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29. Methods 16 System.out.println("main: b = " + b); 17 18 change(a, b); 19 20 System.out.println("main after: s = " + a); 21 System.out.println("main after: b = " + b); 22 23 } 24 } To see this, observe the following output. When we return to the main method, the original string s is unchanged (since it was immutable)... | ComputerScienceOne_Page_462_Chunk2001 |
29.2. Examples on. We could demonstrate good code reuse (as well as good procedural abstraction) by scaling the input value and reusing the functionality already provided in the math library’s Math.round() method. We could further define a roundToCents() method that used our generalized round method. Finally, we could p... | ComputerScienceOne_Page_463_Chunk2002 |
29. Methods is not executable itself. It only provides functionality to other classes in the code base. 430 | ComputerScienceOne_Page_464_Chunk2003 |
30. Error Handling & Exceptions Java supports error handling through the use of exceptions. Java has many different predefined types of exceptions that you can use in your own code. It also allows you to define your own exception types by creating new classes that inherit from the predefined classes. Java uses the standard... | ComputerScienceOne_Page_465_Chunk2004 |
30. Error Handling & Exceptions 4 try { 5 String input = s.next(); 6 n = Integer.parseInt(input); 7 } catch (NumberFormatException nfe) { 8 System.err.println("You entered invalid data!"); 9 System.exit(1); 10 } In this example, we’ve simply displayed an error message to the standard error output and exited the program... | ComputerScienceOne_Page_466_Chunk2005 |
30.1. Exceptions Note that the last catch block was written to catch a generic Exception . This last block will essentially catch any other type of exception. Much like an if-else-if statement, the first type of exception that is caught is the block that will be executed and they are all mutually exclusive. Thus, a “cat... | ComputerScienceOne_Page_467_Chunk2006 |
30. Error Handling & Exceptions Now in our code we can throw and catch this new type of exception. 1 //throw this exception: 2 if( b*b - 4*a*c < 0) { 3 throw new ComplexRootException("Cannot Handle complex roots"); 4 } 1 try { 2 r1 = getComplexRoot01(a, b, c); 3 } catch(ComplexRootException cre) { 4 //handle the except... | ComputerScienceOne_Page_468_Chunk2007 |
30.1. Exceptions 6 //handle the exception here 7 } 8 } or we would need to specify that the method processFile() explicitly throws the exception: 1 public static void processFile() throws FileNotFoundException { 2 Scanner s = new Scanner(new File("data.csv")); 3 } Doing this, however, would force any code that called t... | ComputerScienceOne_Page_469_Chunk2008 |
30. Error Handling & Exceptions exceptions were a mistake and their usage should be avoided. The rationale behind checked exceptions is summed up in the following quote from the Java documentation [7]. Here’s the bottom line guideline: If a client can reasonably be expected to recover from an exception, make it a check... | ComputerScienceOne_Page_470_Chunk2009 |
30.2. Enumerated Types 8 SATURDAY; 9 } In the example, since the name of the enumeration is Day this declaration must be in a source file named Day.java . We can now declare variables of this type. The possible values it can take are restricted to SUNDAY , MONDAY , etc. and we can use these keywords in our program. Howe... | ComputerScienceOne_Page_471_Chunk2010 |
30. Error Handling & Exceptions 1 for(Day d : Day.values() { 2 System.out.println(d.name()); 3 } In the example above, we used another feature: each enum value has a name() method that returns the value as a String . This example would end up printing the following. SUNDAY MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY SATUR... | ComputerScienceOne_Page_472_Chunk2011 |
31. Arrays Java allows you to declare and use arrays. Since Java is statically typed, arrays must also be typed when they are declared and may only hold that particular type of element. Since Java has automated garbage collection, memory management is greatly simplified. Finally, in Java, only locally scoped primitives ... | ComputerScienceOne_Page_473_Chunk2012 |
31. Arrays Each of these initializations creates a new array (allocated on the heap) of the specified size (10, 20, and 5 respectively). These arrays can only hold values of the specified type, int , double , and String respectively. The default value for each element in these new arrays will be zero for the numeric type... | ComputerScienceOne_Page_474_Chunk2013 |
31.1. Basic Usage catch and handle if you choose. To prevent such an exception you can write code that does not exceed the bounds of the array. Java arrays have a special length property that gives you the size of the array. You can access the property using the dot operator, so arr.length would give the value 5 for th... | ComputerScienceOne_Page_475_Chunk2014 |
31. Arrays 31.2. Dynamic Memory The use of the keyword new dynamically allocates memory space (on the heap) for the array. Because Java has automated garbage collection, if the reference to the array goes out of scope, it is automatically cleaned up by the JVM. This process is automated and essentially transparent to u... | ComputerScienceOne_Page_476_Chunk2015 |
31.4. Multidimensional Arrays In Java, arrays are always passed by reference. Though we did not make any changes to the contents of the passed array in the particular example, in general we could have. Any such changes would be realized in the calling method. Unfortunately, there is no mechanism by which we can prevent... | ComputerScienceOne_Page_477_Chunk2016 |
31. Arrays This creates a 2-dimensional array of integers with 10 rows and 20 columns. Once created, we can index individual elements by specifying row and column indices. 1 for(int i=0; i<matrix.length; i++) { 2 for(int j=0; j<matrix[i].length; j++) { 3 matrix[i][j] = 10; 4 } 5 } 31.5. Dynamic Data Structures The Java... | ComputerScienceOne_Page_478_Chunk2017 |
31.5. Dynamic Data Structures would be a compiler error to attempt to add anything other than Integer s to the first list or anything other than String s to the second. Once these lists have been created, you can add and remove elements using the add() method. 1 a.add(42); 2 a.add(81); 3 a.add(17); 4 5 b.add("Hello"); 6... | ComputerScienceOne_Page_479_Chunk2018 |
31. Arrays Any attempt to access an element that lies outside the bounds of the List , will result in an IndexOutOfBoundsException just as with arrays. To stay within bounds you can use the size() method to determine how many elements are in the collection. In this example, values.size() would return an integer value o... | ComputerScienceOne_Page_480_Chunk2019 |
31.5. Dynamic Data Structures 3 } When this code executes we cannot expect any particular order of the three names. Any permutation of the three may be printed. If we executed the loop more than once we may even observe a different enumeration of the names! Finally, Java also supports a Map data structure which allows y... | ComputerScienceOne_Page_481_Chunk2020 |
32. Strings As we’ve previously seen, Java has a String class in the standard JDK. Internally, Java strings are stored as arrays of characters. However, because of the String class, we never directly interact with this representation. Making using strings much easier than in other languages. Java strings have also supp... | ComputerScienceOne_Page_483_Chunk2021 |
32. Strings charAt() method and providing an index. Characters in a string are 0-indexed just as with elements in arrays. 1 String fullName = "Tom Waits"; 2 //access individual characters: 3 char firstInitial = fullName.charAt(0); //'T' 4 char lastInitial = fullName.charAt(4); //'W' 5 6 if(fullName.charAt(8) == 's') { ... | ComputerScienceOne_Page_484_Chunk2022 |
32.2. String Methods 5 //or we can use toCharArray and an enhanced for loop: 6 for(char c : fullName.toCharArray()) { 7 System.out.println(c); 8 } Concatenation Java has a concatenation operator built into the language. The familiar plus sign, + can be used to combine one or more strings by appending them to each other... | ComputerScienceOne_Page_485_Chunk2023 |
32. Strings 1 String firstName = "Tom"; 2 String lastName = "Waits"; 3 4 String formattedName = new StringBuilder(lastName) 5 .append(", ") 6 .append(firstName) 7 .toString(); You can also use the StringBuilder class yourself directly. Computing a Substring There are two methods that allow you to compute a substring of... | ComputerScienceOne_Page_486_Chunk2024 |
32.4. Comparisons We can create our own arrays of strings similar to how we created arrays of int and double types. 1 //create an array that can hold 5 strings 2 String names[] = new String[5]; 3 4 names[0] = "Margaret Hamilton"; 5 names[1] = "Ada Lovelace"; 6 names[2] = "Grace Hopper"; 7 names[3] = "Marie Curie"; 8 na... | ComputerScienceOne_Page_487_Chunk2025 |
32. Strings 6 } The code above will not print anything even though the strings a and b have the same content. This is because a == b is comparing the memory address of the two variables. Since they point to different memory addresses (created by two separate calls to the constructors) they will not be equal. Instead, th... | ComputerScienceOne_Page_488_Chunk2026 |
32.5. Tokenizing 1 String a = "apple"; 2 String b = "apple"; 3 String c = "Hello"; 4 5 boolean result; 6 result = a.equals(b); //true 7 result = a.equals(c); //false 32.5. Tokenizing Recall that tokenizing is the process of splitting up a string using some delimiter. For example, the comma delimited string, "Smith,Joe,... | ComputerScienceOne_Page_489_Chunk2027 |
32. Strings 1 String s = "Alpha Beta \t Gamma \n Delta \t\nEpsilon"; 2 String tokens[] = s.split("[\\s]+"); 3 //tokens is now { "Alpha", "Beta", "Gamma", "Delta", "Epsilon" } 456 | ComputerScienceOne_Page_490_Chunk2028 |
33. File I/O Java provides several different classes and utilities that support manipulating and pro- cessing files. In general, most file operations may result in an IOException , a checked exception that must be caught and handled. 33.1. File Input Though there are several ways that you can do file input, the easiest is ... | ComputerScienceOne_Page_491_Chunk2029 |
33. File I/O 1 String line; 2 while(s.hasNext()) { 3 line = s.nextLine(); 4 //process the line 5 } Once we are done reading the file, we can close the Scanner to free up resources: s.close(); . We could have placed all this code within one large try-catch block with perhaps a finally block to close the Scanner once were... | ComputerScienceOne_Page_492_Chunk2030 |
33.2. File Output 33.2. File Output There are several ways to achieve file output, but we’ll look at the simplest and most useful way. Java provides a PrintWriter class which offers many convenient methods for writing primitive and String types in a formatted manner. It has the usual print() and println() methods that we... | ComputerScienceOne_Page_493_Chunk2031 |
33. File I/O However, if necessary, binary output can be done using a FileOutputStream . Typically, you can load all your data into a byte array and dump it all at once. 1 byte data[] = ...; 2 try (FileOutputStream fos = 3 new FileOutputStream(new File("outfile.bin")) ){ 4 fos.write(data); 5 } catch(IOException ioe) { ... | ComputerScienceOne_Page_494_Chunk2032 |
34. Objects Java is a class-based object-oriented programming language, meaning that it facilitates the creation of objects through the use of classes. Classes are essentially “blueprints” for creating instances of objects. We’ve been implicitly using classes all along since everything in Java is a class or belongs to ... | ComputerScienceOne_Page_495_Chunk2033 |
34. Objects Recall that a package declaration allows you to organize classes and code within a package (directory) hierarchy. Moreover, source code for a class must be in a source file with the same name (and is case sensitive) with the .java extension. Our Student class would need to be in a file named Student.java and ... | ComputerScienceOne_Page_496_Chunk2034 |
34.2. Methods code (through reflection2 or other means) and the values of variables can be accessed or modified. Modifier Class Package Subclass World public Y Y Y Y protected Y Y Y N none (default) Y Y N N private Y N N N Table 34.1.: Java Visibility Keywords & Access Levels We now update our class declaration to incorpo... | ComputerScienceOne_Page_497_Chunk2035 |
34. Objects In contrast to the methods we defined in Chapter 29, when defining a member method, we do not use the static keyword. Making a variable or a method static means that the method belongs to the class and not to instances of the class. We add to our example by providing two public methods that compute and return... | ComputerScienceOne_Page_498_Chunk2036 |
34.2. Methods or mutate (that is, change) an instance’s variables, we can define accessor and mutator methods (or just simply getter and setter methods) to facilitate this. Each getter method returns the value of the instance’s variable while each setter method takes a value and sets the instance’s variable to the new v... | ComputerScienceOne_Page_499_Chunk2037 |
34. Objects 1 public String getFirstName() { 2 return this.firstName; 3 } 4 5 public void setFirstName(String firstName) { 6 this.firstName = firstName; 7 } One advantage to using getters and setters (as opposed to naively making everything public ) is that you can have greater control over the values that your variabl... | ComputerScienceOne_Page_500_Chunk2038 |
34.3. Constructors the class in a multithreaded program without having to worry about threads changing the state of instances on one another. Immutable classes are also safer to use in certain collections such as a Set . Elements in a Set are unique; attempting to add a duplicate element will have no effect on the Set .... | ComputerScienceOne_Page_501_Chunk2039 |
34. Objects 5 this.gpa = 0.0; 6 } Alternatively, we can define constructors that accept a subset of variable values. 1 public Student(String firstName, String lastName) { 2 this.firstName = firstName; 3 this.lastName = lastName; 4 this.id = 0; 5 this.gpa = 0.0; 6 } In both of these examples, we repeated a lot of code. O... | ComputerScienceOne_Page_502_Chunk2040 |
34.4. Usage 34.4. Usage Once we have defined our class and its constructors, we can create and use instances of it. Just as with regular variables, we need to declare instances of a class by providing the type and a variable name. For example: 1 Student s = null; 2 Student t = null; Both of these declarations are simply... | ComputerScienceOne_Page_503_Chunk2041 |
34. Objects The toString() method returns a String representation of the object. However, the default behavior that all classes inherit from the Object class is that it returns a string containing the fully qualified class name (package and class name) along with a hexadeci- mal representation of the JVM memory address ... | ComputerScienceOne_Page_504_Chunk2042 |
34.5. Common Methods this reason, many IDEs provide functionality to automatically generate such methods. The following example was generated by an IDE. 1 public boolean equals(Object obj) { 2 if (this == obj) { 3 return true; 4 } 5 if (obj == null) { 6 return false; 7 } 8 if (!(obj instanceof Student)) { 9 return fals... | ComputerScienceOne_Page_505_Chunk2043 |
34. Objects if two instances are equal (that is, equals() returns true ) then they must have the same hashCode() value. This requirement is necessary to ensure that hash table-based data structures operate properly. It is okay if unequal objects have equal or unequal hash values. This rule only applies when the objects... | ComputerScienceOne_Page_506_Chunk2044 |
34.6. Composition We can take this concept further and design our classes to own collections of other classes. For example, we could define a Course class and then update our Student class to own a collection of Course objects representing a student’s class schedule (this type of collection ownership is sometimes referr... | ComputerScienceOne_Page_507_Chunk2045 |
34. Objects Alternatively, we could make our design a bit more flexible by allowing the construction of a Student instance without having to provide a course schedule. Instead, we could add a method that allowed the outside code to add a course to the schedule . Something like the following. 1 public void addCourse(Cour... | ComputerScienceOne_Page_508_Chunk2046 |
34.7. Example 1 package unl.cse; 2 3 public class Student { 4 5 private String firstName; 6 private String lastName; 7 private int id; 8 private double gpa; 9 10 public Student(String firstName, String lastName, int id, double gpa) { 11 this.firstName = firstName; 12 this.lastName = lastName; 13 this.id = id; 14 this.g... | ComputerScienceOne_Page_509_Chunk2047 |
34. Objects 47 } 48 49 /** 50 * Scales the GPA, which is assumed to be on a 51 * 4.0 scale to a percentage. 52 */ 53 public double getGpaAsPercentage() { 54 return gpa / 4.0; 55 } 56 57 @Override 58 public String toString() { 59 return String.format("%s, %s (ID = %d); %.2f", 60 this.lastName, 61 this.firstName, 62 this... | ComputerScienceOne_Page_510_Chunk2048 |
34.7. Example 93 if (other.firstName != null) { 94 return false; 95 } 96 } else if (!firstName.equals(other.firstName)) { 97 return false; 98 } 99 if (Double.doubleToLongBits(gpa) != Double.doubleToLongBits(other.gpa)) { 100 return false; 101 } 102 if (id != other.id) { 103 return false; 104 } 105 if (lastName == null)... | ComputerScienceOne_Page_511_Chunk2049 |
35. Recursion Java supports recursion with no special syntax necessary. However, as an object-oriented language, recursion is generally expensive and iterative or other non-recursive solutions are generally preferred. We present a few examples to demonstrate how to write recursive methods in Java. The first example of a... | ComputerScienceOne_Page_513_Chunk2050 |
35. Recursion the recursive method call. 1 public static int recSumTail(int arr[], int i, int sum) { 2 if(i == arr.length) { 3 return sum; 4 } else { 5 return recSumTail(arr, i+1, sum + arr[i]); 6 } 7 } As another example, consider the following Java implementation of the naive recursive Fibonacci sequence. An addition... | ComputerScienceOne_Page_514_Chunk2051 |
14 return result; 15 } 16 } Java provides an arbitrary precision data type, BigInteger that can be used to compute arbitrarily large integer values. Since Fibonacci numbers grow exponentially, using an int we could only represent up to to F45. Using BigInteger we can support much larger values. An example: 1 public sta... | ComputerScienceOne_Page_515_Chunk2052 |
36. Searching & Sorting Java provides several methods to search and sort arrays as well as List s of elements of any type. These methods are able to operate on collections of any type because there are several overloaded versions of these functions as well as versions that take a Comparator object that specifies how the... | ComputerScienceOne_Page_517_Chunk2053 |
36. Searching & Sorting something similar, the Comparable interface, that specifies a compareTo() method with the same basic contract. Strings for example, are ordered in lexicographic ordering. Numeric types such as Integer and Double also have compareTo() methods that order elements in ascending order. Java refers to ... | ComputerScienceOne_Page_518_Chunk2054 |
36.1. Comparators this issue by rearranging our cases so that the equality is our final case, avoiding the use of the equality operator. Even better, however, we can exploit the built-in natural ordering of the integers by using the compareTo() method. 1 Comparator<Integer> cmpInt = new Comparator<Integer>(){ 2 public i... | ComputerScienceOne_Page_519_Chunk2055 |
36. Searching & Sorting 9 return a.getFirstName().compareTo(b.getFirstName()); 10 } else { 11 return a.getLastName().compareTo(b.getLastName()); 12 } 13 } 14 }; 1 /** 2 * This Comparator orders Student objects by 3 * last name/first name in descending (Z-to-A) order 4 */ 5 Comparator<Student> byNameDesc = new Comparato... | ComputerScienceOne_Page_520_Chunk2056 |
36.2. Searching & Sorting 4 */ 5 Comparator<Student> byGpa = new Comparator<Student>() { 6 @Override 7 public int compare(Student a, Student b) { 8 return b.getGpa().compareTo(a.getGpa()); 9 } 10 }; 36.2. Searching & Sorting We now turn our attention to the search and sorting methods provided by the JDK. Most of these ... | ComputerScienceOne_Page_521_Chunk2057 |
36. Searching & Sorting public static <T> int binarySearch(T[] a, T key, Comparator<T> c) That is, it takes an array of elements as well as key and a Comparator all of the same type T . It returns an integer representing the index at which it finds the first matching element (there is no guarantee that the first element i... | ComputerScienceOne_Page_522_Chunk2058 |
36.3. Other Considerations 1 ArrayList<Student> roster = ... 2 3 Student castroKey = null; 4 int castroIndex; 5 6 //create a "key" that will match according to the 7 // Student.equals() method 8 castroKey = new Student("Starlin", "Castro", 131313, 3.95); 9 castroIndex = roster.indexOf(castroKey); 10 System.out.println(... | ComputerScienceOne_Page_523_Chunk2059 |
36. Searching & Sorting 1 List<Student> roster = ... 2 Student rosterArr[] = ... 3 Comparator byName = ... 4 Comparator byGPA = ... 5 6 //sort by name: 7 Collections.sort(roster, byName); 8 Arrays.sort(rosterArr, byName); 9 10 //sort by GPA: 11 Collections.sort(roster, byGPA); 12 Arrays.sort(rosterArr, byGPA); Code Sam... | ComputerScienceOne_Page_524_Chunk2060 |
36.3. Other Considerations not. How we handle these is a design decision. We could ignore it, in which case such elements would likely result in a NullPointerException and expect the user to prevent or handle such instances. This may be the preferable choice in most instances, in fact. Alternatively, we could handle nu... | ComputerScienceOne_Page_525_Chunk2061 |
36. Searching & Sorting also important. To illustrate the importance of these methods, consider the following code. 1 Student a = new Student("Joe", "Smith", 1234, 3.5); 2 Student b = new Student("Joe", "Smith", 1234, 3.5); 3 4 Set<Student> s = new HashSet<Student>(); 5 s.add(a); 6 s.add(b); If we do not override the e... | ComputerScienceOne_Page_526_Chunk2062 |
36.3. Other Considerations 36.3.4. Java 8: Lambda Expressions Java 8 introduced a lot of functional-style syntax, including lambda expressions. Lambda expressions are essentially anonymous functions that can be passed around to other methods or objects. One use for lambda expressions is if we want to sort a List with r... | ComputerScienceOne_Page_527_Chunk2063 |
36. Searching & Sorting can be used to modify a Comparator to order null values. 494 | ComputerScienceOne_Page_528_Chunk2064 |
Part III. The PHP Programming Language 495 | ComputerScienceOne_Page_529_Chunk2065 |
37. Basics In the mid-1990s the World Wide Web was in its infancy but becoming more and more popular. For the most part, web pages contained static content: articles and text that was “just-there.” Web pages were far from the fully interactive and dynamic applications that they’ve become. Rasmus Lerdorf had a home page... | ComputerScienceOne_Page_531_Chunk2066 |
37. Basics 1 <?php 2 3 printf("Hello World\n"); 4 5 ?> Code Sample 37.1.: Hello World Program in PHP 1 <html> 2 <head> 3 <title>Hello World PHP Page</title> 4 </head> 5 <body> 6 <h1>A Simple PHP Script</h1> 7 8 <?php printf("<p>Hello World</p>"); ?> 9 10 </body> 11 </html> Code Sample 37.2.: Hello World Program in PHP ... | ComputerScienceOne_Page_532_Chunk2067 |
37.2. Basic Elements 37.2. Basic Elements Using the Hello World! program as a starting point, we will examine the basic elements of the PHP language. 37.2.1. Basic Syntax Rules PHP has adopted many aspects of the C programming language (the interpreter itself is written in C). However, there are some major aspects in w... | ComputerScienceOne_Page_533_Chunk2068 |
37. Basics 37.2.2. PHP Tags PHP code can be interleaved with static HTML or text. Because of this, we need a way to indicate what should be interpreted as PHP and what should be treated as static text. We can do this using PHP tags: the opening tag is <?php and the closing tag is ?> . Anything placed between these tags... | ComputerScienceOne_Page_534_Chunk2069 |
37.2. Basic Elements Function Description abs($x) Absolute value, |x| ceil($x) Ceiling function, ⌈46.3⌉= 47.0 floor($x) Floor function, ⌊46.3⌋= 46.0 cos($x) Cosine functiona sin($x) Sine functiona tan($x) Tangent functiona exp($x) Exponential function, ex, e = 2.71828 . . . log($x) Natural logarithm, ln (x)b log10($x) ... | ComputerScienceOne_Page_535_Chunk2070 |
37. Basics slashes is ignored. With a multiline comment, everything in between the forward slash/asterisk is ignored. Comments are ultimately ignored by the interpreter. Consider the following example. 1 //this is a single line comment 2 $x = 10; //this is also a single line comment, but after some code 3 4 /* 5 This i... | ComputerScienceOne_Page_536_Chunk2071 |
37.3. Variables Internally PHP supports several different types: Booleans, integers, floating point numbers, strings, arrays, and objects. The way that integers are represented may be platform dependent, but are usually 32-bit signed two’s complement integers, able to represent integers between −2, 147, 483, 648 and 2,14... | ComputerScienceOne_Page_537_Chunk2072 |
37. Basics 1 define("PI", 3.14159); 2 define("INSTITUTION", "University of Nebraska-Lincoln"); 3 define("COST_PER_UNIT", 2.50); Constant names are case sensitive. By convention, we use uppercase underscore casing. An attempt to redefine a constant value will raise a script warning, but will ultimately have no effect. Whe... | ComputerScienceOne_Page_538_Chunk2073 |
37.4. Operators 1 $x = 10 % 5; //x is 0 2 $x = 10 % 3; //x is 1 3 $x = 29 % 5; //x is 4 37.4.1. Type Juggling The expectations of an arithmetic expression involving two variables that are either integers or floating point numbers are straightforward. We expect the sum/product/etc. as a result. However, since PHP is dyna... | ComputerScienceOne_Page_539_Chunk2074 |
37. Basics 1 $a = "10"; 2 $b = 5 + $a; //b = 15 3 4 $a = "3.14"; 5 $b = 5 + $a; //b = 8.14 6 7 $a = "ten"; 8 $b = 5 + $a; //b = 5 9 10 //partial conversions also occur: 11 $a = "10ten"; 12 $b = 5 + $a; //b = 15 Code Sample 37.3.: Type Juggling in PHP There are several utility functions that can be used to help determin... | ComputerScienceOne_Page_540_Chunk2075 |
37.4. Operators Value of $var isset($var) empty($var) is_null($var) 42 bool(true) bool(false) bool(false) "" (an empty string) bool(true) bool(true) bool(false) " " (space) bool(true) bool(false) bool(false) false bool(true) bool(true) bool(false) true bool(true) bool(false) bool(false) array() (an empty array) bool(tr... | ComputerScienceOne_Page_541_Chunk2076 |
37. Basics 37.4.2. String Concatenation Strings in PHP can be concatenated (combined) in several different ways. One way you can combine strings is by using the concatenation operator. In PHP the string concatenation operator is a single period. 1 $s = "Hello"; 2 $t = "World!"; 3 $msg = $s . " " . $t; //msg contains "He... | ComputerScienceOne_Page_542_Chunk2077 |
37.6. Examples (short for file get string) using the keyword STDIN (Standard Input). This function will return, as a string, everything the user enters up to and including the enter key (interpreted as the endline character, \n ). To remove the endline character, you can use another function, trim which removes leading... | ComputerScienceOne_Page_543_Chunk2078 |
37. Basics 6 */ 7 8 //TODO: implement this 9 10 ?> It is common for programmers to use a comment along with a TODO note to themselves as a reminder of things that they still need to do with the program. Let’s first outline the basic steps that our program will go through: 1. We’ll first prompt the user for input, asking ... | ComputerScienceOne_Page_544_Chunk2079 |
37.6. Examples printf("Please enter degrees in Fahrenheit: "); In the second step, we’ll use the standard input to read the $fahrenheit variable value from the user. Recall that we can use fgets to read from the standard input, but may have to trim the trailing whitespace. $fahrenheit = trim(fgets(STDIN)); If we want t... | ComputerScienceOne_Page_545_Chunk2080 |
37. Basics 37.6.2. Computing Quadratic Roots Some programs require the user to enter multiple inputs. The prompt-input process can be repeated. In this example, consider asking the user for the coefficients, a, b, c to a quadratic polynomial, ax2 + bx + c and computing its roots using the quadratic formula, x = −b ± √ b2... | ComputerScienceOne_Page_546_Chunk2081 |
37.6. Examples 1 <?php 2 3 /** 4 * This program computes the roots to a quadratic equation 5 * using the quadratic formula. 6 */ 7 8 printf("Please enter a: "); 9 $a = floatval(trim(fgets(STDIN))); 10 printf("Please enter b: "); 11 $b = floatval(trim(fgets(STDIN))); 12 printf("Please enter c: "); 13 $c = floatval(trim(... | ComputerScienceOne_Page_547_Chunk2082 |
38. Conditionals PHP supports the basic if, if-else, and if-else-if conditional structures as well as switch statements. Logical statements are built using the standard logical operators for numeric comparisons as well as logical operators such as negations, And, and Or. 38.1. Logical Operators PHP has a built-in Boole... | ComputerScienceOne_Page_549_Chunk2083 |
38. Conditionals 1 $s = "aardvark"; 2 $t = "zebra"; 3 4 $r = ($s < $t); //true 5 $r = ($s <= $t); //true 6 $r = ($s >= $t); //false 7 $r = ($s > $t); //false However, when these operators are used to compare strings to numeric types, the strings are converted to numbers using the same type juggling that happens when st... | ComputerScienceOne_Page_550_Chunk2084 |
38.2. If, If-Else, If-Else-If Statements Operator(s) Associativity Notes Highest ++ , -- left-to-right increment operators - , ! right-to-left unary negation operator, logical not * , / , % left-to-right + , - left-to-right addition, subtraction < , <= , > , >= left-to-right comparison == , != , === , !== left-to-right... | ComputerScienceOne_Page_551_Chunk2085 |
38. Conditionals 1 //example of an if statement: 2 if($x < 10) { 3 printf("x is less than 10\n"); 4 } 5 6 //example of an if-else statement: 7 if($x < 10) { 8 printf("x is less than 10\n"); 9 } else { 10 printf("x is 10 or more \n"); 11 } 12 13 //example of an if-else-if statement: 14 if($x < 10) { 15 printf("x is less... | ComputerScienceOne_Page_552_Chunk2086 |
38.3. Examples 1 $x = 15; 2 if($x < 10); { 3 printf("x is less than 10\n"); 4 } This PHP code will run without error or warning. However, it will end up printing x is less than 10 , even though x = 15! Recall that a conditional statement binds to the executable statement or code block immediately following it. In this ... | ComputerScienceOne_Page_553_Chunk2087 |
38. Conditionals formula: logb(x) = loga (x) loga (b) If we can compute some base a, then we can compute any base b. Fortunately we have such a solution. Recall that the standard library provides a function to compute the natural logarithm, log() ). This is one of the fundamentals of problems solving: if a solution alr... | ComputerScienceOne_Page_554_Chunk2088 |
38.3. Examples 1 //prompt for income from the user 2 printf("Please enter your Adjusted Gross Income: "); 3 4 $income = floatval(trim(fgets(STDIN))); 5 6 //prompt for children 7 printf("How many children do you have? "); 8 $numChildren = intval(trim(fgets(STDIN))); 9 10 if($income < 0 || $numChildren < 0) { 11 printf("... | ComputerScienceOne_Page_555_Chunk2089 |
38. Conditionals negative). 1 if($baseTax - $credit >= 0) { 2 $totalTax = $baseTax - $credit; 3 } else { 4 $totalTax = 0; 5 } The full program is presented in Code Sample 38.3. 38.3.3. Quadratic Roots Revisited Let’s return to the quadratic roots program we previously designed that uses the quadratic equation to comput... | ComputerScienceOne_Page_556_Chunk2090 |
38.3. Examples 1 <?php 2 3 /** 4 * This program computes the logarithm base b (b > 1) 5 * of a given number x > 0 6 */ 7 8 if($argc != 3) { 9 printf("Usage: %s b x \n", $argv[0]); 10 exit(1); 11 } 12 13 $b = floatval($argv[1]); 14 $x = floatval($argv[2]); 15 16 if($x <= 0) { 17 printf("Error: x must be greater than zer... | ComputerScienceOne_Page_557_Chunk2091 |
38. Conditionals 1 <?php 2 //prompt for income from the user 3 printf("Please enter your Adjusted Gross Income: "); 4 5 $income = floatval(trim(fgets(STDIN))); 6 7 //prompt for children 8 printf("How many children do you have? "); 9 $numChildren = intval(trim(fgets(STDIN))); 10 11 if($income < 0 || $numChildren < 0) { ... | ComputerScienceOne_Page_558_Chunk2092 |
38.3. Examples 1 <?php 2 3 /** 4 * This program computes the roots to a quadratic equation 5 * using the quadratic formula. 6 */ 7 8 if($argc != 4) { 9 printf("Usage: %s a b c\n", $argv[0]); 10 exit(1); 11 } 12 13 $a = floatval($argv[1]); 14 $b = floatval($argv[2]); 15 $c = floatval($argv[3]); 16 17 if($a === 0) { 18 p... | ComputerScienceOne_Page_559_Chunk2093 |
39. Loops PHP supports while loops, for loops, and do-while loops using the keywords while , for , and do (along with another while ). Continuation conditions for loops are enclosed in parentheses, (...) and blocks of code associated with the loop are enclosed in curly brackets. 39.1. While Loops Code Sample 39.1 conta... | ComputerScienceOne_Page_561_Chunk2094 |
39. Loops 1 $i = 1; 2 $flag = true; 3 while($flag) { 4 //perform some action 5 $i++; //iteration 6 if($i>10) { 7 $flag = false; 8 } 9 } Code Sample 39.2.: Flag-controlled While Loop in PHP even worse: the program will enter an infinite loop. To see this, the code is essentially equivalent to the following: 1 while($i <=... | ComputerScienceOne_Page_562_Chunk2095 |
39.3. Do-While Loops 1 $i; 2 for($i=1; $i<=10; $i++) { 3 //perform some action 4 } Code Sample 39.3.: For Loop in PHP 1 $i; 2 do { 3 //perform some action 4 $i++; 5 } while($i <= 10); Code Sample 39.4.: Do-While Loop in PHP 39.3. Do-While Loops PHP also supports do-while loops. Recall that the difference between a while... | ComputerScienceOne_Page_563_Chunk2096 |
39. Loops 4 } In the foreach syntax we specify the array we want to iterate over, $arr and use the keyword as . The last element in the statement is the variable name that we want to use within the loop. This should be read as “ foreach element $x in the array $arr ...”. Inside the loop, the variable $x will be automat... | ComputerScienceOne_Page_564_Chunk2097 |
39.5. Examples 1 $sum = 0; 2 for($i=1; $i<=10; $i++) { 3 $sum += $i; 4 } Code Sample 39.6.: Summation of Numbers using a For Loop in PHP 39.5.3. Nested Loops Recall that you can write loops within loops. The inner loop will execute fully for each iteration of the outer loop. An example of two nested of loops in PHP can... | ComputerScienceOne_Page_565_Chunk2098 |
39. Loops The monthly payment may come out to be a fraction of a cent, say $43.871. For accuracy, we need to ensure that all of the figures for currency are rounded to the nearest cent. The standard math library does have a round() function, but it only rounds to the nearest whole number, not the nearest 100th. However,... | ComputerScienceOne_Page_566_Chunk2099 |
39.5. Examples 1 <?php 2 if($argc != 4) { 3 printf("Usage: %s principle apr terms\n", $argv[0]); 4 exit(1); 5 } 6 7 $principle = floatval($argv[1]); 8 $apr = floatval($argv[2]); 9 $n = intval($argv[3]); 10 11 $balance = $principle; 12 $monthlyInterestRate = $apr / 12; 13 14 //monthly payment 15 $monthlyPayment = ($mont... | ComputerScienceOne_Page_567_Chunk2100 |
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