CPSC 110 - Pascal

1. CPSC 110 - Pascal Chapter 2 – part b Brent M. DingleTexas A&M University

2. Back to the programming- Variables in Pascal • As in math variables are ‘abstract’ representations of values. • Think of a variable as a name. • For example consider your name, it represents you. • So instead of listing out all of your traits to identify you, we can talk about you by name – and the traits are known.

3. Variables (cont) • Unlike in math the value of a variable may change – as the program runs. • Also variables in a program may be one or more characters in length. • Some example variable names are: • X Y Sum N1 N2 Rate time ANS

4. Values • Values may be numbers or other types of data. • Variables are said to contain values.

5. Working up to understanding variables and their values • Shortly we will look at a sample program to demonstrate what variables are and how they contain values. • You may or may not be able to immediately tell what it does – don’t worry it will be explained.

6. Sample Program 1Variables and their values PROGRAM AddTwo VAR n1, n2, sum : integer; BEGIN writeln(‘Enter two numbers’); readln(n1, n2); sum := n1 + n2; writeln(n1, ‘ plus ‘, n2, ‘ = ‘, sum); END.

7. Statements • Between the BEGIN and END lines are 4 instructions to be performed by the computer. Each instruction is called a statement. • Every statement ends with a semi-colon (;)There are some exceptions to the ending semi-colon, but don’t worry too much about that now.

8. Some commands • We will be returning to that program – but before that we need to learn some Pascal commands.

9. writeln • The writeln (pronounced write line) command tells the computer to write a line of text. • Specifically the computer is to output to the screen whatever is in the parentheses after the word writeln. • Anything in single quotes is output “as-is” • Anything not in single quotes is assumed to be a variable and the variable’s value is output.

10. writeln - example 1 • So the statement:writeln(‘Enter two numbers’);will cause the computer to print:Enter two numbersto the computer screen.

11. writeln – example 2 • The statement:writeln(n1);would cause the computer to output the VALUE of n1. • Assuming that n1 holds the value 42, then the output of the above statement would be:42

12. writeln – example 3 • The statement:writeln(‘The value of n1 is’, n1);would cause the computer to output:The value of n1 is 42(again assuming n1 contained the value 42)

13. readln • The readln command causes the computer to read data from the keyboard (or other input device – such as a file) and place it into any variables specified between the parentheses following the readln command. • If no variables are specified (i.e. there are no parentheses following the readln command) then the computer will simply wait for the user to press the [enter] key.

14. readln – example 1 • So the statement:readln(n1);would read ONE number (of 1 or more digits) from the keyboard and place the value of that number into the variable n1. • So if the user typed: 31and pressed enter, the value of n1 would become 31.

15. readln – example 2 • So the statement:readln(n1, n2);would read TWO numbers (of 1 or more digits) from the keyboard. • So if the user typed: 53 27and pressed enter, the value of n1 would be 53 and the value of n2 would be 27.

16. Assignment Operator := • The assignment operator in Pascal is the colon equals, := • So the statement:n1 := 5;would cause the value of n1 to become 5 • Important note: we call the number 5 a constant because it will not change in value.

17. Assignment Op – continued • The statement:sum := n1 + n2;would cause the value of sum to become whatever the value of n1 plus the value of n2 is. • So if n1 contained the value 55 and n2 contained the value 33, then the above statement would set the value of sum to be 88.

18. Expression • Just for reference an assignment statement is always made up of a variable on the left hand side of the := and an expression on the right hand side. • IN THIS CONTEXT, an expression may be a variable, a number, or a more complicated expression made up of variables, numbers and arithmetic operators (like +, -, / or *).

19. Going back to Program 1You should now know what most of it does. PROGRAM AddTwo VAR n1, n2, sum : integer; BEGIN writeln(‘Enter two numbers’); readln(n1, n2); sum := n1 + n2; writeln(n1, ‘ plus ‘, n2, ‘ = ‘, sum); END.

20. Identifiers • In the sample program notice the name of the program is AddTwo – this is an identifier. • The variable names: n1, n2, and sum are also identifiers. • In general a name used in a Pascal program is called an identifier. • Identifiers are defined to be any string made up of letters, digits and the underscore symbol.

21. Identifiers – rules • ALL identifiers MUST start with a letter or an underscore. • Any two identifiers or numbers must be separated by one or more spaces, by a line break, by a punctuation symbol (comma, semi-colon, colon, etc) or by a combination of two or more of these separators.

22. Identifiers – Examples • All the below are valid Pascal identifiers:X x1 x_1 Sum ABC1234q7_abc RATE count DaTa2 Hi_There

23. NOT Identifiers • None of the below are identifiers:21 3x 4Y %change Data.1Bad-Data FIRST.PAS • Do you see why?

24. NOT Identifiers – explanation • 21, 3x, 4Y and %change are NOT identifiers because they do not start with a letter or underscore. • Data.1, Bad-Data and FIRST.PASare NOT identifiers because they contain characters that are not letters or the underscore.

25. Identifiers – length • Technically identifiers in Pascal may be of any length, however Pascal limits you to 63 characters – which should be more than you need. (Who wants to type a variable name that is that long anyway?)

26. Case (In)Sensitivity • In Turbo Pascal there is NO distinction between upper and lower case letters when it comes to identifiers. • For example: Sum, sum and SUMare all the same identifier as far as Turbo Pascal is concerned.

27. Reserved Words • So getting back to our sample program, you should be wondering about the words:program, var, begin and end. • These are all reserved words in Pascal. • A reserved word has a predefined meaning in Pascal.

28. Standard identifiers • You may wonder why words such as readln, writeln, and integer are NOT reserved words… • This is because their meaning, while predefined may be changed – these type of identifiers are called standard identifiers.

29. General Program Layout • Program Heading • Variable Declarations (global) • Program Body

30. Program Heading • This is just the first line of most programs. • For example:PROGRAM AddTwo;

31. Variable Declarations • This is where the (global) variables are declared. • For example:VAR n1, n2, sum : integer; r1 : real;

32. Program Body • The program body is the list of statements to be executed. • This in general are all statements between the BEGIN and END lines. • Side note: the last END is followed by a period.

33. Data Types • A data type is a type or category of data. • All variables are of some data type. • When you declare a variable you also declare its type.

34. Variable Declaration • An example of variable dclaration is:var n1, n2, sum : integer; • This declares 3 variables of type integer.

35. Numbers in Pascal • There are two major number data types in Pascal: • Integer (e.g. –1, 53, 1002, etc) • Real (e.g. –4.3256, 1.045, 120.29801, etc)

36. Integers • Integers are stored as exact values. • Because a computer has finite storage, there does exist a minimum and maximum integer value that Pascal will understand. • Can you write a program to determine these minimum and maximum values?Hint: maxint

37. Reals • Type reals are stored only in an approximate form – again due to the finiteness of a computer. • Type reals may be written using a form of scientific notation: • 5E27 = 5 * 1027 or 6.5E-3 = 6.5*10-3 • Note though the below are NOT valid numbers: • .5E12 because there is no number before the decimal point • 3.78E11.4 because there is a decimal point in the exponent • Reals also have a maximum and minimum value in Pascal.

38. Type Compatibility • Notice that the number 3 and 3.0 are NOT the same type. • 3 is type integer • 3.0 is type real

39. Type Compatibility (cont) • Say you have the following variables:var n1, n2 : integer; r, q : real; • You may assign an integer value to a variable of type integer OR type real. • So the statements:n1 := 3; and r = 3; and q := n1;would all be valid.

40. Type Compatibility (cont 2) • However you may assign a real value ONLY to type reals. • So the statements:r := 4.05 and r := q;are valid. • But the statements:n1 := 4.05; and n1 := q;are NOT valid.

41. Arithmetic Expressions • Arithmetic expressions also have a type associated with them. • For example in:x := 5 * (n + (m/9) + 3*y);the expression is the right hand side of the statement (i.e. 5 * (n + (m/9) + 3*y) ). • The expression returns a type real (because it contains division).

42. Determining Type of Expressions • Combining something of type real with something of type integer or real always yields something of type real.real + real => real real + integer => real

43. Determining Type of Expressions • Combining anything with a division sign always yields something of type real.real / real => realinteger / integer => realreal / integer => realinteger / real => real

44. Determining Type of Expressions • Combining two things of type integer with an addition sign, a subtraction sign, or a multiplication sign yields something of type integer.integer + integer => integerinteger – integer => integerinteger * integer => integer

45. Determining Type of Expressions • Placing a minus sign in front of an arithmetic expression does NOT change its type.- integer => still an integer- real => still a real

46. MOD and DIV • Pascal has two built in arithmetic operators that work ONLY on integer (ordinal) types. • Mod returns the remainder of an integer divided by an integer. • So, 5 mod 3 returns 2. • Div returns the quotient of an integer divided by an integer. • So 5 div 3 returns 1.

47. Char and string data types • Pascal has two types that deal with characters – a character is generally anything that can be typed from the keyboard. • Type char can hold ONE character. • Type string may hold more than ONE character, based on how it is declared.

48. Strings • Variables of type string are declared:var name : string[40]; • This declares a string that can hold up to 40 characters. • To assign a string a value you use a statement like:name := ‘Robert Frost’;

49. Problems with strings • Make sure your strings are declared long enough.Programming as follows:var name : string[4]; : : name := ‘Bob Hope’;will cause errors to develop !(Notice ‘Bob Hope’ is longer than 4 characters)

50. String Trick • To place a single quote inside a string you must type it twice. • For example to set message to have the value Bob’s rock you would say:message := ‘Bob’’s rock’;And that is 2 single quotes between the b and the s, NOT a double quote.