Delphi Programming/Variables and constants

Variables are indispensable in programming. A program wouldn't do much things without variables.

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.

To declare a variable in a program, you have to write:

• var
• The variable name (var1, for example)
• :
• Its type (integer, for example)
• ;

An example:

functionfoo()
var
var1:integer;
var2:integer;

begin
// Some instructions
end;

You can also write:

functionfoo()
var
var1,var2:integer;

begin
// Some instructions
end;

You don't have to worry about lowercase and uppercase as Delphi is case-insensitive.

It's easy to display a variable in an application. In a console application, you use the command

WriteLn(variableToDisplay);

.

Here is the result in a whole application:

programDisplay_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.

Remark: In GUI applications, you display variables in visual components.

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:

programRetrieve_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 variables 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 .

You can set a value to a variable at any time in a program, from another variable for example:

programAssignment;

{$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 are similar to variables, except one point: they can't change their value during the execution.

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 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.

programDeclare_constant;

{$APPTYPE CONSOLE}

uses
SysUtils;
const
const1=12;
var
var1:integer;

begin
// Instructions
end.

programAsk_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.

• Book:Delphi Programming