D Programming - Variables

A variable is nothing but a name given to a storage area that our programs can manipulate. Each variable in D has a specific type, which determines the size and layout of the variable's memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable.

The name of a variable can be composed of letters, digits, and the underscore character. It must begin with either a letter or an underscore. Upper and lowercase letters are distinct because D is case-sensitive. Based on the basic types explained in the previous chapter, there will be the following basic variable types −

D programming language also allows to define various other types of variables such as Enumeration, Pointer, Array, Structure, Union, etc., which we will cover in subsequent chapters. For this chapter, let us study only basic variable types.

Variable Definition in D

A variable definition tells the compiler where and how much space to create for the variable. A variable definition specifies a data type and contains a list of one or more variables of that type as follows −

type variable_list;

Here, type must be a valid D data type including char, wchar, int, float, double, bool, or any user-defined object, etc., and variable_list may consist of one or more identifier names separated by commas. Some valid declarations are shown here −

int    i, j, k; 
char   c, ch; 
float  f, salary; 
double d;

The line int i, j, k; both declares and defines the variables i, j and k; which instructs the compiler to create variables named i, j, and k of type int.

Variables can be initialized (assigned an initial value) in their declaration. The initializer consists of an equal sign followed by a constant expression as follows −

type variable_name = value;

Examples

extern int d = 3, f = 5;    // declaration of d and f.  
int d = 3, f = 5;           // definition and initializing d and f.  
byte z = 22;                // definition and initializes z.  
char x = 'x';               // the variable x has the value 'x'.

When a variable is declared in D, it is always set to its 'default initializer', which can be manually accessed as T.init where T is the type (ex. int.init). The default initializer for integer types is 0, for Booleans false, and for floating-point numbers NaN.

Variable Declaration in D

A variable declaration provides assurance to the compiler that there is one variable existing with the given type and name so that compiler proceed for further compilation without needing complete detail about the variable. A variable declaration has its meaning at the time of compilation only, compiler needs actual variable declaration at the time of linking of the program.

Example

Try the following example, where variables have been declared at the start of the program, but are defined and initialized inside the main function −

import std.stdio; 
 
int a = 10, b = 10; 
int c;
float f;  

int main () { 
   writeln("Value of a is : ", a); 
   
   /* variable re definition: */ 
   int a, b; 
   int c; 
   float f;
   
   /* Initialization */ 
   a = 30; 
   b = 40; 
   writeln("Value of a is : ", a); 
   
   c = a + b; 
   writeln("Value of c is : ", c);  
   
   f = 70.0/3.0; 
   writeln("Value of f is : ", f); 
   return 0; 
}

When the above code is compiled and executed, it produces the following result −

Value of a is : 10 
Value of a is : 30 
Value of c is : 70 
Value of f is : 23.3333

Lvalues and Rvalues in D

There are two kinds of expressions in D −

• lvalue − An expression that is an lvalue may appear as either the left-hand or right-hand side of an assignment.

• rvalue − An expression that is an rvalue may appear on the right- but not left-hand side of an assignment.

Variables are lvalues and so may appear on the left-hand side of an assignment. Numeric literals are rvalues and so may not be assigned and cannot appear on the left-hand side. The following statement is valid −

int g = 20;

But the following is not a valid statement and would generate a compile-time error −

10 = 20;