St. Benedict s High School. Computing Science. Software Design & Development. (Part 1 Computer Programming) National 5

Transcription

1 Computing Science Software Design & Development (Part 1 Computer Programming) National 5

2 VARIABLES & DATA TYPES Variables provide temporary storage for information that will be needed while a program is running. A programmer can imagine a variable as being a box into which information can be placed. When a programmer wants to store something, she or he gives a storage location a name and tells the computer what to put there. Using that name later (the name of the variable), you can get out (recall) the stored information at any time. For example, a program to calculate the area of a rectangle might use the following variables. When the program is running the data can be entered by the user and then stored, or it can be automatically calculated by the computer and then stored. The stored data may change every time the program is run, but the variable names remain the same. A programmer may have as many variables in a program as she or he needs. All that matters is that the variables have different names and use the correct data type. Data Types Data Type Examples Description Integer 3, 7, -2, -99, 54 holds positive and negative whole numbers Real 1.5, 3.14, holds any value in floating point includes decimal numbers and very large numbers. Visual Basic has several options for this including 'decimal', 'single' and 'double'. String Mary, joe, dog, stores data in ASCII - names, addresses and so on are string types. Boolean True, False uses just one bit in memory to store TRUE or FALSE. Before a variable can be used in a program it must be declared. Example 'variable declarations' in Visual Basic: Dim length as Integer Dim average as Decimal Dim firstname as String Dim correct as Boolean 2

3 One-Dimensional Arrays 1D arrays are data structures used to store a group of values or lists. All arrays hold data of the same data type so we can have string arrays or real arrays etc.. Each item of data is known as an element. Arithmetic can be performed still be performed on data held in arrays using +, -, *, /, ^. There are also operations for string arrays. Comparison of separate variables with an array 3

4 Arrays Typical examples of code used Consider the following data: 1 Homer 2 Marge 3 Bart 4 Lisa 5 Maggie Create an array. The code required to create the array could be something like this. Dim firstname(5) As String Note: we specify the name of the array, its size, and its data type. Store a single item of data. The code required to store Bart in element number 3 is firstname(3) = "bart" Display a single item of data. The code required to display the contents of element number 4 could be ListBox1.Items.Add(Firstname(4)) Get the idea? Now look at this 4

5 Display the entire array. The code required to display the entire list of all names can be easily written using a FOR loop as follows. Dim row As Integer For row = 1 To 5 ListBox1.Items.Add(firstname(row)) Next This code would be just as simple with a list of a million names! Store a list of user data. Rather than have all the names entered by the programmer when writing the code, we can also use a FOR loop to allow the user to enter their own names into the array. Dim person As Integer For person = 1 To 5 firstname(person) = InputBox("Enter a name") Next Prompt for, and display a single item of data. We can even retrieve individual items of information. Dim element As Integer element = InputBox("Enter a number between 1 and 5") MsgBox("You picked " & firstname(element)) 5

6 COMPUTATIONAL CONSTRUCTS Looping (Iteration) There are two types of loop: fixed and conditional. Fixed Loop (For Next) With a fixed loop you know exactly how many times something will be done. For instance a routine to enter 20 names into a program: DIM name$(20) FOR person = 1 TO 20 Input Please enter name :name$(person) NEXT person Conditional Loops (While Endwhile, Do... Until) With a conditional loop you do not know how many times the loop will be executed - it depends on a condition. You can have the condition at the beginning or the end. If the condition is checked at the beginning of the loop it is possible the loop will never be executed. If the condition is checked at the end of the loop then the loop will execute at least once. Here is a validation subroutine that checks the condition at the beginning. The program is taking in 20 marks in a test out of 100: Input number WHILE number < 0 OR number > 100 DO Input number ENDWHILE If the user keeps entering valid numbers like 56, 87, 23 etc. then the while loop above will never be executed. If they enter 105, then they will be asked to re-enter. If they keep entering such numbers then the loop will keep executing. We do not know how many times. 6

7 The condition can also be at the end of the loop meaning that the loop will be executed at least once. This simple example only accepts a number between 0 and 100, although it doesn t give an error message if it isn t. REPEAT Input mark UNTIL mark >= 0 AND mark <= 100 7

8 Complex Conditions AND, OR, NOT A "complex condition" contains two or more simple conditions linked by logic operators (AND, OR, NOT). They are frequently used in IF statements and some types of programming loops. Examples When might I wear shorts and a T-shirt? IF it is summer AND it is hot THEN I ll wear shorts and a T-shirt END IF Any other times? IF (it is summer AND it is hot) OR (it is winter AND I ve gone completely mad!) THEN I ll wear shorts and a T-shirt END IF Some example code dealing with test marks might require a complex condition like this If score1 > 50 AND score2 > 50 Then TextBox1.Text = "You passed both the tests." End If Our examples in the last section examples also use complex conditions Input number WHILE number < 0 OR number > 100 DO Input number ENDWHILE NOTE: The example above uses OR. This means that only one of the conditions need to be true for the code inside the loop to run repeatedly. REPEAT Input mark UNTIL mark >= 0 AND mark <= 100 NOTE: The example above uses AND. This means that both of the conditions need to be true for the code inside the loop to run repeatedly. NOT is often used in database and web searches Harry Potter AND book NOT film 8

9 Selection (IF THEN ELSE) A 'selection statement' tests if a condition is true or not and then does one thing or another. A simple example is adding 1 to a score if an answer is correct: IF answer = Paris THEN MsgBox( Well done ) Score = score + 1 ELSE MsgBox( No, Paris is the answer ) END IF When there are a number of alternatives, a big list of ELSE, ELSE, ELSE can be difficult to understand. Also even when one is true, the program still checks the remaining alternatives. Programming in this way is not pretty(!) and is very difficult to read. Look at the following horrible example: If mark>= 90 Then Listbox1.Items.Add("Grade A") Else If mark>= 80 Then Listbox1.Items.Add("Grade B") Else If mark>= 70 Then Listbox1.Items.Add("Grade C") Else If mark>= 60 Then Listbox1.Items.Add("Grade D") Else Listbox1.Items.Add("Fail") End If End If End If End If It's horrible!!! You should try to avoid this in your own code. So multi IF.. THEN.. ELSEs are inefficient and make a program less readable (and so less maintainable). (NOTE: An IF statement coded entirely inside another IF statement is called a 'nested IF'.) 9

10 Multiple Selection (CASE) When there are many possible outcomes, the IF method of selection becomes cumbersome. All those ELSE s make a program difficult to understand. As an alternative there is the CASE statement: Based upon a variable called mark, this code displays the appropriate Grade. CASE mark OF WHEN >=70 Listbox1.Items.Add( Grade A ) WHEN >=60 Listbox1.Items.Add( Grade B ) WHEN >=50 Listbox1.Items.Add( Grade C ) WHEN >=45 Listbox1.Items.Add( Grade D ) OTHERWISE Listbox1.Items.Add( Fail ) END CASE Note that if the mark was 90, the first case would be true and the program would skip the rest (unlike ELSEs where it would check every condition). 10

11 String Functions Arithmetic can be performed on numeric variables using +, -, *, / etc. There are also operations for string variables. String Concatenation When you use '&' (or '+' depending on the programming language) it joins two strings together. This is called concatenation. E.g. wholename = firstname & " " & surname Other String Operations You can also extract part of a string - a substring. Where name="scotland" LEN(name) LCASE(name) UCASE(name) LEFT(name,2) RIGHT(name,3) MID(name,2,3) MID(name,4,2) RESULT => 8 scotland SCOTLAND Sc and cot tl Note: Len gives you how many characters are in the string. Lcase and Ucase give lower or upper case letters. Right (name,3) gives the last 3 characters. Left(name,2) gives the first 2 Mid(name,2,3) means start at position 2 and give the next 3 characters. 11

12 Pre-defined Functions A pre-defined function is a piece of pre-written code that is actually part of the programming language. Many programming languages will actually have a library of these pre-defined functions, each of which is designed to perform a frequently needed task for the programmer. Here are a few useful Visual Basic pre-defined functions: Function Example Code Description Val Int number = Val("2457") 'sets number to number = Val(" ") 'sets number to 2457 number = Val("24 and 57") 'sets number to 24. number = Val(InputBox("Enter a number")) converts an empty inputbox value to 0. MyNumber = Int(99.8) ' Returns 99. MyNumber = Int(-99.8) ' Returns MyNumber = Int(-99.2) ' Returns The Val function actually returns the numbers contained in the data. Val stops converting at the first character that cannot be interpreted as a numeric digit or white space. It is particularly useful for converting empty textboxes to the value 0 in order to avoid program crashes. The Int function returns integer portions of numbers. In the case of a negative number, the Int function returns the first negative integer less than or equal to the number Math.Round mynumber = Math.Round(4.3) ' Returns 4. mynumber = Math.Round(4.4) ' Returns 4. mynumber = Math.Round(4.5) ' Returns 4. mynumber = Math.Round(4.6) ' Returns 5. mynumber = Math.Round(3.14, 1) ' Returns 3.1 The Math.Round function rounds a value to the nearest integer or to the specified number of decimal digits depending on the data type used. Very useful for calculations involving money. UCase City = InputBox("Enter capital city of France...") If UCase(City) = "PARIS" Then Correct = Correct + 1 Else MsgBox("Wrong!") End If UCase converts lowercase letters to uppercase. Very useful for text comparisons (e.g. quiz programs) where a match is required but, the programmer cannot anticipate the exact use of capital letters made by the user. 12

13 TESTING AND DOCUMENTING SOLUTIONS Program Testing Once a program has been written, there then follows the testing stage. The most crucial aspect is to test that the program does what it is supposed to do and does not crash. Test Data A programmer should create sets of test data to feed the software to find out how it behaves. Here are some of the important points about making up such sets: The expected results from a test data set must be known in advance, so that they can be checked against the actual results. One set should test normal operation of the software, to make sure that there are no unexpected results in ordinary use. One set should test the operation extremes of the software, to make sure that the boundary conditions are handled properly. One set should test exceptions to the normal operating conditions. This set will show whether or not the software can react to unexpected inputs in an effective way, without crashing. 13

14 Testing must be systematic and comprehensive. It is systematic when you plan a table of test data with expected outcomes and then test the program with that data. It is comprehensive when you test as many eventualities as possible. For example if you were testing a program that took in marks out of 100 in a test you must test: Normal Data (valid data) Test Data Expected Result Actual Result Comment 45 Data accepted As expected. Program appears OK 78 Data accepted As expected. Program appears OK 26 Data accepted As expected. Program appears OK Extreme (data on the boundary of being acceptable) Test Data Expected Result Actual Result Comment 0 Data accepted As expected Program ppears OK 100 Data accepted As expected Program appears OK Exceptions (data that should not be acceptable) Test Data Expected Result Actual Result Comment -5 Not Accepted Not Accepted Prompt to re-enter 109 Not Accepted Accepted Prompt to re-enter Not Accepted Not Accepted Program continues!!! Five Not accepted Not Accepted Program crash!!! x Not accepted Not accepted Program crash!!! 14

15 ERROR TYPES There are three main types of error in a program: LOGIC ERROR when the program doesn t crash, but doesn t do what it is supposed to do. e.g. IF age < 18 THEN display You can vote SYNTAX ERROR where the program won t work because of a mistake in the code (usually a typing mistake). e.g. Listbox1.Items.Add( Hello) RUN-TIME ERROR when the program crashes because of a fault e.g. Division by zero: Average = total / howmany and howmany has the value 0 Note: Division by zero is impossible! Seriously! Think about it... 15

16 READABILITY There are several factors which can affect a program s readability. Meaningful Variable Names This makes a program more readable and easier to understand. It is therefore easier for a programmer who has to make changes in the future to understand the code. Internal Commentary Internal commentary explains what each section of the code does. Again, this makes it easier for a programmer who has to make changes in the future to understand the code. Structured Listing When a listing is structured it provides good visual cues to the programmer about the purpose of sections of code. For example, Indenting loops and selection (IF) structures and lower case for variables all help make a program more readable. Modularity A problem should be broken down into smaller sub-problems. Each sub-problem can be thought of as a single module. Good modularity allows sections of the program to be altered without breaking other parts of the program. 16

17 Readability - Example of Good Practice The example below shows meaningful variable names and internal commentary. The indentation of code and use of white space also makes the structure easy to follow. Sub find_max() 'age example Dim max As Integer ' highest value Dim position As Integer ' element number in array with highest value max = age(1) ' make first value in array initial highest position = 1 ' record position of highest so far For row = 1 To 5 Next If age(row) > max Then ' a new maximum value has been found End If max = age(row) ' assign new value to maximum position = row ' update position of maximum found ListBox1.Items.Add("The oldest age is " & max) ListBox1.Items.Add("The oldest person is " & name(position)) End Sub 17

18 Readability - Example of Bad Practice The code below does exactly the same as the code above. Unfortunately there are no meaningful variable names, comments, and no indentation to show the structure. There isn't even any colour. Can you read it and understand it? Sub find_max() Dim a As Integer Dim b As Integer a = c(1) b = 1 < For x = 1 To 5 If c(x)> a Then a = c(x) b = x End If Next ListBox1.Items.Add("The oldest age is " & a) ListBox1.Items.Add("The oldest person is " & d(b)) End Sub 18

19 ALGORITHM SPECIFICATION Input Validation Input validation is commonly used to ensure that a number lies within an acceptable range of values. E.g. acceptable exam score data should be between 0 and 100 inclusive. Data outwith this range should be rejected and the user should be prompted to enter the data again. This should continue until an acceptable score is entered by the user. DESIGN: 1. Take in value 2. While value not in range 3. Ask to re-enter 4. Take in value 5. End while CODE: number = InputBox("Please enter a number between 0 and 100 While number <= 0 Or number >= 100 End While MsgBox("Please enter a number between 0 and 100",, "Error!") number = InputBox("Please enter a number between 0 and 100 ") 19

20 Linear Search To find a particular item of data in an array... DESIGN 1. Take in target 2. For each item in list 3. If target = item then 4. Report found 5. Report position 6. End if 7. Next value CODE Dim target As String 'name to be found Dim found As Boolean = False Dim found_location As Integer target = InputBox( "Enter name to be found...") For row = 1 To 5 Next If target = name(row) Then End If found = True found_location = row 'take note of element number of found item [NOTE: Code like the following could then be used after the standard algorithm:] If found = True Then ListBox1.Items.Add(target & is & age(found_location) & years old. ) Else ListBox1.Items.Add( "Not found!") End If 20

21 Find Max/Min Finding the largest or smallest value in a list of numbers held in a one-dimensional array. FIND MAX DESIGN 1. Set max to first value 2. For each other value in list 3. If the value > max then 4. Set max to that value 5. End if 6. Next value 7. Display max CODE Sub find_max() 'age example Dim max As Integer ' highest value max = age(1) ' make first value in array initial highest For row = 1 To 5 Next End Sub If age(row) > max Then ' a new maximum value has been found max = age(row) ' assign new value to maximum End If ListBox1.Items.Add("The oldest age is " & max) 21

22 FIND MIN DESIGN 1. Set min to first value 2. For each other value in list 3. If the value < min then 4. Set min to that value 5. End if 6. Next value 7. Display min CODE Sub find_min() 'age example Dim min As Integer ' smallest value min = age(1) ' make first value in array initial smallest For row = 1 To 5 Next If age(row) < min Then ' a new smallest value has been found max = age(row) ' assign new value to smallest End If ListBox1.Items.Add("The youngest age is " & min) End Sub 22

23 Count Occurrences To count certain items of data in an array... DESIGN 1. Set count to zero 2. Take in target 3. For each item in list 4. If target = item then 5. add 1 to count 6. End if 7. Next value 8. Display count CODE Dim count As Integer count = 0 target = InputBox( "Enter age to be found...") For row = 1 To 5 If age(row) = target Then count = count + 1 ' incremenent count End If Next ListBox1.Items.Add("There are " & count & " records matching your search.") 23

24 DESIGN NOTATIONS Once the needs of the user are clearly understood and documented, software development can move onto the next stage, involving the DESIGN of the system. We are now moving from asking what needs to be done to asking how are we going to do it. We need to use a structured and methodical approach to the planning of our solution. Top Down Design A top-down approach to design allows us to think simply about the overall task to be done and then split the task up into smaller, more manageable sub-problems. This is sometimes called stepwise-refinement. The structured design approach of stepwise refinement also allows you to manage a very large project, by sub-dividing it into modules or subroutines. Each module can be further refined, producing a hierarchical structure. These structures are much simpler to develop, to understand and to maintain. The important point is that there must be some documented evidence of this stage of the design. It is far easier to correct logical mistakes, or to upgrade a piece of software, by using the structure charts or pseudocode listings, than by referring to the actual program code, because the structure charts or pseudocode listings make it easier to understand how the software was built in the first place. 24

25 Pseudocode Pseudocode is an outline of a program, written in a form that can easily be converted into real programming statements. Here are some examples of Pseudocode: Calculate the area of a rectangle TOP LEVEL 1. Get sizes 2. Calculate area 3. Display area REFINEMENTS 1.1 Get length from keyboard 1.2 Get breadth from keyboard 2.1 Set Area to length * breadth 3.1 Clear screen 3.2 Set text colour to blue 3.3 Send message and answer to screen Calculate the area of 5 rectangles (notice only the top level changes from above) TOP LEVEL 1. Do 5 times 2. Get sizes 3. Calculate area 4. Display area 5. End loop REFINEMENTS 1.1 Get length from keyboard 1.2 Get breadth from keyboard 2.1 Set Area to length * breadth 3.1 Clear screen 3.2 Set text colour to blue 3.3 Send message and answer to screen 25

26 A quiz question: 1. Ask question 2. Get answer 3. IF answer correct THEN 4. display well done message 5. ELSE 6. display fail message and correct answer 7. END IF 26

27 Structure Diagrams / Flowcharts The examples here represent structure diagrams of the same programs shown in the pseduocode page for comparison. Calculate the area of a rectangle Calculate the area of 5 rectangles 27

28 A quiz question: 28