens if you enter something that isn't an integer? What happens if you enter a random word like "banana"?

continued on next slide...

Exercise : Working with Scanners

Now modify the code so that it prompts the user to enter their name, stores the user's String input in a String variable, then prints the user's name rather than their age.

Example output:

What's your name? Brett
You are Brett

The solution is on the next slide, if you get stuck!

Exercise : Solution

This is one possible solution:

import java.util.*; // needed to use Scanners

public class Introduction {
   public static void main(String[] args) {
      Scanner input = new Scanner(System.in);
      System.out.print("What's your name? ");
      String name = input.next();
      System.out.println("You are " + name);
   }
}

Can you still enter an integer in this modified program? What about a double? Why?

Cumulative algorithms

A cumulative algorithm involves incrementally accumulating a value by repeatedly adding, multiplying, dividing, etc., while storing the result in a variable.

Key aspects of a cumulative algorithm: A loop, and a variable declared outside the loop whose value is modified inside the loop.

Example: Cumulative algorithm to sum the numbers 1-100:

int sum = 0;                // safe default value, 0 doesn't affect a sum
for (int i = 1; i <= 100; i++) {
    sum = sum + i;
}
System.out.println(sum);    // 5050

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`Conditional` Statements

Conditional statements lets your program choose to do different things based on the results of tests, which evaluate certain parts of a program. There are 3 structures that we can use:

if statements
else statements
else if statements

`if` statements

if statements are composed of a test, and some code to execute if that test is true (ex. 2 + 2 = 4 is true).

if (test) {
    statement(s);  // executes if test is true
} 
System.out.println("yay!")  // prints yay, business as usual outside the if  statement

Example:

if (gpa >= 2.0) { 
    System.out.println("Welcome to Mars University!");  // perform this code is gpa >= 2.0
}

Exercise : `if` statements

What does the following code output?

x = 5;
if (x < 0) {
    System.out.println("inside if branch!");
} 
System.out.println("outside if branch!");

Output:

outside if branch!

Exercise : `if` statements

What does the following code execute?

x = -2;
if (x < 0) {
    System.out.println("inside if branch!");
} 
System.out.println("outside if branch!");

Output:

inside if branch!
outside if branch!

Exercise : `if` statements

What does the following code execute?

x = -2;
if (x < 0) {
    System.out.println("inside if branch!");
} 
if (x < -1) {
    System.out.println("inside this branch!");
}
System.out.println("outside if branch!");

Output:

inside if branch!
inside this branch!
outside if branch!

Note: it's possible for any if statement to execute, or not. If you have a couple if statements next to each other, 0 of them, 1 of them, or both of them could execute.

`if/else` statements

if/else statements are statements that include two branches, one of which is always executed. They allow us to control our program's behavior when the if statement's test evaluates to false.

if (test) {
    statement
} else {              //  implicitly, test is false
    statement
}

Example:

if (gpa >= 2.0) {
    System.out.println("Welcome to Mars University!");  // perform this code is gpa >= 2.0
} else { 
    System.out.println("Please apply again soon.");     // perform this code is gpa < 2.0
}

Exercise : `if-else` practice

What does the following program output?

public static void main(String[] args) {
   int x = 5;
   if (x < 5) {
      System.out.println("abc");
   } else {
      System.out.println("def");
   }
   System.out.println("ghi");
}

Output:

def
ghi

Exercise : `if-else` practice

What does the following program output?

public static void main(String[] args) {
   int x = 2;
   if (x < 5) {
      System.out.println("abc");
   } else {
      System.out.println("def");
   }
   System.out.println("ghi");
}

Output:

abc
ghi

Note: With if-else structures, exactly one branch will execute. It is impossible for neither the if branch or the else branch to execute. It is also impossible for both to execute.

nested `if/else` statements

Nested if/else statements allow you to write code that executes if its test is met, but only in the case that an if statement before it has already evaluated to false.

if (test1) {
   statement  // executes if test1 == true
} else if (test2) {
   statement  // executes if test1 -= false AND test 2 == true
}

Example:

if (gpa >= 2.0) { 
    System.out.println("Welcome to Mars University!");  // perform this code is gpa >= 2.0
} else if (gpa < 0.7) 
    System.out.println(":*(");  // perform this code when gpa < 2.0 and < 0.7 
}

Exercise : `else-if` practice

What does the following code output?

public static void main(String[] args) {
   int x = 5;
   if (x > 6) { 
      System.out.println("abc");
   } else if (x < 6) {
      System.out.println("def");
   }
   System.out.println("ghi");
}

Output:

def
ghi

Exercise : `else-if` practice

What does the following code output?

public static void main(String[] args) {
   int x = 4;
   if (x < 5) { 
      System.out.println("abc");
   } else if (x < 6) {
      System.out.println("def");
   }
   System.out.println("ghi");
}

Output:

abc
ghi

Exercise : `else-if` practice

What does the following code output?

public static void main(String[] args) {
   int x = 8;
   if (x < 5) { 
      System.out.println("abc");
   } else if (x < 6) {
      System.out.println("def");
   }
   System.out.println("ghi");
}

Output:

ghi

Note that even if both the if and else if tests are satisfied, we still only execute 1 of the branches at most! It's also possible for neither the if or else if branches to execute.

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`if`/`else` factoring

Recall that with if/else, exactly 1 branch executes. This make a new kind of redundancy possible!

if (x < 30) {
    a = 2;
    x++;
    System.out.println("CSE 142 TAs are awesome! " + x);
} else {
    a = 2;
    x--;
    System.out.println("CSE 142 TAs are awesome! " + x);
}

Because the red code will happen no matter what (in the if and the else case), it can be factored out:

a = 2;
if (x < 30) {
    x++;
} else {
    x--;
}
System.out.println("CSE 142 TAs are awesome! " + x);

[an error occurred while processing this directive]

Checkpoint: Congratulations!

Nice job making it this far--labs are tough! Feel free to work with the person next to you for the remaining slides. Labs are a unique opportunity (unlike homework) to collaborate directly on ideas, and practice peer programming.

These next problems get a little more challenging as we explore earlier concepts further.

We put a lot of problems in here so that you have plenty to refer back to later when working on homework. Don't feel bad if you don't finish all of them--Brett can't finish them all in a 50 minute lab, either! :)

Forest the cat says good job!

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If you finish them all...

If you finish all the exercises, try out our Practice-It web tool. It lets you solve Java problems from our Building Java Programs textbook.

You can view an exercise, type a solution, and submit it to see if you have solved it correctly.

Choose some problems from the book and try to solve them!

University of Washington, CSE 142

Lab 4: Return, if/else, and Scanner

University of Washington, CSE 142

Lab 4: Return, if/else, and Scanner

Basic lab instructions

Today's lab

Exercise : Check Canvas scores

Returning Values

Exercise : Returns

Exercise : Returns

Math expression syntax

User input and Scanner

Scanner method examples

⚠ CAUTION: do not mix .next() and .nextLine()! ⚠

Exercise : Working with Scanners!

Exercise : Working with Scanners

Exercise : Solution

Cumulative algorithms

Conditional Statements

if statements

Exercise : if statements

Exercise : if statements

Exercise : if statements

if/else statements

Exercise : if-else practice

Exercise : if-else practice

nested if/else statements

Exercise : else-if practice

Exercise : else-if practice

Exercise : else-if practice

if/else factoring

Checkpoint: Congratulations!

If you finish them all...

User input and `Scanner`

⚠ CAUTION: do not mix `.next()` and `.nextLine()`! ⚠

`Conditional` Statements

`if` statements

Exercise : `if` statements

Exercise : `if` statements

Exercise : `if` statements

`if/else` statements

Exercise : `if-else` practice

Exercise : `if-else` practice

nested `if/else` statements

Exercise : `else-if` practice

Exercise : `else-if` practice

Exercise : `else-if` practice

`if`/`else` factoring