PASCAL PROGRAMMING

INTRODUCTION PASCAL is a programming language named after the 17th century mathematician Blaise Pascal. The Pascal programming language: Provides a teaching language that highlights concepts common to all computer languages. It standardizes the language in such a way that it makes programs easy to write. Basic format of every Pascal program Every Pascal program has the same essential format, which is illustrated below: Program Title(input , output); Begin program statement(s); End.

KEYWORDS AND IDENTIFIERS Keywords are reserved words, i.e. You cannot use keywords to describe variables. Some examples include: Identifiers: In the previous example, TITLE is the name the programmer gave to the Pascal program. It is an identifier. Identifiers begin with a letter, then followed by any digit, letter or the underscore character ( _ ). Which of the following are valid Pascal identifiers? birthday Too_hot? First_Initial grade 1stprogram down.to.earth see you OldName Multiplication2 Program Begin End If Else While Do Repeat Until For Const Var Procedure Function Then

VARIABLES A variable is an item that allows you to store data into it. Variable names are a maximum of 32 alphanumeric characters. Older Pascal compilers recognizes the first 8 characters. Naming a variable : The 1st letter of the variable must be alphabetic (A to Z) or (a to z), followed by either letters, a number or an underscore eg. num, age2, avg, sum, first_num etc. Variable (Data) Types: Integer – to store integer or ‘whole’ numbers, eg.) 45 234 1245 -99 Real – to store fractional numbers, eg.) 3.45 567.78 -99.99 Char - to store a single character such as a letter, digit or punctuation mark, eg.) A, b, 4, x, Y String – to store a collection of characters, eg. Apple abc123 Boolean - used for values that are True/False or Yes/No type responses

INPUT, PROCESS & OUTPUT The majority of programs will follow the Input - Process - Output cycle. INPUT In Pascal we use two commands together in order to receive input from the user. We first use a writeln or write statement to display a message explaining the type of input required and then a readln statement to store the data in an appropriate variable, for example: writeln('What is your name?'); readln(name); N.B. the single quote (‘) characters around the question. There is no need to use single quotes in the readln line. Each line ends with a semi-colon ; writeln – displays a message and sends the cursor in the output screen to the next line without pressing ‘Enter’. write – displays a message but does not sent the cursor to the next line readln – reads the data to be stored on separate lines read – reads the data to be stored on the same line. Each piece of data is usually separated by a space.

INPUT, PROCESS & OUTPUT CONT’D PROCESS Processing usually involves changing values of variables or creating extra variables by combining other variables e.g. adding two numbers together to come up with third. In Pascal we use the assign := sign to perform calculation. While it looks very similar to the equals = symbol it is different. Equals (=) is used for comparisons while assign (:=) is for storing values. All types of variables can have values placed in them using the assign sign, for example: age := 2000 - yearborn; name := 'Fred Fruit'; total := number1 + number2; N.B. Single quotation marks are only required for storing strings or characters

INPUT, PROCESS & OUTPUT CONT’D OUTPUT The final stage is the outputting of processed data or information to the user. Once again we use the writeln or write statements. The basic rule to follow is that text that you create in contained within single quotation marks (') and variables are contained between commas (,) unless they are at the start or end of the output, for example: writeln('Thank you for using this program.'); writeln(name,' is in Year 10'); writeln(name,' will turn ',age,' in 2000'); When displaying real type variables it is necessary to format the output by specifying how many spaces you wish to leave and how many decimal places you would like to display. As a general rule 4 spaces and 2 decimal places works in most cases, for example: writeln('The amount owing is $',amountowed:4:2);

SYNTAX Syntax is a set of rules for combining the various elements that make up a programming language. It the very precise way in which the statements in a program must be written in order to be understood. The syntax allows the programmer to create a correctly structured statement that is ‘legal’ although it does not guarantee that the statement will be useful: All program statements and lines are terminated with a semi-colon (;) A semi-colon is not required after any of the Pascal keywords The program name and variables must be one word, no spaces The program name and variable names cannot match Variables of each type must be declared separately The use of indentation and spacing helps in the presentation and readability of the coding After the last END, a full stop (.) is required

PROGRAM STRUCTURE Nearly all structured programs share a similar overall structure: Statements to establish the start of the program Variable declarations Program statement(s) – blocks of code Example 1: Example 2: Program MYFIRST(output); Begin writeln(‘Hello. How are you?’); End. Program AddNumbers(input,output); Var a,b,sum:integer Begin a:=2; b:=5; sum:= a + b; writeln(‘The sum is: ‘,sum); End.

COMMENTS Comments are inserted into Pascal programs by enclosing the comment within { } braces. Comments are ignored by the computer, but are helpful to explain how the program works to other programmers. program DEMOPROG (output); begin write('Hello there.'); {the write statement does not set the cursor to the beginning of the next line. } writeln('This is getting boring.') { This is printed on the same line as Hello there, but now the cursor moves to the beginning of the next line, because this time we used writeln instead of write } end.

CONVERTING PSEUDOCODE TO PASCAL CODE Eg. 1) Write an program which prompts the user to input his/her name and prints a welcome message to the user. PASCAL CODE Program Welcome; {This program prints a welcome message for the user} Begin writeln(‘Welcome to the world of problem- solving!’); End. ALGORITHM Algorithm Welcome This algorithm prints a welcome message for the user Start Print (‘Welcome to the world of problem- solving !’) Stop

CONVERTING PSEUDOCODE TO PASCAL CODE CONT’D Eg. 2) Write a program to prompt the user to input 3 numbers and calculate and print the total and average Algorithm FindAverage This algorithm finds the total and average of 3 numbers which the user inputs. num1,num2,num3 – stores the 3 numbers sum – stores the sum of the 3 numbers average – stores the average of the 3 numbers Start Set num1,num2,num3 to 0 Print(‘Please enter 3 numbers’) Input (num1,num2,num3) sum = num1+num2+num3 average = Sum/3 Print(‘The sum is: ‘,sum) Print(‘The average is: ‘,average) Stop Program FindAverage(input,output); {This program finds the total and average of 3 numbers which the user inputs} Var num1, num2, num3, Sum: integer; Average: real; Begin num1:=0; num2:=0; num3:=0; writeln(‘Please enter 3 numbers’); readln(num1,num2,num3); sum:= num1+num2+num3; average:= sum/3; writeln(‘The sum is: ‘,sum); writeln(‘The average is: ‘,average:4:2); End.

THE ‘CONST’ DECLARATION The const declaration is used to define symbolic constants. Constants are like variables except that their values can't change. A value is assigned to a constant when you create it. Const is used instead of Var when declaring a constant. Constants are used for values that do not change, such as the value of pi. Eg.) Write a program which prompts the user to enter the radius of a circle and calculate the area of the circle. [Area of a circle = πr2] Program CircleArea(input , output); Const pi = 3.1416; Var r: integer; area: real; Begin writeln(‘Please enter the radius of the circle’); readln(r); area:= pi*r^2; writeln(‘Area = ‘,area:4:2); End.

EXERCISE 5 1. Code the algorithms from Exercise 1 in Pascal code. 2. Write a program to request a number and print the number and its square. 3. Write a program which requests a user to enter a weight in kilograms and converts it into pounds. [1kg = 2.2 lbs] 4. Write a program to which prompts the user to enter two integer values L and B, which represents the length and breadth of a rectangle, and calculates the area of the rectangle. 5. A ball is thrown vertically upwards with an initial speed of U metres per second. Its height H after time T is given by: H = UT – 4.9T2 Write a program which request U and T, and prints the height of the ball after T seconds.

IF...THEN...ELSE The if-then-else statement – a branching method which chooses between two alternative courses of action Syntax: IF (Boolean expression) THEN statement1 ELSE statement2 ; N.B. the semi-colon is placed after the statement following the ELSE. Eg.) The following piece of code checks the age of the person to see if he/she is eligible to vote. The person must be 18 years and over to vote. IF (age >= 18) THEN writeln(‘You are eligible to vote’) writeln(‘You are too young to vote’);

IF...THEN...ELSE CONT’D Eg.1) Write a program to read the score of a student in an examination and determine whether the student has passed. If the score is greater than or equal to 50, the student has passed. Print whether the student has passed or failed. Algorithm TestScore This algorithm determines whether a student has passed of failed a test. score – stores the student’s mark Start Print (‘Please enter the score’) Input Score IF Score >= 50 THEN Print ‘PASS’ ELSE Print ‘FAIL’ Stop Program TestScore(input , output); {This program determines whether a student has passed or failed a test} Var score: integer; Begin writeln(‘Please enter the score’); readln(score); IF score >= 50 THEN writeln(‘PASS’) ELSE writeln(‘FAIL’); End.

IF...THEN...ELSE CONT’D Eg.2) Write a program which reads the age of a person and determine whether he/she is a student. The person is considered a student if he/she is between the ages of 5 and 18. If the person is under the age of 5 then he/she is a ‘toddler’. Otherwise, the person is an adult. Print the age of the person. Program CheckAge(input , output); {This program prompts the user to input the age of a person and prints whether he/she is a student, a toddler or an adult} var age: integer; Begin writeln(‘Please enter the age of the person’; readln(age); IF (age <= 18) AND (age >=5) THEN writeln(‘You are a student’) ELSE IF (age < 5) THEN writeln(‘You are a toddler’) ELSE writeln(‘You are an adult’); End. Algorithm CheckAge This algorithm prompts the user to input the age of a person and prints whether he/she is a student, a toddler or an adult age – stores the age of the person Start Print(‘Please enter the age of the person’) Input(age) IF (age <= 18) AND (age >=5) THEN Print(‘You are a student’) ELSE IF (age < 5) THEN Print(‘You are a toddler’) ELSE Print(‘You are an adult’) Stop

EXERCISE #1 1. Code the algorithms from Exercise 2 in Pascal. 2. Write a program to request a score in a test and print a letter grade based on the following: score < 50 F 50 ≤ score ≤ 65 C 65 ≤ score < 80 B score ≥ 80 A 3. Given 3 integer values representing the sides of a triangle, write a program to print: Not a triangle – if the any of the values are negative or zero Sorry !– if the length of any side is greater than or equal to the sum of the other 2 sides Isosceles – if the triangle has two equal sides Equilateral – if all three sides are the same Scalene – if all three sides are different

THE ‘FOR’ LOOP Eg.) A man works 40 hours per week and he is paid an hourly rate of $50.00 per hour. Write a program to read the names and hours worked for 10 employees. Determine and print their weekly wages along with their names. ALGORITHM Start FOR x = 1 to 10 DO Print (‘Please enter a name and hours worked’) Input Name, Hours Wage = Hours * 50 Print (Name,’ $’,Wage) ENDFOR Stop PASCAL CODE Begin FOR x = 1 to 10 DO writeln(‘Please enter a name and hours worked’); readln(Name, Hours); Wage = Hours * 50; writeln(Name,’ $’,Wage); End; End.

THE ‘WHILE’ LOOP Eg.)Write Pascal code to accept an unspecified amount of integers. Calculate and print the sum of the numbers entered. The program should be terminated when the number 999 is entered. Algorithm Start Print(‘Please enter a number’) Read(num) WHILE (num <> 999) DO sum = sum + num Read (num) ENDWHILE Print(‘The sum is: ‘, sum) Stop Pascal Code Begin writeln(‘Please enter a number’); readln(num); WHILE (num <> 999) DO sum:= sum + num; End; writeln(‘The sum is: ‘, sum); End.

THE ‘REPEAT...UNTIL’ LOOP Eg.)Write Pascal code to accept an unspecified amount of integers. Calculate and print the sum of the numbers entered. The program should be terminated when the number 999 is entered. Algorithm Start Print(‘Please enter a number’) Read(num) REPEAT sum = sum + num UNTIL (num = 999) Print(‘The sum is: ‘, sum) Stop Pascal Code Begin writeln(‘Please enter a number’); readln(num); REPEAT sum:= sum + num; UNTIL (num = 999); writeln(‘The sum is: ‘, sum); End.

ARRAYS An array is a structure which holds many variables, all of the same data type. Each element of the array is capable of storing one piece of data (a variable). A subscript is used to refer to a particular location. Declaring an array An array is usually declared in the ‘var’ section. The syntax is as follows: Pseudocode: ArrayName[lower..upper] of data type Pascal Code: ArrayName = Array[lower..upper] of data type Eg.) numbers = Array[1..5] of integer; Here is a visual representation of an array with 5 elements: numbers[1] numbers[2] numbers[3] numbers[4] numbers[5] 23 12 56 19 34

INITIALISING ARRAYS Pascal Code Pseudocode Initialising an array of integers or real : FOR x:= 1 to n DO ArrayName[x] := 0; Initialising an array of string: ArrayName[x] := ‘ ‘; Initialising an array of char: ArrayName[x] := ‘ ‘; Initialising an array of integers or real: FOR x = 1 to n DO ArrayName[x] = 0 Initialising an array of string: ArrayName[x] = ‘ ‘ Initialising an array of char: FOR x = 1 to n DO ArrayName[x] = ‘ ‘

INPUTTING/OUTPUTTING TO/FROM AN ARRAY Pseudocode Pascal Code Inputting data into an array: FOR x = 1 to n DO Print(‘Please enter.......’) Read(ArrayName[x]) EndFOR Outputting from an array: Print(ArrayName[x]) Inputting data into an array: FOR x := 1 to n DO Begin writeln(‘Please enter.....’); readln(ArrayName[x]); End; Outputting from an array: writeln(ArrayName[x]);

Eg 1.) Read in the marks of 10 students and store them in an array MARKS. Calculate the sum an average of those numbers. Algorithm Calculate This algorithm calculates the sum and average of 10 numbers using an array sum – holds the value of sum average – holds the value of average Marks[1..10] of integer – array of marks Start Set sum to 0 FOR n = 1 to 10 DO Print(‘Please enter a mark’) Read(Marks[n]) Set sum to sum + Marks[n] EndFOR Set average to sum/10 Print(‘The sum is: ‘, sum) Print(‘The average is: ‘, average) Stop Program Calculate; {this program calculates the sum and average of 10 marks using an array} var sum, n: integer; average: real; Marks = Array[1..10] of integer; Begin sum := 0; FOR n := 1 to 10 DO writeln(‘Please enter a mark’); readln(Marks[n]); sum:= sum + Marks[n]; end; average:= sum/10; writeln(‘The sum is: ‘, sum); writeln(‘The average is: ‘, average); End.

SEARCHING AN ARRAY The following is a standard algorithm for searching for a particular item in an array: Set Found to 0 FOR x = 1 to n DO IF item = ArrayName[x] THEN set Found to x set x to n EndIF EndFOR IF (Found = 0) THEN Print(item,’ is not found’) ELSE Print(item,’ is found in ‘, Found) ‘item’ can be an integer, character or a string and refers to an element in an array which is to be located. setting x to n ‘forces’ exit out of the FOR loop once the item is found

Eg 2.) Read in the marks of 10 students and store them in an array MARKS. Determine and print the maximum and minimum mark. Program Calculate2; {This program reads in the marks of 10 students and stores them in an array. It also finds the maximum and minimum marks} var min, max, n: integer; Marks = Array[1..10] of integer; Begin max := 0; min := 99999; FOR n := 1 to 10 DO writeln(‘Please enter a mark’); readln(Marks[n]); IF (Marks[n] > max) THEN max := Marks[n]; IF (Marks[n] <main THEN min := Marks[n]; End; writeln(‘The maximum mark is: ‘, max); writeln(‘The minimum mark is: ‘, min); End.

EXERCISE #2 Consider the following array of integers: 1 2 3 4 5 6 7 1. Write code to declare the above array of marks. 2. What is the contents of Marks[4]? 3. Draw a diagram to illustrate how the contents of Marks[3] and Marks[6] can be exchanged. 4. Write code to: i. Swap the contents of Marks[3] and Marks[6]. Use a temporary variable called ‘temp’ ii. Increase each mark by 10% iii. Print each of the marks that is greater than 20 10 7 12 8 26 19 21

PROCEDURES Procedures are sub-programs that can be called from the main part of the program. Procedures are declared outside of the main program body using the procedure keyword. Procedures must also be given a unique name. Procedures have their own begin and end. Program Procedures; Procedure Hello; Begin Writeln('Hello'); End; {Main Program} Begin Hello; End.

Eg 3.)Using Eg.2, re-write the program using procedures. Program Calculate3; {This program finds the max and min of 10 marks which have been read into an array} Var min, max, n: integer; Marks = Array[1..10] of integer; Procedure Initialise; {This procedure initialises all variables used} Begin max := 0; min := 99999; End; Procedure InputData; {This procedure reads the data into the array} writeln(‘Please enter a mark’); readln(Marks[n]); Procedure Determine; {This procedure searches the array element element until the max and min marks are found} Begin FOR n := 1 to 10 DO InputData; IF (Marks[n] > max) THEN max := Marks[n]; IF (Marks[n] <main THEN min := Marks[n]; End; {FOR} End;

EXERCISE #3 Procedure OutputData; {This procedure outputs the data on the screen} Begin writeln(‘The maximum mark is: ‘, max); writeln(‘The minimum mark is: ‘, min); End; {Main Program} Initialise; Determine; OutputData; End. EXERCISE #3 Using Exercise #2, write each part of Q4 as separate procedures and incorporate them into a single program.