Delphi Programming/Variables and constants
Variables are indispensable in programming. A program wouldn't do much things without variable.
A variable links a name to a value. You must not confuse its name and its value. A variable is not constant. It may change during the application execution.
Contents
Variables and program[edit]
Variable declaration in the program[edit]
To declare a variable in a program, you have to write:
- The variable name (var1, for example)
- :
- Its type (integer)
- ;
An example:
-
function foo()
-
var -
var1: integer; -
var2: integer; -
-
begin -
// Some instructions -
end;
You can also write:
-
function foo()
-
var -
var1, var2: integer; -
-
begin -
// Some instructions -
end;
Right syntax for the variable names[edit]
| Rules | Wrong identifiers | Right identifiers |
|---|---|---|
| Must not start with a number | 1name | name1 |
| Dots are not allowed | name.2 | name_2 |
| Dashes are not allowed | -name-3 | _name_3 |
| Spaces are not allowed | Variable name | Variable_name |
| Accented characters are not allowed | déjà_vu | deja_vu |
You don't have to worry about lowercase and uppercase as Delphi is case-insensitive.
Display a variable[edit]
It's easy to display a variable in an application. You must call the
WriteLn();
function. Once done, to properly display an integer, you must call the
WriteLn(variableToDisplay);
function. Here is the result in a whole application:
-
program Display_a_variable;
-
-
{$APPTYPE CONSOLE} -
-
uses -
SysUtils; -
var -
var1:integer;
-
-
begin -
var1:= 12
-
WriteLn (var1);
-
ReadLn; -
end.
So this code will display 12.
- Remark: If you don't want the display of a new line, use the Write function rather than WriteLn .
- Remark: You can use the ReadLn function to avoid the console from closing too quickly, but the actual feature of this function is described below.
Retrieve a variable[edit]
It's easy too. You have to call the ReadLn(variable); function.
You have to first declare the variable you want to use. Here is a whole code:
-
program Retrieve_a_Variable;
-
-
{$APPTYPE CONSOLE} -
-
uses -
SysUtils; -
var -
var1:integer;
-
-
begin -
ReadLn (var1);
-
end.
In the next pages, we will see how to operate variable additions, use it in loops and conditions, etc...
- Remark: If you don't want to skip a line after the entry, use the Read function rather than ReadLn .
Assignment[edit]
You can set a value to a variable at any time in a program, from another variable for example:
-
program Assignment;
-
-
{$APPTYPE CONSOLE} -
-
uses -
SysUtils; -
var -
sourceVariable:integer;
-
targetVariable:integer;
-
-
begin -
ReadLn (sourceVariable);
-
targetVariable := sourceVariable;
-
end.
The changed variable is on the left and the variable whose value is duplicated is on the right. Do not confuse.
The constants[edit]
Introduction[edit]
The constants are similar to variables, except one point: they can't change their value during the execution.
The constants of the system[edit]
Those constants specify all the values that are native and defined in the header files.
Example:
- stdout points on the screen buffer
- stdin points on the keyboard buffer
The symbolic constants[edit]
The symbolic constants are defined by the developer. They work as the variables, except for their declaration.
To declare a constant, you have to declare it after the reserved keyword const instead of var.
-
program Declare_constant;
-
-
{$APPTYPE CONSOLE} -
-
uses -
SysUtils; -
const -
const1 = 12;
-
var -
var1:integer;
-
-
begin -
// Instructions -
end.
Write an application that asks the user its age and then display it.
-
program Ask_your_age;
-
-
{$APPTYPE CONSOLE} -
-
uses -
SysUtils; -
var -
age:integer;
-
-
begin -
WriteLn ('How old are you?');
-
ReadLn (age);
-
Write ('You are ');
-
Write (age);
-
WriteLn (' year(s) old.');
-
end.
