C-Union

A union is a special data type available in C that enables you to store different data types in the same memory location. You can define a union with many members, but only one member can contain a value at any given time. Unions provide an efficient way of using the same memory location for multi-purpose.

Defining a Union

To define a union, you must use the union statement in very similar was as you did while defining structure. The union statement defines a new data type, with more than one member for your program. The format of the union statement is as follows:

union [union tag]
{
   member definition;
   member definition;
   ...
   member definition;
} [one or more union variables];  

The union tag is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. At the end of the union's definition, before the final semicolon, you can specify one or more union variables but it is optional. Here is the way you would define a union type named Data which has the three members i, f, and str:

union Data
{
   int i;
   float f;
   char  str[20];
} data;  

Now, a variable of Data type can store an integer, a floating-point number, or a string of characters. This means that a single variable ie. same memory location can be used to store multiple types of data. You can use any built-in or user defined data types inside a union based on your requirement.
The memory occupied by a union will be large enough to hold the largest member of the union. For example, in above example Data type will occupy 20 bytes of memory space because this is the maximum space which can be occupied by character string. Following is the example which will display total memory size occupied by the above union:

#include <stdio.h>
#include <string.h>
 
union Data
{
   int i;
   float f;
   char  str[20];
};
 
int main( )
{
   union Data data;        

   printf( "Memory size occupied by data : %d\n", sizeof(data));

   return 0;
}

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

Memory size occupied by data : 20

Accessing Union Members

To access any member of a union, we use the member access operator (.). The member access operator is coded as a period between the union variable name and the union member that we wish to access. You would use union keyword to define variables of union type. Following is the example to explain usage of union:

#include <stdio.h>
#include <string.h>
 
union Data
{
   int i;
   float f;
   char  str[20];
};
 
int main( )
{
   union Data data;        

   data.i = 10;
   data.f = 220.5;
   strcpy( data.str, "C Programming");

   printf( "data.i : %d\n", data.i);
   printf( "data.f : %f\n", data.f);
   printf( "data.str : %s\n", data.str);

   return 0;
}

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

data.i : 1917853763
data.f : 4122360580327794860452759994368.000000
data.str : C Programming

Here, we can see that values of i and f members of union got corrupted because final value assigned to the variable has occupied the memory location and this is the reason that the value if str member is getting printed very well. Now let's look into the same example once again where we will use one variable at a time which is the main purpose of having union:

#include <stdio.h>
#include <string.h>
 
union Data
{
   int i;
   float f;
   char  str[20];
};
 
int main( )
{
   union Data data;        

   data.i = 10;
   printf( "data.i : %d\n", data.i);
   
   data.f = 220.5;
   printf( "data.f : %f\n", data.f);
   
   strcpy( data.str, "C Programming");
   printf( "data.str : %s\n", data.str);

   return 0;
}

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

data.i : 10
data.f : 220.500000
data.str : C Programming

Here, all the members are getting printed very well because one member is being used at a time.

C-Structure

C arrays allow you to define type of variables that can hold several data items of the same kind but structure is another user defined data type available in C programming, which allows you to combine data items of different kinds.
Structures are used to represent a record, Suppose you want to keep track of your books in a library. You might want to track the following attributes about each book:

• Title
  
• Author
• Subject
  
• Book ID

Defining a Structure

To define a structure, you must use the struct statement. The struct statement defines a new data type, with more than one member for your program. The format of the struct statement is this:

struct [structure tag]
{
   member definition;
   member definition;
   ...
   member definition;
} [one or more structure variables];  

The structure tag is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. At the end of the structure's definition, before the final semicolon, you can specify one or more structure variables but it is optional. Here is the way you would declare the Book structure:

struct Books
{
   char  title[50];
   char  author[50];
   char  subject[100];
   int   book_id;
} book;  

Accessing Structure Members

To access any member of a structure, we use the member access operator (.). The member access operator is coded as a period between the structure variable name and the structure member that we wish to access. You would use struct keyword to define variables of structure type. Following is the example to explain usage of structure:

#include <stdio.h>
#include <string.h>
 
struct Books
{
   char  title[50];
   char  author[50];
   char  subject[100];
   int   book_id;
};
 
int main( )
{
   struct Books Book1;        /* Declare Book1 of type Book */
   struct Books Book2;        /* Declare Book2 of type Book */
 
   /* book 1 specification */
   strcpy( Book1.title, "C Programming");
   strcpy( Book1.author, "Nuha Ali"); 
   strcpy( Book1.subject, "C Programming Tutorial");
   Book1.book_id = 6495407;

   /* book 2 specification */
   strcpy( Book2.title, "Telecom Billing");
   strcpy( Book2.author, "Zara Ali");
   strcpy( Book2.subject, "Telecom Billing Tutorial");
   Book2.book_id = 6495700;
 
   /* print Book1 info */
   printf( "Book 1 title : %s\n", Book1.title);
   printf( "Book 1 author : %s\n", Book1.author);
   printf( "Book 1 subject : %s\n", Book1.subject);
   printf( "Book 1 book_id : %d\n", Book1.book_id);

   /* print Book2 info */
   printf( "Book 2 title : %s\n", Book2.title);
   printf( "Book 2 author : %s\n", Book2.author);
   printf( "Book 2 subject : %s\n", Book2.subject);
   printf( "Book 2 book_id : %d\n", Book2.book_id);

   return 0;
}

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

Book 1 title : C Programming
Book 1 author : Nuha Ali
Book 1 subject : C Programming Tutorial
Book 1 book_id : 6495407
Book 2 title : Telecom Billing
Book 2 author : Zara Ali
Book 2 subject : Telecom Billing Tutorial
Book 2 book_id : 6495700

Structures as Function Arguments

You can pass a structure as a function argument in very similar way as you pass any other variable or pointer. You would access structure variables in the similar way as you have accessed in the above example:

#include <stdio.h>
#include <string.h>
 
struct Books
{
   char  title[50];
   char  author[50];
   char  subject[100];
   int   book_id;
};

/* function declaration */
void printBook( struct Books book );
int main( )
{
   struct Books Book1;        /* Declare Book1 of type Book */
   struct Books Book2;        /* Declare Book2 of type Book */
 
   /* book 1 specification */
   strcpy( Book1.title, "C Programming");
   strcpy( Book1.author, "Nuha Ali"); 
   strcpy( Book1.subject, "C Programming Tutorial");
   Book1.book_id = 6495407;

   /* book 2 specification */
   strcpy( Book2.title, "Telecom Billing");
   strcpy( Book2.author, "Zara Ali");
   strcpy( Book2.subject, "Telecom Billing Tutorial");
   Book2.book_id = 6495700;
 
   /* print Book1 info */
   printBook( Book1 );

   /* Print Book2 info */
   printBook( Book2 );

   return 0;
}
void printBook( struct Books book )
{
   printf( "Book title : %s\n", book.title);
   printf( "Book author : %s\n", book.author);
   printf( "Book subject : %s\n", book.subject);
   printf( "Book book_id : %d\n", book.book_id);
}

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

Book title : C Programming
Book author : Nuha Ali
Book subject : C Programming Tutorial
Book book_id : 6495407
Book title : Telecom Billing
Book author : Zara Ali
Book subject : Telecom Billing Tutorial
Book book_id : 6495700

Pointers to Structures

You can define pointers to structures in very similar way as you define pointer to any other variable as follows:

struct Books *struct_pointer;

Now, you can store the address of a structure variable in the above defined pointer variable. To find the address of a structure variable, place the & operator before the structure's name as follows:

struct_pointer = &Book1;

To access the members of a structure using a pointer to that structure, you must use the -> operator as follows:

struct_pointer->title;

Let us re-write above example using structure pointer, hope this will be easy for you to understand the concept:

#include <stdio.h>
#include <string.h>
 
struct Books
{
   char  title[50];
   char  author[50];
   char  subject[100];
   int   book_id;
};

/* function declaration */
void printBook( struct Books *book );
int main( )
{
   struct Books Book1;        /* Declare Book1 of type Book */
   struct Books Book2;        /* Declare Book2 of type Book */
 
   /* book 1 specification */
   strcpy( Book1.title, "C Programming");
   strcpy( Book1.author, "Nuha Ali"); 
   strcpy( Book1.subject, "C Programming Tutorial");
   Book1.book_id = 6495407;

   /* book 2 specification */
   strcpy( Book2.title, "Telecom Billing");
   strcpy( Book2.author, "Zara Ali");
   strcpy( Book2.subject, "Telecom Billing Tutorial");
   Book2.book_id = 6495700;
 
   /* print Book1 info by passing address of Book1 */
   printBook( &Book1 );

   /* print Book2 info by passing address of Book2 */
   printBook( &Book2 );

   return 0;
}
void printBook( struct Books *book )
{
   printf( "Book title : %s\n", book->title);
   printf( "Book author : %s\n", book->author);
   printf( "Book subject : %s\n", book->subject);
   printf( "Book book_id : %d\n", book->book_id);
}

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

Book title : C Programming
Book author : Nuha Ali
Book subject : C Programming Tutorial
Book book_id : 6495407
Book title : Telecom Billing
Book author : Zara Ali
Book subject : Telecom Billing Tutorial
Book book_id : 6495700

Bit Fields

Bit Fields allow the packing of data in a structure. This is especially useful when memory or data storage is at a premium. Typical examples:

• Packing several objects into a machine word. e.g. 1 bit flags can be compacted.
• Reading external file formats -- non-standard file formats could be read in. E.g. 9 bit integers.

C allows us do this in a structure definition by putting :bit length after the variable. For example:

struct packed_struct {
  unsigned int f1:1;
  unsigned int f2:1;
  unsigned int f3:1;
  unsigned int f4:1;
  unsigned int type:4;
  unsigned int my_int:9;
} pack;

Here, the packed_struct contains 6 members: Four 1 bit flags f1..f3, a 4 bit type and a 9 bit my_int.
C automatically packs the above bit fields as compactly as possible, provided that the maximum length of the field is less than or equal to the integer word length of the computer. If this is not the case then some compilers may allow memory overlap for the fields whilst other would store the next field in the next word

C-Bits Manipulation

Bit manipulation is the act of algorithmically manipulating bits or other pieces of data shorter than a byte. C language is very efficient in manipulating bits.
Here are following operators to perform bits manipulation:

Bitwise Operators:

Bitwise operator works on bits and perform bit by bit operation.
Assume if B = 60; and B = 13; Now in binary format they will be as follows:
A = 0011 1100
B = 0000 1101
-----------------
A&B = 0000 1000
A|B = 0011 1101
A^B = 0011 0001
~A  = 1100 0011
Show Examples
There are following Bitwise operators supported by C language

Operator	Description	Example
&	Binary AND Operator copies a bit to the result if it exists in both operands.	(A & B) will give 12 which is 0000 1100
|	Binary OR Operator copies a bit if it exists in eather operand.	(A | B) will give 61 which is 0011 1101
^	Binary XOR Operator copies the bit if it is set in one operand but not both.	(A ^ B) will give 49 which is 0011 0001
~	Binary Ones Complement Operator is unary and has the efect of 'flipping' bits.	(~A ) will give -60 which is 1100 0011
<<	Binary Left Shift Operator. The left operands value is moved left by the number of bits specified by the right operand.	A << 2 will give 240 which is 1111 0000
>>	Binary Right Shift Operator. The left operands value is moved right by the number of bits specified by the right operand.	A >> 2 will give 15 which is 0000 1111

The shift operators perform appropriate shift by operator on the right to the operator on the left. The right operator must be positive. The vacated bits are filled with zero.
For example: x << 2 shifts the bits in x by 2 places to the left.

if x = 00000010 (binary) or 2 (decimal) then: x >>= 2 => x = 00000000 or just 0 (decimal) Also: if x = 00000010 (binary) or 2 (decimal) then x <<= 2 => x = 00001000 or 8 (decimal)

Therefore a shift left is equivalent to a multiplication by 2. Similarly a shift right is equal to division by 2. Shifting is much faster than actual multiplication (*) or division (/) by 2. So if you want fast multiplications or division by 2 use shifts.
To illustrate many points of bitwise operators let us write a function, Bitcount, that counts bits set to 1 in an 8 bit number (unsigned char) passed as an argument to the function.

int bitcount(unsigned char x) { int count; for ( count=0; x != 0; x>>=1); { if ( x & 01) count++; } return count; }

This function illustrates many C program points:

• for loop not used for simple counting operation.
• x >>= 1 => x = x>> 1;
• for loop will repeatedly shift right x until x becomes 0
• use expression evaluation of x & 01 to control if
• x & 01 masks of 1st bit of x if this is 1 then count++

Bit Fields

Bit Fields allow the packing of data in a structure. This is especially useful when memory or data storage is at a premium. Typical examples:

• Packing several objects into a machine word. e.g. 1 bit flags can be compacted.
• Reading external file formats -- non-standard file formats could be read in. E.g. 9 bit integers.

C allows us do this in a structure definition by putting :bit length after the variable.For example:

struct packed_struct { unsigned int f1:1; unsigned int f2:1; unsigned int f3:1; unsigned int f4:1; unsigned int type:4; unsigned int my_int:9; } pack;

Here the packed_struct contains 6 members: Four 1 bit flags f1..f3, a 4 bit type and a 9 bit my_int.
C automatically packs the above bit fields as compactly as possible, provided that the maximum length of the field is less than or equal to the integer word length of the computer. If this is not the case then some compilers may allow memory overlap for the fields whilst other would store the next field in the next word

Count Nouns vs. Non-Count Nouns

Count nouns

Can be counted as one or more.

• pen, computer, bottle, spoon, desk, cup, television, chair, shoe, finger, flower, camera, stick, balloon, book, table, comb, etc.

Take an s to form the plural.

• pens, computers, bottles, spoons, desks, cups, televisions, chairs, shoes, fingers, flowers, cameras, sticks, balloons, books, tables, combs, etc.

Work with expressions such as (a few, few, many, some, every, each, these, and the number of).

• a few pens, a few computers, many bottles, some spoons, every desk, each cup, these televisions, the number of chairs, a few shoes, a few fingers, many flowers, some cameras, every stick, each balloon, these books, the number of tables, many combs, etc.

Work with appropriate articles (a, an, or the).

• a pen, the computer, a bottle, the spoon, a desk, the cup, a television, the chair, a shoe, the finger, a flower, the camera, a stick, the balloon, a book, the table, a comb, etc.

Do NOT work with much (for example, you would never say much pens or much computers).

Non-count nouns

Cannot be counted. They usually express a group or a type.

• water, wood, ice, air, oxygen, English, Spanish, traffic, furniture, milk, wine, sugar, rice, meat, flour, soccer, sunshine, etc.

Generally cannot be pluralized.

Work both with and without an article (a, an, or the), depending on the context of the sentence.

• Sugar is sweet.
• The sunshine is beautiful.
• I drink milk.
• He eats rice.
• We watch soccer together.
• The wood is burning.

Work with expressions such as (some, any, enough, this, that, and much).

• We ate some rice and milk.
• I hope to see some sunshine today.
• This meat is good.
• She does not speak much Spanish.
• Do you see any traffic on the road?
• That wine is very old.

Do NOT work with expressions such as (these, those, every, each, either, or neither).

[Quiz 2.1]

Choose all of the non-count nouns in the following list:

wine, student, pen, water, wind, milk, computer, furniture, cup, rice, box, watch, potato, wood

View Answers

[2.1]
wine, water, wind, milk, furniture, rice, wood

Singular and Plural Nouns

A noun names a person, place, thing, or idea.

Usually, the first page of a grammar book tells you about nouns. Nouns give names of concrete or abstract things in our lives. As babies learn "mom," "dad," or "milk" as their first word, nouns should be the first topic when you study a foreign language.

For the plural form of most nouns, add s.

• bottle – bottles
• cup – cups
• pencil – pencils
• desk – desks
• sticker – stickers
• window – windows

For nouns that end in ch, x, s, or s sounds, add es.

• box – boxes
• watch – watches
• moss – mosses
• bus – buses

For nouns ending in f or fe, change f to v and add es.

• wolf – wolves
• wife – wives
• leaf – leaves
• life – lives

Some nouns have different plural forms.

• child – children
• woman – women
• man – men
• mouse – mice
• goose – geese

Nouns ending in vowels like y or o do not have definite rules.

• baby – babies
• toy – toys
• kidney – kidneys
• potato – potatoes
• memo – memos
• stereo – stereos

A few nouns have the same singular and plural forms.

• sheep – sheep
• deer – deer
• series – series
• species – species

[Quiz 1.1]

Choose the correct form of the noun in each sentence.

1) I have three (child, children).
2) There are five (man, men) and one (woman, women).
3) (Baby, Babies) play with bottles as toys.
4) I put two big (potato, potatoes) in the lunch box.
5) A few men wear (watch, watches).
6) I put a (memo, memos) on the desk.
7) I saw a (mouse, mice) running by.
8) There are few (bus, buses) on the road today.

View Answers

[1.1]
1) children
2) men, woman
3) Babies
4) potatoes
5) watches
6) memo
7) mouse
8) buses

C - Strings

The string in C programming language is actually a one-dimensional array of characters which is terminated by a null character '\0'. Thus a null-terminated string contains the characters that comprise the string followed by a null.
The following declaration and initialization create a string consisting of the word "Hello". To hold the null character at the end of the array, the size of the character array containing the string is one more than the number of characters in the word "Hello."

char greeting[6] = {'H', 'e', 'l', 'l', 'o', '\0'};

If you follow the rule of array initialization then you can write the above statement as follows:

char greeting[] = "Hello";

Following is the memory presentation of above defined string in C/C++:
Actually, you do not place the null character at the end of a string constant. The C compiler automatically places the '\0' at the end of the string when it initializes the array. Let us try to print above mentioned string:

#include <stdio.h>

int main ()
{
   char greeting[6] = {'H', 'e', 'l', 'l', 'o', '\0'};

   printf("Greeting message: %s\n", greeting );

   return 0;
}

When the above code is compiled and executed, it produces result something as follows:

Greeting message: Hello

C supports a wide range of functions that manipulate null-terminated strings:

S.N.	Function & Purpose
1	strcpy(s1, s2); Copies string s2 into string s1.
2	strcat(s1, s2); Concatenates string s2 onto the end of string s1.
3	strlen(s1); Returns the length of string s1.
4	strcmp(s1, s2); Returns 0 if s1 and s2 are the same; less than 0 if s1<s2; greater than 0 if s1>s2.
5	strchr(s1, ch); Returns a pointer to the first occurrence of character ch in string s1.
6	strstr(s1, s2); Returns a pointer to the first occurrence of string s2 in string s1.

Following example makes use of few of the above-mentioned functions:

#include <stdio.h>
#include <string.h>

int main ()
{
   char str1[12] = "Hello";
   char str2[12] = "World";
   char str3[12];
   int  len ;

   /* copy str1 into str3 */
   strcpy(str3, str1);
   printf("strcpy( str3, str1) :  %s\n", str3 );

   /* concatenates str1 and str2 */
   strcat( str1, str2);
   printf("strcat( str1, str2):   %s\n", str1 );

   /* total lenghth of str1 after concatenation */
   len = strlen(str1);
   printf("strlen(str1) :  %d\n", len );

   return 0;
}

When the above code is compiled and executed, it produces result something as follows:

strcpy( str3, str1) :  Hello
strcat( str1, str2):   HelloWorld
strlen(str1) :  10

You can find a complete list of c string related functions in C Standard Library.

• In C language Strings are defined as an array of characters or a pointer to a portion of memory containing ASCII characters. A string in C is a sequence of zero or more characters followed by a NULL '\0' character:
• It is important to preserve the NULL terminating character as it is how C defines and manages variable length strings. All the C standard library functions require this for successful operation.
• All the string handling functions are prototyped in: string.h or stdio.h standard header file. So while using any string related function, don't forget to include either stdio.h or string.h. May be your compiler differes so please check before going ahead.
• If you were to have an array of characters WITHOUT the null character as the last element, you'd have an ordinary character array, rather than a string constant.
• String constants have double quote marks around them, and can be assigned to char pointers as shown below. Alternatively, you can assign a string constant to a char array - either with no size specified, or you can specify a size, but don't forget to leave a space for the null character!

char *string_1 = "Hello"; char string_2[] = "Hello"; char string_3[6] = "Hello";

Reading and Writing Strings:

One possible way to read in a string is by using scanf. However, the problem with this, is that if you were to enter a string which contains one or more spaces, scanf would finish reading when it reaches a space, or if return is pressed. As a result, the string would get cut off. So we could use the gets function
A gets takes just one argument - a char pointer, or the name of a char array, but don't forget to declare the array / pointer variable first! What's more, is that it automatically prints out a newline character, making the output a little neater.
A puts function is similar to gets function in the way that it takes one argument - a char pointer. This also automatically adds a newline character after printing out the string. Sometimes this can be a disadvantage, so printf could be used instead.

#include <stdio.h> int main() { char array1[50]; char *array2; printf("Now enter another string less than 50"); printf(" characters with spaces: \n"); gets(array1); printf("\nYou entered: "); puts(array1); printf("\nTry entering a string less than 50"); printf(" characters, with spaces: \n"); scanf("%s", array2); printf("\nYou entered: %s\n", array2); return 0; }

This will produce following result:

Now enter another string less than 50 characters with spaces: hello world You entered: hello world Try entering a string less than 50 characters, with spaces: hello world You entered: hello

String Manipulation Functions

• char *strcpy (char *dest, char *src) - Copy src string string into dest string.
• char *strncpy(char *string1, char *string2, int n) - Copy first n characters of string2 to stringl .
• int strcmp(char *string1, char *string2) - Compare string1 and string2 to determine alphabetic order.
• int strncmp(char *string1, char *string2, int n) - Compare first n characters of two strings.
• int strlen(char *string) - Determine the length of a string.
• char *strcat(char *dest, const char *src); - Concatenate string src to the string dest.
• char *strncat(char *dest, const char *src, int n); - Concatenate n chracters from string src to the string dest.
• char *strchr(char *string, int c) - Find first occurrence of character c in string.
• char *strrchr(char *string, int c) - Find last occurrence of character c in string.
• char *strstr(char *string2, char string*1) - Find first occurrence of string string1 in string2.
• char *strtok(char *s, const char *delim) - Parse the string s into tokens using delim as delimiter.

C-Recursion

                            

Number Factorial

Following is an example, which calculates factorial for a given number using a recursive function:

#include <stdio.h>

int factorial(unsigned int i)
{
   if(i <= 1)
   {
      return 1;
   }
   return i * factorial(i - 1);
}
int  main()
{
    int i = 15;
    printf("Factorial of %d is %d\n", i, factorial(i));
    return 0;
}

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

Factorial of 15 is 2004310016

Fibonacci Series

Following is another example, which generates Fibonacci series for a given number using a recursive function:

#include <stdio.h>

int fibonaci(int i)
{
   if(i == 0)
   {
      return 0;
   }
   if(i == 1)
   {
      return 1;
   }
   return fibonaci(i-1) + fibonaci(i-2);
}

int  main()
{
    int i;
    for (i = 0; i < 10; i++)
    {
       printf("%d\t%n", fibonaci(i));
    }
    return 0;
}

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