Basic Concepts
Advanced Programming
SUT • Spring 2019
Review
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Variables
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Primitive data types
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Operators
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Methods
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Parameter passing
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Call by value
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Conditions
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If, else, else if
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Loops
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while
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do-while
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for
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Outline
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Review
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User input
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Scanner
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Strong type checking
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Other flow-control structures
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switch
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break & continue
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Strings
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Arrays
User input
User input
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Print on console
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System.out.println
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How to read from console?
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Scanner
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import java.util.Scanner;
public class UserInput {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int n = scanner.nextInt();
double d = scanner.nextDouble();
}
public static void power() {
Scanner scanner = new Scanner(System.in);
int a = scanner.nextInt();
int b = scanner.nextInt();
long pow = (long)Math.pow(a, b);
System.out.println(pow);
}
}
Type Checking
Type Checking
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Java has a strong type-checking mechanism
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Some assignment is not permitted
public class TypeChecking{
public static void main(String[] args){
int intVal = 2;
long longVal =12;
// intVal = longVal; // Syntax Error
longVal = intVal; // OK
intVal = (int)longVal; // OK (Type Casting)
}
}
Direct Type Conversion
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The arrows are transitive
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All other conversions need an explicit cast
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boolean is not convertible
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char is a special type
short
byte
char
int
long
float
double
boolean
Type Conversion Grid
From | boolean | byte | short | char | int | long | float | double |
---|---|---|---|---|---|---|---|---|
boolean | - | N | N | N | N | N | N | N |
byte | N | - | Y | C | Y | Y | Y | Y |
short | N | C | - | C | Y | Y | Y | Y |
char | N | C | C | - | Y | Y | Y | Y |
int | N | C | C | C | - | Y | Y* | Y |
long | N | C | C | C | C | - | Y* | Y* |
float | N | C | C | C | C | C | - | Y |
double | N | C | C | C | C | C | C | - |
Type Conversion
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N : the conversion cannot be performed
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Y : the conversion is performed automatically and implicitly by Java
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C : the conversion is a narrowing conversion and requires an explicit cast
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Y* : the conversion is an automatic widening conversion, but that some of the least significant digits of the value may be lost by the conversion
Example
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floating-point types are approximations of numbers
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They cannot always hold as many significant digits as the integer types
public class TypeConversion {
public static void main(String[] args) {
int i = 123456789; //a big integer
double f = i; //f stores and approximation of i
System.out.println(f);//output : 1.23456792E8
i = (int) f;
System.out.println(i); //output : 123456792
}
}
Floating Point, Some Notes
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Double.NaN
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Infinity
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Negative infinity
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Formatting a double
public class FloatingPoint{
public static void main(String[] args){
double nan = 0.0 / 0.0;
double pinf = Double.MAX_VALUE * 2;
double ninf = Double.MAX_VALUE * (-2);
System.out.println(Double.NEGATIVE_INFINITY);
System.out.println(Double.POSITIVE_INFINITY);
System.out.format("min double = %.2f", 12.216213);
}
}
Comparison
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Compare doubles
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Using == with float or double is an anti-pattern
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An infinite loop:
public class InfiniteLoop{
public static void main(String[] args){
for (float f = 0f; f != 10f; f += 0.1){
System.out.println(f);
}
}
}
Numeric Assignments
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Numeric Suffix
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Assignment Overflow
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Large long to int
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Lower bits are used
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No runtime error
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Large double to integer
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Brings a max int
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public class NumericSuffix{
public static void main(String[] args){
Double d = 123.54d;
Float f = 123f;
Long l = 123123l;
byte b = 127; // Nothing
}
}
Switch Statement
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An alternative to if-else
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Better structure
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Before Java 1.7
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When the condition is a numeric or an ordinal variable
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With Java 1.7
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Strings are also allowed
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Switch Example
import java.util.Scanner;
public class Switch {
public static void main(String[] args) {
int j = 1;
switch (j) {
case 1:
System.out.println("1");
break;
case 2:
System.out.println("2");
break;
default:
System.out.println("default");
}
}
}
Switch Example
import java.util.Scanner;
public class Switch {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
boolean again = true;
while (again) {
System.out.println("1: Play");
System.out.println("2: Setting:");
System.out.println("3: Exit");
System.out.print("Enter Your Choice:");
int i = scanner.nextInt();
switch (i) {
case 1:
// play();
break;
case 2:
// setting();
break;
case 3:
again = false;
break;
default:
System.out.println("Enter a valid number");
}
}
}
}
Break
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Breaks the execution of a loop
import java.util.Scanner;
public class ControlLoop {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
while (true) {
int nextInt = scanner.nextInt();
if (nextInt == 0)
break;
}
}
}
Continue
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Stops the execution of the body of the loop and continues from the beginning of the loop
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Difference between continue in for and while
import java.util.Scanner;
public class ControlLoop {
public static void main(String[] args) {
for (int i = 0; i < 10; i++) {
if (i == 4)
continue;
System.out.println(i);
}
}
}
Nested Loops
- How to break or continue from outer loop?
import java.util.Scanner;
public class ControlLoop {
public static void main(String[] args) {
int nextInt;
do {
nextInt = scanner.nextInt();
for (int i = 0; i < nextInt; i++) {
System.out.println(i);
}
} while (nextInt > 0);
}
}
Label
import java.util.Scanner;
public class ControlLoop {
public static void main(String[] args) {
outer:
for (int i = 0; i < 10; i++) {
inner:
for (int j = 0; j < 10; j++) {
if (j == 2)
break outer;
else {
System.out.println(i);
System.out.println(j);
continue inner;
}
}
}
}
}
Tip of the Day: Indentation
import java.util.Scanner;
public class Indentation {
public static void main(String[] args) {
Scanner scanner=new Scanner(System.in);
int nextInt;
do{
nextInt=scanner.nextInt();
for(int i=0; i<nextInt; i++){
System.out.println(i);
}
}while(nextInt>0);
}
}
Tip of the Day: Indentation
import java.util.Scanner;
public class Indentation {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int nextInt;
do {
nextInt = scanner.nextInt();
for (int i = 0; i < nextInt; i++) {
System.out.println(i);
}
} while (nextInt > 0);
}
}
Comments
import java.util.Scanner;
public class Indentation {
public static void main(String[] args) {
// nextInt = scanner.nextInt();
/*
nextInt = scanner.nextInt();
for(int i=0;i<nextInt;i++){
System.out.println(i);
}
*/
/**
* ... text ...
*/
}
}
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Comments are ignored by compiler
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One-line comment
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Multiple-line comment
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Javadoc comments
Example
import java.util.Scanner;
public class Comment {
/**
* @author Ali Alavi
* If the input is a prime number, it returns true
*/
public boolean isPrime(int number) {
if (number < 1)
return false;
/*if(isEven(number))
return false;
*/
for (int i = 2; i < number / 2; i++) //searching for a divisible
if (number % i == 0)
return false;
return true;
}
}
String
Contents
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A sequence of characters
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Character
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Strings
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String is not a primitive type
import java.util.Scanner;
public class StringExample {
public static void main(String[] args) {
char ch;
ch = 'a';
ch = '1';
ch = '#';
String st;
st = "Ali";
st = "123";
st = "1";
st = "";
}
}
String
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String in C and C++
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char* and char[]
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\0 at the end of String
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Some functions
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strlen, strcpy, …
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String in java is String in java is a class
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not equal to char[]
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Constant strings
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“salam!”
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“Hellow World!”
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Example
import java.util.Scanner;
public class StringExample {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
String input;
input = scanner.next();
switch (input) {
case "Salam":
System.out.println("Hi!");
break;
case "Khdahafez":
System.out.println("Bye!");
break;
default:
System.out.println("Ha?!");
break;
}
System.out.println(input);
}
}
Example
import java.util.Scanner;
public class StringExample {
public static void main(String[] args) {
String input = "Nader and Simin, A Separation";
char ch = input.charAt(0);
int i = input.indexOf("Nader");
int j = input.lastIndexOf("Simin");
String newS = input.replace("Separation", "Reconciliation");
String sth = newS + ch + i + j;
System.out.println(sth);
}
}
String Methods
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charAt
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concat: plus (+) operator
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contains
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startsWith
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endsWith
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indesxOf: first index of sth
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lastIndexOf
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replace
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substring
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length
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split
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Regular Expression or Regex
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Regex is a way to describe a set of strings
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Based on their common characteristics
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Regex can be used to search, edit, or manipulate text
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You must learn a specific syntax to create regex
Regex Examples
Regex | Meaning | Ex .1 | Ex .2 | Ex .3 |
---|---|---|---|---|
Salam | Salam | Salam | ||
\d | A digit | 4 | 5 | 9 |
. | Any character | 2 | A | # |
[afg] | a or f or g | a | f | g |
[a-zA-Z] | Range: a through z or A through Z, inclusive | A | a | m |
Salam|bye | Salam or bye | Salam | bye | |
a+ | One or more a | aaaaa | aa | a |
[a-z]+[\d]* | a lowercase string and an optional integer number | ali | ali24 | a43 |
String and Regex
public class Regex {
public static void main(String[] args) {
String input = "Nader and Simin";
boolean noDigitString = input.matches("[\\D]+");
System.out.println(noDigitString);
String[] array = input.split("[ ,]");
for (int i = 0; i < array.length; i++) {
System.out.println(array[i]);
}
}
}
Regex Usage
public class Regex {
public static void main(String[] args) {
String input = "Nader and Simin, A Separation.";
input = input.replace(".", "*");
//input = Nader and Simin, A Separation*
input = input.replaceAll(".", "*");
//i
}
}
Immutable String
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String in java is an immutable class
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After creating a string, you can not change it
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If you want to change it, you should create a new string
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There is no such methods for strings:
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setCharAt(int)
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setValue(String)
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Methods like replace and replaceAll, do not change the value
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They return a new String
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Example
- What is the output of this code?
public class Regex {
public static void main(String[] args) {
String str = "Gholi";
str.replaceAll("li", "lam");
System.out.println(str);
String replaced = str.replaceAll("li", "lam");
System.out.println(replaced);
}
}
Data Hierarchy
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Bit
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Byte
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Character
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Word
Java Characters
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A Java character has two bytes
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Java supports Unicode character set standard
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ASCII
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Java uses UTF-16 encoding
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Other unicode encodings:
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UTF-8
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UTF-16
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Other non-unicode encodings
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Windows-1256
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Java Special Characters
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Some characters are special characters
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Special characters are shown using backslash
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Examples:
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New line: \n
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Tab : \t
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Double-quote : \”
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Single-quote : \’
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Backslash : \\
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Java Special Characters
public class Regex {
public static void main(String[] args) {
String s = "Salam!\nI am S\tA";
System.out.println(s);
s = "\\ \' \"";
System.out.println(s);
/* output:
Salam!
I am S A
\ ' "
*/
}
}
Array
Array
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Collections of related data items
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related data items of the same type
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Arrays are fixed-length entities
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they remain the same length once they are created
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An array is a group of variables
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called elements
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containing values that all have the same type
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The position number of the element is it’s index
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Array elements are sequentially located in memory
Array
Example
public class ArrayExample {
public static void main(String[] args) {
// Create an array of 10 integer elements
int[] array = new int[10];
int array[] = new int[10]; // equal
// Create an array of n characters
char[] characters = new char[n];
// Change value of 5’th element
array[5] = 12;
// Retrieving value of n’th element
char ch = array[n];
}
}
Example
import java.util.Scanner;
public class ArrayExample {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int n = scanner.nextInt();
double numbers[] = new double[n];
for (int i = 0; i < numbers.length; i++) {
numbers[i] = scanner.nextDouble();
}
for (int i = 0; i < numbers.length; i++) {
double d = numbers[i];
System.out.println(d);
}
}
}
Array Creation Shortcut
import java.util.Scanner;
public class ArrayExample {
public static void main(String[] args) {
char[] array = new char[3];
array[0] = 'a';
array[1] = 's';
array[2] = 't';
// The above code can be rewritten as:
char[] array2 = {'a', 's', 't'};
// Other examples:
int[] numbers = {1, 2, 3, 5, 9, 123};
boolean[] b = {true, true, false, true};
}
}
Multidimensional Arrays
import java.util.Scanner;
public class ArrayExample {
public static void main(String[] args) {
int[][] matrix = new int[3][4];
matrix[2][3] = 2;
System.out.println(matrix[2][1]);
int[][] matrix = new int[3][];
matrix[0] = new int[2];
matrix[1] = new int[5];
matrix[2] = new int[4];
matrix[2][3] = 2;
System.out.println(matrix[2][1]);
matrix[0][3] = 2; // Runtime Error (ArrayIndexOutOfBoundsException)
}
}
Passing Arrays to Methods
public class ArrayParameter {
public static void main(String[] args) {
int[] array = {1, 2, -4, 0};
System.out.println(max(array));
}
public static int max(int[] numbers) {
if (numbers == null || numbers.length == 0)
return -1;
int max = numbers[0];
for (int i = 1; i < numbers.length; i++)
if (max < numbers[i])
max = numbers[i];
return max;
}
}
Multi-Dimensional Array Parameters
int determinant(int[][] matrix){…}
int [][] matrix = { {1,2}, {3,4}} ;
int de = determinant(matrix);
void check(int[][] array){…}
int [][] unbalanced = { {1,2}, {3,4,5,6,7,8}};
check(unbalanced);
boolean f(double[][][] cube){…}
Call by Element Values?
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No
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If the method has an array parameter
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Array elements are not copied on method invocations
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A reference to the array is passed to the method
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More about this topic later
Exercises
Write a method for sorting an array of integers
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Write a method that compares two arrays
returns true if elements of the arrays are equal
returns false, otherwise
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Write a method that returns determinant of a matrix
Matrix is a two-dimensional array as the method parameter
Title Text
Basic Concepts
By Behnam Hatami
Basic Concepts
Basic Concepts / Advanced Programming Course @ SUT, Spring 2019
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