3.0 Table of Contents

3.1 Session 2 Review

3.1.1 Review of Topics

  • Variables and Data Types
  • Operators
  • Type Conversion and Type Casting
  • Methods and parameters
  • Conditionals

3.1.2 Solution to Assignment 2

View Solution

3.1.3 Frequent Error #1

The declaration of the main method should be and can only be:

public static void main(String[] args) {
    // code
}

3.1.4 Frequent Error #2

Return values: if a method has a return type, it must return a value of that type

public static double calculateWage(double hourlyWage, int hours) {
    if (hourlyWage < 15.0)
        return -1.0
        else if (hours > 60)
        return -1.0
        else {
        double wage = 0.0;
        ...
        return wage;
    }
}

3.1.5 Frequent Error #3

variables can only be declared once, and no two variables can have the same name

public static int pay(double hourlyWage, int hours) {
    int wage = 0; // OK
    …
    int wage = 0; // salary already defined!!
    …
    double wage = 0; //salary already defined!!
    …
}

3.2 Good Programming Style

  • The goal of good style is to make your code more readable by you and by others

3.2.1 Rule #1: Naming Conventions

Bad:

String a1;
int a2;
double b;

Good:

String firstName;
String lastName;
int age;
  • Use meaningful names for variables, methods, and classes
  • variable and method names should be lowerCamelCase
  • class names should be UpperCamelCase

3.2.2 Rule #2: Indentation

Bad:

public static void main(String[] args) {
int x = 5;
x = x * x;
if (x > 20) {
System.out.println("x is greater than 20");
}
}

Good:

public static void main(String[] args) {
    int x = 5;
    x = x * x;
    if (x > 20) {
        System.out.println("x is greater than 20");
    }
}
  • Indentation is important for readability
  • although indentation is not required for the compiler, it is required for the human reader

3.2.3 Rule #3: Whitespace

put spaces between operators and operands

Bad:

double celcius=5/9*(fahrenheit-32);

Good:

double celcius = 5 / 9 * (fahrenheit - 32);

3.2.4 Rule #4: Comments

  • Comments are used to explain what your code does
  • they are ignored by the compiler
  • denote comments with // for single line comments and /* */ for multi-line comments
// this is a single line comment

/* this
is
a
multi-line
comment */

3.3 Loops

static void main(String[] args) {
    System.out.println("Rule #1");
    System.out.println("Rule #2");
    System.out.println("Rule #3");
    System.out.println("Rule #4");
}

  • if I wanted to print out the rules 100 times, I would have to copy and paste the System.out.println() statement 100 times

  • Loops are used to repeat a block of code
  • there are two types of loops in Java: for loops and while loops

3.3.1 while Loops

while (condition) {
    // code to be repeated
}

Example:

int i = 0;
while (i < 10) {
    System.out.println("Rule #" + i);
    i++;
}
  • the while loop will repeat the code inside the block as long as the condition is true
  • therefore it is important to update the condition inside the loop and make sure that it will eventually become false
  • otherwise, the loop will run forever (infinite loop)

3.3.2 for Loops

for (initialization; condition; update) {
    // code to be repeated
}

Example:

for (int i = 0; i < 10; i++) {
    System.out.println("Rule #" + i);
}
  • for loops combine the initialization, condition, and update into one line, making it easier to read and write
  • the for loop is equivalent to the while loop above

3.3.3 Branching Statements

  • Branching statements are used to control the flow of a program

  • break and continue are two branching statements that are used with loops

  • break terminates a for or while loop
for (int i = 0; i < 10; i++) {
    if (i == 5) {
        break;
    }
    System.out.println("Rule #" + i);
}

continue skips the current iteration of a loop and proceeds directly to the next iteration

for (int i = 0; i < 10; i++) {
    if (i == 5) {
        continue;
    }
    System.out.println("Rule #" + i);
}

3.3.4 Embedded Loops

  • Embedded loops are loops inside of loops
  • they can also be called nested loops
for (int i = 0; i < 10; i++) {
    for (int j = 0; j < 10; j++) {
        System.out.println("i = " + i + ", j = " + j);
    }
}

the variable i will be incremented 10 times, and for each iteration of i, the variable j will be incremented 10 times

  • therefore, the code inside the inner loop will be executed 100 times
  • each variable declared inside the loop is only accessible inside the loop

3.4 Arrays

  • An array is an ordered and indexed collection of elements of the same type
  • arrays are used to store multiple values in a single variable
  • an array can be of any type
    • int
    • double
    • String
    • etc.

Example: an array of ints

3.4.1 Declaring Arrays

  • arrays are declared using TYPE[] NAME = new TYPE[SIZE]
  • they are just another type of variable
  • int[] is an array of ints
  • int[][] is an array of int[]s
int[] numbers = new int[5];
int size = 5;
int[] numbers = new int[size];

3.4.2 Array Initialization

  • curly braces {} are used to initialize arrays
  • it can be done in two ways
int[] numbers = {1, 2, 3, 4, 5};
int[] numbers = new int[] {1, 2, 3, 4, 5};

3.4.3 Array Indexing

  • The index of an array starts at 0 and ends at the length of the array minus 1
  • to access an element of an array, use array[index]
int[] numbers = new int[5];
numbers[0] = 1;
numbers[1] = 2;
numbers[5] = 3; // this will cause an error

int[] numbers = {1, 2, 3, 4, 5};
numbers[3] = 10; // this will change the value of the 4th element to 10
int x = numbers[3] + 3; // x = 13
  • the length of an array can be accessed using array.length
int[] numbers = {1, 2, 3, 4, 5};
int size = numbers.length; // size = 5

int[] numbers = new int[7];
int size = numbers.length; // size = 7

3.5 Combine Loops and Arrays

3.5.1 Example 1

int[] values = new int[5];

for (int i = 0; i < values.length; i++) {
    values[i] = i;
    int y = values[i] * values[i];
    System.out.println(y);
}

3.5.2 Example 2

int[] values = new int[5];
int i = 0;
while (i < values.length) {
    values[i] = i;
    int y = values[i] * values[i];
    System.out.println(y);
    i++;
}

3.6 Summary

  1. Programming Style
  2. Loops
  3. Arrays

3.7 Assignment 3 - Marathon Runners

A group of students decided to run in a Marathon. Their names and finishing times are given below. Write a program that print out the name of the fastest runner.

Name Time (minutes)
Elena 341
Thomas 273
Hamilton 278
Suzie 329
Phil 445
Matt 402
Alex 388
Emma 275
John 243
James 334
Jane 412
Emily 393
Daniel 299
Neda 343
Aaron 317
Kate 265

Specifications

  • Write a method that takes as input an array of integers and returns the index corresponding to the person with the lowest time. Run this method on the array of times. Print out the name and time corresponding to the returned index.

  • Write a second method to find the second-best runner. The second method should use the first method to determine the best runner, and then loop through all values to find the second-best (second lowest) time.

Program Skeleton

class Marathon {
    public static void main (String[] args) {
        String[] names = {
            "Elena", "Thomas", "Hamilton", "Suzie", "Phil", "Matt", "Alex",
            "Emma", "John", "James", "Jane", "Emily", "Daniel", "Neda",
            "Aaron", "Kate"
        };

        int[] times = {
            341, 273, 278, 329, 445, 402, 388, 275, 243, 334, 412, 393, 299,
            343, 317, 265
        };

        // call method to find the index of the fastest runner
        // print out the name and time of the fastest runner
        int fastestIndex = findFastestRunner(times);
        System.out.println("Fastest runner: " + names[fastestIndex] + " (" + times[fastestIndex] + " minutes)");

        // call method to find the index of the second-best runner
        // print out the name and time of the second-best runner
        int secondBestIndex = findSecondBestRunner(times);
        System.out.println("Second-best runner: " + names[secondBestIndex] + " (" + times[secondBestIndex] + " minutes)");
    }
}

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