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

Today's lab

Goals for today:

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

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
Math.round(grade);                             //    grade      =   2.7
long roundedGrade = Math.round(grade);         // roundedGrade  =   3
roundedGrade = Math.max(roundedGrade, 4);      // roundedGrade  =   4

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

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

Exercise : area practice-it

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);

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 : Scanner sum

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 sum

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.

Exercise : Scanner sum solution

import java.util.*;

public class SumNumbers {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        
        System.out.print("low? ");
        int low = console.nextInt();
        System.out.print("high? ");
        int high = console.nextInt();
        
        int sum = 0;
        for (int i = low; i <= high; i++) {
            sum += i;
        }
        System.out.println("sum = " + sum);
    }
}

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? 
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2
3
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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? 
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2
3
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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? 
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2
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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? 
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2
3
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5
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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? 
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2
3
4
5
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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 practice-it

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.

Exercise : pow practice-it

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. practice-it

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 icon 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 practice-it

  • 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!