Lecture 4: Introduction to PASCAL

PASCAL is a french acronym for "Program Appliqué à la Selection et la Compilation Automatique de la Literature" a high-level ("fourth generation") computer-programming language. Designed by Niklaus Wirth in the 1960s as an aid to teaching programming. It is still widely used as such in universities, and as a good general-purpose programming language. Most professional programmers, however, now use C or C++. Pascal was named after the 17th century French mathematician Blaise Pascal.
 

Blaise Pascal (1623-1662) French philosopher and mathematician. He contributed to the development of hydraulics, the calculus, and the mathematical theory of probability. His most famous invention was probably the "Pascal Triangle": Each number is the sum of the two numbers immediately above it, left and right, like this:        1       1 1      1 2 1     1 3 3 1    1 4 6 4 1

A program is a sequence of instructions, or statements which inform the computer of a specific task we want it to do.
Most modern program languages are in a very readible format, close to English, making it easy for humans to read and write programs. This in contrast to earlier programming languages, which were closer to things the computer understand. See for example the assembler language (aula 2). PASCAL is the language which most resembles a natural human language and as such is best suited for explaining the art of programming.

Every PASCAL program has the same essential format, (called the template):

PROGRAM Title;
begin
  program_statement_1;
  program_statement_2;
         |
  program_statement_n;
end.

Let's take a look at this program.

• Every program starts with the word PROGRAM followed by the title of the program.
• begin defines the starting point of the program.
• end. defines the end of the file and thus the end of the program. Note the full stop "." at the end. This is only used for the "end" statement at the end of the program. All other statements end with a semicolon ";"
• The combination of "begin" and "end" is a way to group instructions so they form a block. All the statements between "begin" and "end" are treated by the compiler as one statement.
• In between "begin" and "end" we can put our instructions. These instructions can be either contain instructions that PASCAL already knows, or instructions that we are going to define (so called procedures or functions, which we will dsicuss later).
• PASCAL is not case sensisitive: "Program", "PROGRAM", "program", etc, are all the same.

Identifiers, as the name already says, are used for identifying things. This can be name of programs (as above), name of procedures and functions and names of variables and constants. This we will see in later aulas. Like in most languages, names of identifiers have some restrictions:

• They should start with a letter; "PROGRAM 20hours" is not allowed.
• Followed by any combination of letters, digits or the underscore character "_".
• Spaces are not allowed, nor are characters like "(", "{", "[", "%", "#", "?", etc, except "_". The reason why this is so is that these characters are used for other things in PASCAL. They are called reserved characters.

{ } [ ] ( ) - = + / ? < > . , ; : ' " ! @ # $ % ^ & * ~ ` \ |

• Identifiers cannot be equal to reserved keywords of PASCAL, such as word like "program", "integer", "begin". Note that identifiers like "program1", or "program_" are allowed, although it is advised to avoid such confusing names. Note: In many programming environments (like the Turbo Pascal 7 that we will use in the practical lectures), we will notice when we are using a reserved keyword because they will change color when we type them in.
• Choose your identifiers well. When a program is calculating interest rates, call it, for example "PROGRAM InterestRates" and not "PROGRAM program1". Although it is not an error to give a program the name "program1", it is much more intelligent to give it a more meaningful name. This helps other people to understand your program (or yourself when you come back to the program after a long time).
• The minimum length of identifiers is 1, the maximum length 255. Make use of this possibility of long names, but also remember that also long names can make the program unreadable. Choose a "golden middle". Which of the following do you think is best:

  r := r + a;
  money := money + interest;
  themoneyintheaccountofpersonwithnameJohnson := themoneyintheaccountofpersonwithnameJohnson + thecurrentinterestrateatthetimeofthiswriting;
• They are not case sensitive, "i" is equal to "I", etc. To make programs more readable, follow a convention all through your program(s). The most often used convention is lowercase for variables and UPPERCASE for CONSTANTS.

• Use meaningful names for variables, procedures and functions.
• Use indentation. Compare the following two programs:

  PROGRAM Myprogram;begin writeln("Hello world!");end.
and
  PROGRAM MyProgram;

  begin
    writeln("Hello world!");
  end.
Both programs do exactly the same, but the second one is much more readible. The difference is

• Only put one statement per line.
• Use indent. Put (2) extra spaces in the beginning of the line every time we are one level "deeper" in the structures.
• Seperate blocks of text (functions and procedures) with blank lines.
• Avoid the use of "goto label" statements. With these statements, the program rapidly starts looking like spaghetti. Whereas in BASIC (Beginner's All-purpose Symbolic Instruction Code) the use of the GOTO statement is nearly unavoidable, in any itself-respecting language, the goto statement should be avoided.
• Comment. Since PASCAL is nearly like English, the program itself should be self-explanatory. Still, in places where the idea of the program might not be clear to the programmer, use comments. In PASCAL comments are placed within a set of accolades: "{" and "}". The compiler stops interpreting the text after the first "{" and resumes compiling after it encounters the matching "}". Therfore, any text can be placed within this context.

Alternatively, in Turbo Pascal, comments can be placed within a combination of "(*" and "*)". Note that in that case even the accolades "{" and "}" are skipped by the compiler.
• Use procedures and functions wherever it makes the text more organized. If at many different places the program has to do basically the same thing (for instance reading a line of text form a file), consider putting it in a procedure or function (for example PROCEDURE FileReadLn;). This will make the program more readable, more efficient and shorter.

The following words are related to variables and constants. Use of these words for identifiers is disadvised:
 

boolean

byte

char

double

integer

real

string

text

word