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

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Talk to your classmates for help.
You may want to bring your textbook to future labs to look up syntax and examples.
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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.
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Today's lab

Goals for today:

use return values to send data between methods
use if, else if and else to have different branches of execution
use Scanner to create interactive programs that read user input
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 (none) 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

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);

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 
}

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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);

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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.

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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

Math expression syntax

User input and Scanner

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

if/else factoring

If you finish them all...

User input and `Scanner`

`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

`if`/`else` factoring