Variables - Objective-C

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A variable is nothing but a name given to a storage area that our programs can manipulate. Each variable in Objective-C 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 operation 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 letter are distinct because Objective-C is case-sensitive. Based on the basic types explained in previous post.

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Type                        Description

char                         Typically char type is considered single byte.
int                            The most natural size of integer for the machine.
float                         A single-precision floating point value.
double                     A double-precision floating point value.
void                         Represents the absence of type.

Objective-C programming language also allow to define various other types of variables, which we will cover in subsequent chapters like Enumeration, Pointer, Array, Structure, Unioun, etc.

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Variable Definition in Objective-C:

A variable definition means to tell the compiler where and how much to create the storage for the variable. A variable definition specifies a data type and contain a list of one or more varaible of tha type as follows:

type variable_list;

Here,type must be a valid Objective-C data type including char w_char, 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 declaration 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 create variable names i, j and k of type int.
Variables can be initialized in their declaration. The initializer consist of an equal sign followed by a constant expression as follows:

type variable_name = value;

some expression are;

extern int d=3, f=5;  //declaration of d and f
int d=5, f=6;             //definition and initializing d anf f
byte z=22;                //definition and initialize z
char x='x';                //the variable x has the value 'x'

For definition without an initializer: varaibles with static storage,duration are implicitly initialized with NULL; the initial value of all other variable is undefined.

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Variable Declaration in Objective-C

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 need actual variable declaration at the time of linking of the program.

A variable declaration is useful when you are using multiple files and you define your variable in one of the files, which will be available at the time of linking the program. You will use extern keyword to declare a variable at any place Though you can declare a variable multiple times in your objective-C program but it can be defined only once in file, a function or a block of code.

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Example
Try the following example,where the variables have been declared at the top, but they have been defined and initialized inside the main function.

Program:

#import <Foundation/Foundation.h>
//variable declaration
extern int a,b;
extern int c;
extern float f;

int main()
{
        //variable defination
        int a,b;
        int c;
        float f;

         //actual initialization
         a = 7;
         b = 29;
         c = a + b;
         NSLog(@"value of c : %d \n", c);
 
         f = 70.0/3;
         NSLog(@"value of f : %f ", f);
       
         return 0;
}

some concept applies  function declaration where you provide a function name at the time of its declaration and its actual definition can be given anywhere else. In the following example, It;s explained using C function and as you know Objective-C supports C style Function also;

//function declaration
int func();

int main()
{
         //function call
          int  i = func();
}
//function definition
int func()
{
      return 0;
}

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Lvalues and Rvalues in Objective-C:

There are two kind of expressions in Objective-C:

1.  lvalue : Expressions that refer to memory location is called "lvalue" expression. An lvalue may appear as either the left-hand  or right-hand side of an assignment.
2.  rvalue : The term rvalue refer to data value that is stored at some address in memory. An rvalue is an expression that cannot have a value assigned to it which means 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 rvalue and so may not be assigned and can not appear on the left-hand side.
Valid statement:

int g = 20;