[2. Basics of Delphi grammar] 3 Object Pascal constants and variables

3.Object Pascal Constants and variables

3.1 Constant

stay Object Pascal in , A constant is an amount whose value cannot be changed during the execution of a program , Have certain data types . The representation of constants ：

• Direct constants ： Constant values

• declare constant ： Denote with a declared identifier , Include ： Symbolic constant 、 Type constant

3.1.1 Direct constants

Direct constants are constants used directly in programs .

1. integer constants

   Such as ：10、-3、0、1、2 etc.

2. Real constant

   • Fixed point real number , Number in decimal form , Such as ：0.06、3.14、-2.58、0.0 etc.

   • Floating point real , A number expressed in scientific notation , Such as ：1.234E-4 Express 1.234 ride 10 Negative 4 Power ;, -3.546E12 Express -3.546 ride 10 Of 12 Power .

3. Character type constants

   A single character enclosed in single quotes . Such as ：'d'、'E'、'2'、'=' etc. .

4. String constant

   A string enclosed in single quotation marks . Such as ：'Hello world.'、' Hello '、'3.1415926' etc. .

5. Boolean constants

   True and False

3.1.2 declare constant

1. Symbolic constant

Declare the constants represented by the defined identifier in advance , Also known as pure constants . Define the syntax of symbolic constants ：

Const
    < Constant names 1> = < Constant values 1>;
    < Constant names 2> = < Constant values 2>;
    ......
    < Constant names n> = < Constant values n>;

Const The keyword is used to declare constants .

Object Pascal Judge the type of constant according to the value of constant . Once a constant is declared , Can be quoted .

Code example ：

Const
    PI = 3.1415926;
    Msg = 'Out of memory';
    ErrMsg = 'Error:' + Msg + '.';

2. Type constant

Type constants cannot appear in constant expressions . Syntax for declaring type constants ：

Const
    < Type constant name >:< type > = < Constant values >;

Code example ：

Const
    PI:real = 3.1415926;
    EnterKey:char = #13;
    Heading:String = 'Hello world';

3.2 Variable

A variable is an amount that can change its value when a program is running , In order to represent and reference variables, variables must be named with identifiers , And call this identifier variable name . All variables must follow the rule of declaration before reference .

1. Declaration of variables

stay Object Pascal in , Variables in unit 、 The declaration part of a function or procedure , The location of a declared variable determines its scope . Syntax format of variable declaration ：

Var
    < Variable name 1>: < Type name 1>
    < Variable name 2>: < Type name 2>
    ......
    < Variable name n>: < Type name n>

When multiple variables have the same type , Compact formats can also be used ：

Var < Variable name 1>, < Variable name 2>, ......, < Variable name n>: < Type name >

among ,Var Keyword is used to declare variables , When declaring variables in a compact format , Variable names are separated by commas .

Code example ：

Var
    a,b,c: Integer;
    x,y,z: Real;
    ch: Char;
    city: String;

When variables are declared , Except to conform to Object Pascal Beyond the language , Try to use meaningful names that are easy to remember and read as variable names .

2. Use of variables

Once declared, variables can be used in programs , Generally, you can assign values to variables through assignment statements , Then use variables in expressions .

Object Pascal It's a compiled language , The program can only run after compilation . When the program is compiled , The compiler will allocate memory space to variables according to their types , But the compiler does not automatically initialize variables , The value of an uninitialized variable is an uncertain random value , therefore , After variable declaration , It should be initialized in time .

The easiest way to initialize variables is to assign values to variables .