Operator precedence and associativity in C language

Below is the table for operator precedence and associativity in C programming language.

[ ] ( ) . -> ++ --	Array subscript Function Call Structure reference Structure dereference Postfix increment/Postfix decrement	Left to right
++ -- + - ! ~ (type) *  & sizeof	Prefix increment/Prefix decrement Unary plus/Unary minus Logical negation/One's complement Typecast operator Pointer dereference/Address of Size of type/variable in bytes	Right to left
*  /  %	Multiplication/Division/Modulo	Left to Right
+ -	Addition/Subtraction	Left to Right
<<   >>	Bitwise left shift/ Bitwise right shift	Left to Right
<    > <= >=	Comparison less than/Comparision greater than Comparison less than or equal to Comparison greater than or equal to	Left to Right
==   !=	Comparison equal to/Comparison not equal to	Left to Right
&	Bitwise AND	Left to Right
^	Bitwise XOR	Left to Right
|	Bitwise OR	Left to Right
&&	Logical AND	Left to Right
||	Logical OR	Left to Right
?:	Ternary Conditional Operator	Right to Left
= *=    /=   %= +=    -= <<=   >>= &=    ^= |=	Assignment Operator Mulplication/division/modulo assignment Addition/Subtraction assignment Bitwise left shift/right shift assignment Bitwise AND/XOR assignment Bitwise OR assignment	Right to Left
,	Comma Operator	Left to Right

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#ifdef, #ifndef and #endif directives in C

#ifdef
If the named macro is defined, then the code between #ifdef and #endif will be compiled.

#ifndef
If the named macro is not defined, then the code between #ifndef and #endif will be compiled.


#ifdef, #ifndef and #endif example in C

 
  #include <stdio.h>
  #define NAME "Raj"

  int main() {

        #ifdef NAME
                printf("#ifdef: Value of NAME is %s\n", NAME);
        #endif

        #ifndef VAL
                printf("#ifndef: macro VAL is not defined\n");
        #endif
        return 0;
  }

  Output:

  jp@jp-VirtualBox:~/$ ./a.out
  #ifdef: Value of NAME is Raj
  #ifndef: macro VAL is not defined

In the above program, macro NAME is defined.  So, the code between #ifdef and #endif is executed. Similarly, the code between #ifndef and #endif is executed as the macro VAL is not defined.

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#if, #elif, #else and #endif example

#if
If the resultant value of the arithmetic expression is non-zero, then the code between #if and #endif will be compiled.

Example:
#if 10 > 5
     printf("10 is greater than 5");
#endif

The code between #if and #endif will be compiled since the resultant value of the expression(10 > 5) is non-zero.


#elif
This provides an alternate expression to evaluate

Example:
#if 10 < 5
     printf("10 is less than 5");
#elif 10 > 5
     printf("10 is greater than 5");
#endif

The expression at #if directive evaluates to 0.  So, the expression at #elif is evaluated. If it is non-zero, then the code between #elif and #endif will be compiled.


#else
If the resultant value of the arithmetic expression is false for #if, #ifdef or #ifndef, then the code between #else and #endif will be compiled.

Example:
#if 10 < 5
     printf("10 is less than 5");
#else
     printf("10 is greater than 5");
#endif


#endif
This acts as an end directive for #if, #ifdef, #ifndef, #elif or #if

#if 100 < 50   // resultant value of expression is 0
   printf("#if directive");
#elif 50 < 10  // resultant value of the expression is 0
    printf("#elif directive");
#else
    printf("#else directive");  // this statement would be executed
#endif

Example C program to illustrate #if #elif #else #endif usage in C:

  #include <stdio.h>
  #define VAL1 10
  #define VAL2 20
  #define VAL3 30

  int main() {
    #if VAL1 > VAL2
          printf("%d is greater than %d\n", VAL1, VAL2);
    #elif VAL1 > VAL3
          printf("%d is greater than %d\n", VAL1, VAL3);
    #else
          printf("%d is less than %d and %d\n", VAL1, VAL2, VAL3);
    #endif
    return 0;
  }

  Output:

  jp@jp-VirtualBox:~/$ ./a.out
  10 is less than 20 and 30

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How to access union members in C

Dot operator is used to access(or assign) the data members of a union variable.  Consider the following example,

union value {
         int x;
         float y;
};

union value obj;
Here, obj is a union variable of type union value.  x and y are data members of the union value. Let us see how to assign value to or access data members of union using dot operator.

obj.x = 10;

The above statement assigns value 10 to the data member x of the union variable obj.

obj  - object name(union variable name)
x     - data member in union value
.     - dot operator


Example C program to illustrate accessing union members using dot operator

  #include <stdio.h>
  union value {
        int x;
        float y;
  }obj;

  int main() {
        obj.x = 5;
        printf("Value of x is %d\n", obj.x);
        obj.y = 19.22;
        printf("Value of y is %f\n", obj.y);
        printf("Value of x is %d\n", obj.x);
        return 0;
  }

  Output:

  jp@jp-VirtualBox:~/$ ./a.out
  Value of x is 5
  Value of y is 19.219999
  Value of x is 1100595855

The value of data member y overwrites the value of x and that is reason why we see junk value in x.  Basically, all data members in a union shares same storage place.

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How to initialize union variables in C

Let us see all possible ways to initialize a union variable.  Please remember that we can access one data member at a time in the case of union.

Method 1:

union value {

     int x;

     float y;

};

union value obj = {5};

Here, initialization is done after union definition.  5 is assigned to the member x of union variable obj.

Method 2:

union value {

     int x;

     float y;

}obj = {5};

Here, initialization is done while defining the union.  5 is assigned to the member x of union variable obj.

Method 3:

union value {

     int x;

     float y;

};

union value obj = {.y = 5.0};

Here, initialization is done after union definition.  5.0 is assigned to the member y of union variable obj.  And we have used dot(.) operator for initialization.

Method 4:

union value {

     int x;

     float y;

};

union value obj = {y:5.0};

This method is similar to method 3.  Here, color(:) is used instead of '=' and period before union member is removed

Example C program to illustrate union variable initialization:

  #include <stdio.h>
  union value {
        int x;
        float y;
  }obj1 = {5};  //initializing union variable

  int main() {
        /* declaration and initialization for union variables */
        union value obj2 = {6};
        union value obj3 = {.y = 7.1};
        union value obj4 = {y:8.2};

        /* printing the values */
        printf("obj1.x : %d\n", obj1.x);
        printf("obj2.x : %d\n", obj2.x);
        printf("obj3.y : %f\n", obj3.y);
        printf("obj4.y : %f\n", obj4.y);
        return 0;
  }

  Output:

  jp@jp-VirtualBox:~/$ ./a.out
  obj1.x : 5
  obj2.x : 6
  obj3.y : 7.100000
  obj4.y : 8.200000

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