# University of Washington, CSE 142

## Lab 4: Return, if/else, and Scanner

Except where otherwise noted, the contents of this document are Copyright 2013 Stuart Reges and Marty Stepp.

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

# Basic lab instructions

• Mouse over 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
• 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! # 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`

# Exercise - Math expressions

Fill in the value of the variable asked about in the comment. Use the proper type (such as `.0` for a `double`). Note that a variable's value changes only if you re-assign it using the `=` operator. You can put an expression into the `Interactions` pane if you're stuck! (Remember to leave off the ; s in Interactions!)

```int min = Math.min(2, 5);                  //   min   = 2
int max = Math.max(4, 9);                  //   max   = 9

double x = Math.pow(2, 4);                 //    x    = 16.0
x = Math.sqrt(64);                         //    x    = 8.0

int count = 25;
double root = Math.sqrt(count);            //  root   = 5.0
count = (int) Math.sqrt(count);            //  count  = 5

int abs = Math.abs(-8);                    //   abs   = 8
```

# Exercise - Math expressions

Fill in the value of the variable asked about in the comment. Use the proper type (such as `.0` for a `double`). Note that a variable's value changes only if you re-assign it using the `=` operator. You put an expression in the Interactions pane if you're stuck! (Remember to leave off the ; s in Interactions!)

```long rounded = Math.round(2.9);                //   rounded     =   3
rounded = Math.round(6.4);                     //   rounded     =   6

double grade = 2.7;                            //    grade      =   2.7

double floor = Math.floor(2.9);                //    floor      =   2.0

double ceiling = Math.ceil(8.4);               //     ceil      =   9.0

```

# 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 : area Consider the following method for converting milliseconds into days:

```// converts milliseconds to days
public static double toDays(double millis) {
return millis / 1000.0 / 60.0 / 60.0 / 24.0;
}
```

Write a similar method named `area` that takes as a parameter the radius of a circle and that returns the area of the circle. For example, the call `area(2.0)` should return `12.566370614359172`. Recall that area can be computed as π times the radius squared and that Java has a constant called `Math.PI`.

# 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);
```

# Exercise : `Scanner`

Copy and paste the following code into jGrasp.

```public class SumNumbers {
public static void main(String[] args) {
int low = 1;
int high = 1000;
int sum = 0;
for (int i = low; i <= high; i++) {
sum += i;
}
System.out.println("sum = " + sum);
}
}
```

continued on next slide...

# Exercise : `Scanner`

Modify the code to use a `Scanner` to prompt the user for the values of `low` and `high`. Below is a sample execution in which the user asks for the same values as in the original program (1 through 1000):

```low? 1
high? 1000
sum = 500500
```

Below is an execution with different values for `low` and `high`:

```low? 300
high? 5297
sum = 13986903
```

You should exactly reproduce this format.

# `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?
 ```1 2 3 4 5``` ```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?
 ```1 2 3 4 5``` ```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?
 ```1 2 3 4 5 6 7 8``` ```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?
 ```1 2 3 4 5 6 7 8 9 ``` ```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?
 ```1 2 3 4 5 6 7 8 9 ``` ```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 : `if`/`else` mystery

```public static void mystery3(int x, int y) {
int z = 4;
if (z <= x) {
z = x + 1;
} else {
z = z + 9;
}
if (z <= y) {
y++;
}
System.out.println(z + " " + y);
}
```
Fill in the boxes with the output produced by each of the method calls.
 `mystery3(3, 20);` `13 21` `mystery3(4, 5);` `5 6` `mystery3(5, 5);` `6 5` `mystery3(6, 10);` `7 11`

# Exercise : `if`/`else` mystery

```public static void mystery(int n) {
System.out.print(n + " ");
if (n > 10) {
n = n / 2;
} else {
n = n + 7;
}
if (n * 2 < 25) {
n = n + 10;
}
System.out.println(n);
}
```
Fill in the boxes with the output produced by each of the method calls.
 `mystery(40);` `40 20` `mystery(0);` `0 17` `mystery(12);` `12 16` `mystery(20);` `20 20`

# Exercise : `if`/`else` mystery

```public static void mystery2(int a, int b) {
if (a < b) {
a = a * 2;
}
if (a > b) {
a = a - 10;
} else {
b++;
}
System.out.println(a + " " + b);
}
```

Fill in the boxes with the output produced by each of the method calls.

 `mystery2(10, 3);` `0 3` `mystery2(6, 6);` `6 7` `mystery2(3, 4);` `-4 4` `mystery2(4, 20);` `8 21`

# Exercise : AgeCheck

• Copy/paste and save the following program in jGRASP, then see the instructions on the next slide.
```public class AgeCheck {
public static void main(String[] args) {
int myAge = 19;   // I am 19; let me see if I can drive
message(myAge);
}

// Displays message about driving to user based on given age
public static void message(int age) {
if (myAge >= 16) {
System.out.println("I'm old enough to drive!");
}
if (myAge <= 16) {
System.out.println("Not old enough yet... :*(");
}
}
}
```

# Exercise - things to fix

• The program has a few syntax errors. Fix them until it compiles.
• The code has a logic problem. (For some value(s), it prints the wrong answer.) Find any such problems and fix them. (You may need to run the program a few times and try different values to see which ones fail.)
• The program uses `if` and `else` in a clumsy way. Improve the style of the code.

# Exercise - answer

• The following is a corrected version of the program:
```public class AgeCheck {
public static void main(String[] args) {
int myAge = 19;   // I am 19; let me see if I can drive
message(myAge);
}

// Displays a message about driving to user based on given age
public static void message(int age) {
if (age >= 16) {
System.out.println("I'm old enough to drive!");
} else {
System.out.println("Not old enough yet... :*(");
}
}
}
```

# Exercise : pay Write a method named `pay` that accepts two parameters: a real number for a TA's salary, and an integer for the number of hours the TA worked this week. The method should return how much money to pay the TA. For example, the call `pay(5.50, 6)` should return `33.0`.

The TA should receive "overtime" pay of 1 ½ normal salary for any hours above 8. For example, the call `pay(4.00, 11)` should return (4.00 * 8) + (6.00 * 3) or `50.0`.

# 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
```

# Exercise : pow Write a method named `pow` that accepts a base and an exponent as parameters and returns the base raised to the given power. For example, the call `pow(3, 4)` returns 3 * 3 * 3 * 3 or 81. Do not use `Math.pow` in your solution; use a cumulative algorithm instead. Assume that the base and exponent are non-negative. See ch4 lecture slides on cumulative sums for a hint.

• (Try solving this problem in Practice-It by clicking the icon above.)
• For added challenge, try turning your solution into a second version `pow2` that works with real number bases and negative exponents, as in book Exercise 4.11. # `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);
```

# Exercise : `if`/`else` Factoring

• Download the following program FactorExample.java to your machine and open it with jGrasp.
• The program's method has a few issues with external correctness and redundancy. Fix the code, and factor the method, restructuring the code to eliminate unnecessary statements while retaining the same behavior. This might involve changing conditional statements (ex. changing an `if` to an `else if`).
• (When fixed,) running the program should produce the following output:

• continued on the next slide...

# Exercise : `if/else` Factoring

Expected Output
```I'm valedictorian for this class! Woo hoo!
I made the dean's list for this class!
I received 5 credits for this class.

I made the dean's list for this class!
I received 5 credits for this class.

I received 5 credits for this class.

Uh-oh..I probably should have studied a little more.
I received 0 credits for this class.

Uh-oh..I probably should have studied a little more.
I received 5 credits for this class.```

# Exercise : `if/else` Factoring

```    public static void factoring(double gpa) {
int credits = 5;  // since we want credits = 5 in all cases except gpa <= 0.7
if (gpa == 4.0) {
credits = 5;
System.out.println("I'm valedictorian for this class! Woo hoo!");
System.out.println("I made the dean's list for this class!");
}
else if (gpa >= 3.5) {  // we want the same behavior for all gpa >= 3.5 cases (4.0 or not)
credits = 5;
System.out.println("I made the dean's list for this class!");
} else {
credits = 5;
} else if (gpa < 1.5) {  // gpa can be < 1.5, >= 3.5, or neither
System.out.println("Uh-oh..I probably should have studied a little more.");
}
if (gpa <= 0.7) {
System.out.println("Uh-oh..I probably should have studied a little more.");
credits = 0;
}
System.out.println("I received " + credits + " credits for this class.");
System.out.println();
}```

# Exercise : season • Write a method named `season` that takes two integers as parameters representing a month and day and that returns a String indicating the season for that month and day. Assume that months are specified as an integer between 1 and 12 (1 for January, 2 for February, and so on) and that the day of the month is a number between 1 and 31.
• If the date falls between 12/16 and 3/15, you should return `"Winter"`. If the date falls between 3/16 and 6/15, you should return `"Spring"`. If the date falls between 6/16 and 9/15, you should return `"Summer"`. And if the date falls between 9/16 and 12/15, you should return `"Fall"`.
• Try solving this problem in Practice-It! using the link above.

# 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!