Unit 9 Managing Complexity Through Modularity Sunday March

Unit 9 : Managing Complexity Through Modularity Sunday, March 01, 2009 Presented by Mais M. Fatayer

1. Introduction • In M 150 Part one, you have been introduced to using functions to structure your programs and to facilitate the reuse of the code you had written.

In this unit 1. 2. 3. 4. 5. 6. 7. 8. the advantages of structuring large complex programs into separate code modules; the benefits of black box programming and the importance of documentation; how to import a function library into a Java. Script program and select and use suitable functions to solve simple programming tasks; how to create a fully documented function library; the scoping rules that apply to variables and function names; how to use objects in Java. Script; to understand describe the differences between object types and primitive types; how to create new object types.

unsigned int index; int count=0; int max =0; unsigned int y = 0; for(index=0; index<choice; index++)for(double x = 0; choice > y; y++) //y is the # that cycles through the array { if(nums[index]==nums[index+1]) { x += nums[y]; //adds everything to x count++; } else double n = 0; //use { long hold = 0; if(count>max) long other = 0; max=count; double holder = 0; //use count=0; double m = 0; //use if (choice % 2) //If number is odd means the middle number is } median` } { count=0; for(index=0; index<choice; index++) n = (choice/2); //finds the very middle number n +=. 5; //it will return a #. 5 so we add another. 5 to counter { that if(nums[index]==nums[index+1]) hold = static_cast<int>(n); // we convert #. 0 to just # count++; holder = nums[hold]; //holder = the # stored at nums[hold] else } { else { if(count==max) hold = (choice/2); //if choice is even then hold is equal to the cout<<"Mode(s): t" higher middle # <<nums[index]<<"n"; other = (hold - 1); //other is the lower middle number count=0; n = nums[hold]; //assign the value in hold to n } m = nums[other]; //asign the value in other to m holder = ((n+m)/2); //find the average of the 2 middle numbers } return holder; //return the average Main Program: Session #5 Statistical Information monolithic is difficult to read or modify. . I need a better way to write my code.

Breaking down a program in to series of manageable subtasks to avoid unnecessarily repeating the same code in one program Mean Median double avg(double* nums, unsigned int choice) { double median(double* nums, unsigned int choice) { double n = 0; //use long hold = 0; long other = 0; double holder = 0; //use double m = 0; //use if (choice % 2) //If number is odd means the middle number is median { n = (choice/2); //finds the very middle number n +=. 5; //it will return a #. 5 so we add another. 5 to counter that hold = static_cast<int>(n); // we convert #. 0 to just # holder = nums[hold]; //holder = the # stored at nums[hold] } else { hold = (choice/2); //if choice is even then hold is equal to the higher middle # other = (hold - 1); //other is the lower middle number n = nums[hold]; //assign the value in hold to n m = nums[other]; //asign the value in other to m holder = ((n+m)/2); //find the average of the 2 middle numbers } return holder; //return the average } unsigned int y = 0; for(double x = 0; choice > y; y++) //y is the # that cycles through the array { x += nums[y]; //adds everything to x } return(x/choice); //divides it by how many numbers they entered } Mod Standard Deviation T-Test chart Main Program: Session #5 Statistical Information

Modules and modular programming • simplest concept of a module is an identifiable piece of program code that implements a particular task. • A Module has a name by which it can be identified and so used. • A module can be one of a number of identifiable ‘chunks’ within a single file, or it may be a separate file. • This description originally made the term ‘module’ synonymous with language-specific terms such as subroutine, function and procedure.

2. Separate Code Modules • Java. Script you learned to construct a program from a number of separate files, each containing Java. Script code. • A main program file can indicate that it wants to use Java. Script code from an external file by the simple inclusion of a line of HTML naming that file. • By convention an extension of. js (short for Java. Script) is used as a suffix to the name of such an external file. • The most common type of. js file is a function library which is a file containing a collection of related functions. • See example

What else. . • (. js files) can also be used to hold the code needed to implement new types of objects.

SAQ 2. 1 • Name two types of module that can be implemented using. js files. • Answer to SAQ 2. 1 Function libraries and object types.

SAQ 2. 2 • Give at least three advantages of using modules. • Answer to SAQ 2. 2 1. Different aspects of a programming problem may be developed separately. 2. Locating and isolating problems is much easier than would be the case with a monolithic program in a single file. 3. It allows many people to work on the same project, so shortening development time. 4. Modules can be reused. 5. Modules can be considered as replaceable components which can be easily replaced by improved versions without affecting or making changes to other parts of the program.

3. Function Libraries • Now we will discuss some of the features of function libraries and their use in Java. Script.

Write once , use many times… • When you develop a solution for doing something in any programming language you should keep in mind that you may encounter similar problem in future. • Do you think that a simple copy and paste the required function from the old program into the new one is a the idea of reuse? • A far better solution is to collect reusable functions into files which can then be imported into programs as and when needed. • Such a file is called a function library and it is an example of a module.

Documentation ( Black Box Programming ) • To make it possible for other programmers to /****************** use your function , you need to document it /* ***************/ Function draws a square, 10 stars wide and precisely. You need to specify: 10 stars high. */ /* The function takes two – – arguments: */ /* colour -a string that The name of the function; specifies draw. Square the colour of the rectangle. */ /* -a whole number which specifies the What arguments must beindent passed to the function number of spaces */ /* the rectangle is indented from be; the left-hand side of the page. and what type of value each must */ /* Function returns no value. */ (colour, indent) Draw /****************** Square() ***************/ – What the function does; – The value returned, if any, and what it represents.

How to import a function library § Suppose we have a function library called drawing. Library. js (the extension. js stands for java script file). § To import this library into our code we need a special version of the SCRIPT tags as follows: § <SCRIPT SRC = drawing. Library. js> </SCRIPT> § Note that NOTHING should be written between these tags, our code and the call for functions from this library will be written in a separate pair of SCRIPT tags.

4. Programming with function libraries • Manipulating the Date object using the date. Library. js provided with the course CD. • As you are already familiar with Java. Script objects such as Array, Document, etc… • Date is a Java. Script object that follows the same rules of creation • The Array object has a related function length() which is called using a dot notation. • Date also has related functions for further manipulations.

Creating an object of type Date var today = new Date(); • Explain the above code? –Creates a new instance of type Date and stores it in the variable today. – This instance has the following properties : year, month, day of the month, hour, minute, second, millisecond. – Date() is referred to as a constructor function note it has the same name as the object. Nice to know When it is called without parameters, Date() reads the computer’s internal clock setting, and initializes the properties of the newly created instance to the read values.

More on Dates • A Date object can also be created for a specific date (past or future). • Example : All the arguments are optional except year and month, if not filled, Java. Script sets them to 0.

Rules on using Date() • The year is written with 4 digits. • Months are numbered from 0 till 11, so 0 is January, 1 is February, etc… • The 24 h clock system is adopted so 1 pm is referred to as 13. • The name of the day of the week cannot be set as a parameter since it follows the prefixed rules of the Gregorian calendar.

Create and Initialize a Date • Two notations can be used to create and initialize a Date object instance, using multiple arguments or a single argument: • var my. Date = new Date(2004, 4, 3, 11, 45, 21); • Or • var my. Date = new Date(‘May 3 2004 11: 45: 21’);

Date Methods • Several methods are associated with the Date object to retrieve properties, example: – get. Full. Year() → Returns a 4 digit number indicating the year – get. Month() → Returns the month a number from 0 to 11 – get. Date() → Returns the day of the month a number from 1 to 31 – get. Day() → Returns the week day from 0 (Sunday) to 6 (Saturday) – get. Hours() → Returns the hour a number from 0 to 23 – get. Minutes() → Returns the minutes a number from 0 to 59 – get. Seconds() → Returns the seconds a number from 0 to 59

Date Methods ( cont. ) • Several methods are associated with the Date object to set properties, example: – set. Full. Year(year) → Sets the year of the Date object (4 digit number) – set. Month(month) → Sets the month of the Date object (0 to 11) – set. Date(day) → Sets the day of the month of the Date object (1 to 31) – set. Hours(hour) → Sets the hour in the Date object (0 to 23) – set. Minutes(minute) → Sets the minute in the Date object (0 to 59) – set. Seconds(second) → Sets the second in the Date object (0 to 59)

Activity 4. 1

Activity 4. 1 Solution Importing the function library <SCRIPT SRC=date. Library. js> </SCRIPT> <SCRIPT> Separate /********************************/ SCRIPT tags for the code /* Program to prompt the user for the year, month and day of their birth */ /* and prints out the day of the week on which they were born. */ /********************************/ var day; var month; var year; var birth. Day; year = parse. Float(window. prompt('Enter a year as a four digit number', '')); Subtracting 1 from month = parse. Float(window. prompt('Enter a month number from 1 to 12', '')); the month entered by the user month = month - 1; day = parse. Float(window. prompt('Enter a day as a number from 1 to 31', '')); day. Name is a function from the birth. Day = new Date(year, month, day); document. write('You were born on a ' + day. Name(birth. Day)); </SCRIPT> date. Library if given a parameter of type Date returns the name of the week day property

Extending a function library • Being useful to prompt to user to enter a year, a month, and a day, why not add it as a function to the date. Library. js instead of having to write it entirely over and over again? • How do we do that?

Extending a function library • Adding a function to a Library is not only adding lines of code. • Others may find use of such a function, so appropriate specification needs to be provided along with the lines of code. • Note that the function must be written in a way that minimizes the errors of usage, all user exceptions must be taken into consideration and must be overcome while programming.

Exercise 4. 2 Write a specification for the function, which we will call prompt. For. Date(), in the style of those you have already seen in drawing. Library. js and date. Library. js.

Exercise 4. 2 Solution function prompt. For. Date() /*******************************/ /* Function takes no arguments. Prompts user for: */ /* a year number from 1900 to 3000 */ /* a month number from 1 to 12 */ /* a day number from 1 to 31 */ /* Returns a Date object with the given year, month, and day */ /*******************************/

Activity 4. 5

Activity 4. 5 Solution function prompt. For. Date() { var day. Number; var month. Number; var year. Number; Asking the user to enter the year As long as the user enters an invalid year number Asking the user to enter the year AGAIN year. Number = parse. Float(window. prompt('Enter year number from 1900 to 3000', '')); while (year. Number < 1900 || year. Number > 3000) { year. Number = parse. Float(window. prompt('Enter year number from 1900 to 3000', '')); Asking the user to }; enter the month. Number = parse. Float(window. prompt('Enter month number from 1 to 12', '')); while (month. Number < 1 || month. Number > 12) As long as the user enters an invalid month number { month. Number = parse. Float(window. prompt('Enter month number from 1 to 12', '')) ; }; Asking the user to day. Number = parse. Float(window. prompt('Enter day number from 1 to 31', '')); enter the day while (day. Number < 1 || day. Number > 31) As long as the user enters an invalid day number { day. Number = parse. Float(window. prompt('Enter day number from 1 to 31', '')); }; Returning the required Date return new Date(year. Number, month. Number - 1, day. Number); object }

Activity 4. 6 • Something is missing? ? • The previously written function contains a major flaw. • It is true that it verifies if the user enters a valid year, month, and day. • But it doesn’t verify if the day is valid relatively to the month!!! • Not all months have 31 days!!! • February has only 28 or 29 days depending whether or not it is a leap year, and June has always 30 days.

Activity 4. 6 • Repeat the first two steps exactly as in Activity 4. 5. • Before prompting the user to enter a day number: • Create a Date object with the year and the month like so: • var a. Date = new Date(year. Number, month. Number - 1); • Once this is done, use a function from date. Library. js that when given a Date object as a parameter returns the number of days in that date’s month, store the returned value in a variable called days. • Then change the code, and instead of prompting the user to enter a day number from 1 to 31, you will prompt him to enter a day number from 1 to days.

Activity 4. 6 Solution Just before prompting the user to enter a day number: Create a Date object with the prompted year and month a. Date = new Date(year. Number, month. Number -1); days = no. Of. Days. In. Month(a. Date); no. Of. Days. In. Month is a function from the date. Library if given a Date object as a parameter returns the number of days in its month property day. Number = parse. Float(window. prompt('Enter day number from 1 to ' + days, '')); while (day. Number < 1 || day. Number > days) { day. Number = parse. Float(window. prompt('Enter day number from 1 to ' + days, '')); }; return new Date(year. Number, month. Number -1, day. Number); Using the number of days associated with the chosen month instead of 31

Creating a function library • You have learned how to import and use a library function. • Creating one is not difficult : ü Functions are written as lines of code in a text file. ü The text file is saved with the extension. js if the functions are written in Java. Script. • Remember that no function library is complete without proper documentation, each function needs to be commented since you will not be the only users of this library.

The scope of variables • Where a variable is declared in Java. Script determines where it can be accessed. • The scope of a variable is the extent of the access to it. • If a variable is declared using var in your Java. Script code OUTSIDE a function, then it is called a global variable, it is known to all the functions in the code, and they all can have access to it.

Activity 4. 7 Take a look at the following piece of code, the variable declared as var a, is a global variable: <SCRIPT LANGUAGE = "Java. Script"> var a; a = 'dog'; document. write('Before calling scope. Test(), the value of a is '+ a + '<BR>'); scope. Test(); document. write('After calling scope. Test(), the value of a is '+ a + '<BR>'); function scope. Test() { a = 'cat'; document. write('Inside the function, the value of a is '+ a + '<BR>'); } </SCRIPT>

Activity 4. 7 • The variable a is initially assigned a value, 'dog', in the first line of code in the main program. • After this value has been displayed, the function scope. Test() is called. • This function assigns a new value, 'cat' to a, and displays this value. • Finally, control returns to the main program, which displays the current value of a. As the output demonstrates, this value is 'cat', demonstrating that the variable has retained the value it was assigned during execution of the function scope. Test().

Activity 4. 7 When a variable is global and a function alters its value, the new value will stick to the variable even after the function finishes executing.

Activity 4. 8 Lets modify the previous code, we will leave the first var a as a global variable but we will declare another variable var a but this time inside the function scope. Test 2(). <SCRIPT LANGUAGE = "Java. Script"> var a; a = 'dog'; document. write('Before calling scope. Test(), the value of a is '+ a + '<BR>'); scope. Test(); document. write('After calling scope. Test(), the value of a is '+ a + '<BR>'); function scope. Test() { a = 'cat'; document. write('Inside the function, the value of a is '+ a + '<BR>'); } </SCRIPT> function scope. Test 2() { var a; a = 'cat'; document. write('Inside the function, the value of a is '+ a + '<BR>'); } </SCRIPT>

Activity 4. 8 • The variable a is initially assigned a value, 'dog', in the first line of code in the main program. • After this value has been displayed, the function scope. Test() is called. • This function declares a new variable a and assigns, 'cat' to a, and displays this value. • Finally, control returns to the main program, which displays the value of the initial variable a this value is ‘dog‘.

So what really happened? üThe variable a declared in the function scope. Test 2() is a different variable, even though it has the same name. ü This variable is only accessible to code statements within the function. ü Once the function has finished executing, the Java. Script system will once more understand a as referring to the variable with global scope. ü The variables declared in a function are said to be local to that function and are out of scope outside the body of the function. ü In other words, any variable born inside a function will die once the function finishes executing.

Careful notice • Avoid using the same name for different variables. • Never use a variable without declaring it first using the keyword var. • Always declare the variables you need at the top of your program or your function. • Always keep track of how the value of a global variable varies throughout your code.

Data types • A data type defines a collection of values along with the operations that can be performed on these values. • 1, 27, 56, and 34 belong to the number data type, 34 is a particular instance of this data type, allowable operations on this data type are +, -, *, /, etc… • True and False are the only instances of the Boolean data type, allowable operations are &&, ||, etc… • These data types are primitive data types since they are not made out of other data types.

5. Objects in Java. Script • Object types are a different type of data, an instance of such a type is called an object. • We already used several instances of object types in previous programs such as Array, String, and Date. • Objects are like containers that hold multiple values of different data types, these are called properties.

Categories of objects In Java. Script there are three categories of objects: • Native objects that are already defined in the language such as Array, String, and Date. • Host objects that are supplied by the browser (ex: Netscape and Internet Explorer) such as Document and Window. • User-Defined objects are types defined by you as a programmer.

Objects in Java. Script • An object defines a collection of properties and methods: • Properties are the variables that can be either primitive data types or object data types. • Methods are functions that act on an object’s properties and collectively implement the objects behaviour.

Creating new object types • In order to create a new object type, we use a constructor function that has the same name as the object. • If we wish to create a Student object with two properties name and course. Code, we will need something that looks like this: function Student(a. Name, a. Course) { //object properties this. name = a. Name; this. course. Code = a. Course; } • Note that the constructor function is like any other function except that its name starts with a capital letter.

Creating new object types • name and course. Code are the object’s properties. • The properties are the object’s variables, we didn’t need the keyword var to create these variables. • The this reserved word will specify the scope of the variables. • The this keyword refers to the instance of the object that has just been created using the new keyword. • When using this. name tells Java. Script to give the object referenced by this a property called name. • When the constructor function is called with the parameter value in a. Name the assignment statement will cause this. name to reference the value of the argument.

Creating new object types Ø Using the constructor function that we previously defined we wish to create a student Student. A with the name ‘Joe’ and the course. Code ‘M 150’, and another student Student. B with the name ‘Jill’ and the course. Code ‘M 225’. var Student. A = new Student(‘Joe’, ‘M 150’); var Student. B = new Student(‘Jill’, ‘M 225’);

Homework 1 • Solve activity 5. 1 and 5. 2 pages 46 and 47 • Bring the answers next session “ printed”

How is data stored in memory • In the case of primitive data types, the memory location holds the data value. This is known as value semantics. • Each primitive type has a fixed size and, as primitive data types are single data values, it tends to be small. • Java. Script allocates an eight-byte block of memory for the storage of any primitive value.

How is data stored in memory • The size of Object data types is not predictable, they will typically be much larger than eight bytes and so cannot be stored directly in the same way as primitive data types. • If you assign an object to a variable, the variable doesn’t hold the value of the object. • It holds a further memory address and it is this further address which gives the address of the block of memory where the object can actually be found. • In other words, the variable holds a reference to the object, rather than the object itself, inside is the address of the object along with the number of consecutive blocks of memory that it occupies. This is known as reference semantics.

How is data stored in memory Ø Examples how a number type and a Student type are stored in memory: var a. Number = 99; var student. A = new Student(‘Meena’, ‘M 254’, 27634); 61

How is data stored in memory How data is stored in memory affects the way assignment statements work, take a look at this example with primitive data types: var a = 56; var b = a; a = 22; This is what happens in the computer memory, variable a is stored in a memory location, its value was changed from 56 to 22, variable b is stored in another memory location and in it is copied the old value of variable a which is 56.

How is data stored in memory The following example is for the Student object data type: var Student. A = new Student(‘Meena’, ‘M 254’, 76549); var Student. B; Student. B = Student. A; This is what happens in the computer memory, in the memory location Student. A is stored the address and the number of blocks where the object’s instance is located, when the content of Student. A was assigned to Student. B, the address and the number of blocks were copied to the memory location Student. B and not the object itself. Both locations Student. A and Student. B refer to the location of the same Student object.

How is data stored in memory • If the following instruction is executed: Student. A. student. ID = 27634; Or Student. B. student. ID = 27634; • Java. Script will follow the address in Student. A or Student. B location, and when it finds the Student object, it will change the student. ID to 27634. Both locations Student. A and Student. B have the address of the same object, it is like writing down your address on two different pieces of paper.

Homework 2 • solve Exercise 5. 1 page 52 • Bring the answers next session “ printed”

Adding property of type Object to a Constructor • As we defined them objects are a collection of variables called properties and functions called methods. • First we will add to our Student object an additional property which is an Array of TMA scores, the size of the Array is fixed using an additional argument to the constructor called no. Of. Tmas. • Our object becomes like so: function Student(a. Name, a. Course, id. Number, no. Of. Tmas) { this. name = a. Name; this. course. Code = a. Course; this. student. ID = id. Number; this. tma. Scores = new Array(no. Of. Tmas); }

Adding behavior-object’s methods Ø What we need now is a method to update the TMA scores in the Array. Ø First we will write a function called update. Tma. Results(), this function needs two arguments, the number of the TMA and the score, our function will look like this: function update. Tma. Results(tma. No, score) { // arrays in Java. Script are zero based // so we deduct 1 from the TMA number this. tma. Scores [tma. No – 1] = score; } 69

Adding behavior-object’s methods • We need now to link the update. Tma. Results() function to a method that we will call update. Tma. Scores(), parenthesis are omitted when assigning a function to a method name, inside the Object’s constructor, like so: function Student(a. Name, a. Course, id. Number, no. Of. Tmas) { // object properties this. name = a. Name; this. course. Code = a. Course; this. student. IDc = id. Number; this. tma. Scores = new Array(no. Of. Tmas); // object methods this. update. Tma. Scores = update. Tma. Results; } • Notice that properties and methods seem rather similar? They are both variables! Properties hold values such as numbers or references to other objects, while methods are variables that hold references to functions. 70

Object types and. js files • The code of Activity 5. 6 includes the Student object that we created along with all its associated methods. • We used them while writing them between our SCRIPT tags. • It might be useful for other programmers to use these codes. • A good idea will be creating a library (. js) file in which we place the object’s constructor itself and all its associated functions. • For future use, we would only have to import the library in separate SCRIPT tags into our code without having to rewrite the entire code from the beginning. • The library is on the course CD under the name Student 1. js.

What’s next Unit ten : Software development.

Review questions Ø What are the disadvantages of old fashioned programming? Ø What is the new trend to overcome these disadvantages? Ø Give a definition of the term “function library”. Ø How do we import a function library into our Java. Script code? Ø Define the term “specification” in the modular programming context. Ø What is the Date object? Ø Why can’t we set the name of the week day ourselves when we create a Date object? Ø How do we create a function library? Ø Define the terms “global variable” and “local variable”, and state the main differences between them. Ø What is the difference between “primitive data types” and “object data types”? Ø What are three categories of objects in Java. Script? Ø What is a “property” and what is a “method”? Ø What is a constructor function? Ø What is the importance of the “this” keyword? Ø What is the difference between “value semantics” and “reference semantics”?