Lesson 13 · 13 of 20

Procedures · Pascal

Group named blocks of code into Pascal procedures, with parameters and local variables, to print a bakery receipt.

Pascal · Procedures in DMarlon Studio: the example's code and its output in the terminal

A procedure is a named block of code that you write once and call as often as you need. It is declared before the main begin, with `procedure`, its name and, in brackets, its parameters.

PrintLine(width: Integer; symbol: Char) takes two pieces of data: how many characters to draw and which one. It is called as PrintLine(26, '=') and PrintLine(26, '-'): the same code draws two different lines. In the declaration, parameters are separated by semicolons; in the call, by commas.

i is a local variable: it is declared in the procedure's own var block and only exists while the procedure runs. Another procedure can have its own i without the two getting mixed up.

PrintItem prints one line of the receipt and works out the amount, qty * price, without the main program having to know how. Greet welcomes each customer. The main program stays short and reads like a list of steps.

In the Lazarus desktop apps that DMarlon Studio builds, every button click calls a procedure just like these: its event handler.

Code · main.pas

program main;// A procedure is a named block of code that you can call again and again.procedure PrintLine(width: Integer; symbol: Char);var  i: Integer;  // a local variable: it only exists insidebegin  for i := 1 to width do    Write(symbol);  WriteLn;end;procedure PrintItem(name: String; qty: Integer; price: Double);begin  WriteLn(name:8, qty:3, ' x', price:6:2, qty * price:8:2);end;procedure Greet(customer: String);begin  WriteLn('Welcome, ', customer, '!');end;begin  Greet('Marta');  PrintLine(26, '=');  PrintItem('Bread', 2, 1.20);  PrintItem('Milk', 3, 0.95);  PrintItem('Cheese', 1, 4.50);  PrintLine(26, '-');  Greet('Luis');end.

Output · fpc main.pas && ./main

Welcome, Marta!
==========================
   Bread  2 x  1.20    2.40
    Milk  3 x  0.95    2.85
  Cheese  1 x  4.50    4.50
--------------------------
Welcome, Luis!

Key points

procedure Name(parameters); declares a procedure.
Call it by name, with the values in brackets.
Variables in its var block are local.
A procedure saves you from repeating the same code.

Exercise

Write a PrintTotal(amount: Double) procedure that prints the total line aligned with the amounts, and call it at the end with 9.75.