# Month: December 2014

### Union

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Union, Difference between structure & union

Unions are quite similar to the structures in C. Union is also a derived type as structure. Union can be defined in same manner as structures just the keyword used in defining union in union where keyword used in defining structure was struct.

`union car{`
`  char name[50];`
`  int price;`
`};`

Union variables can be created in similar manner as structure variable.

`union car{`
`  char name[50];`
`  int price;`
`}c1, c2, *c3;`
` `
`OR;`
` `
`union car{`
`  char name[50];`
`  int price;`
`};`
`-------Inside Function-----------`
`union car c1, c2, *c3;`

In both cases, union variables c1, c2 and union pointer variable c3 of type union car is created.

##### Accessing members of an union

The member of unions can be accessed in similar manner as that structure. Suppose, we you want to access price for union variable c1 in above example, it can be accessed as c1.price. If you want to access price for union pointer variable c3, it can be accessed as (*c3).price or as c3->price.

#### Difference between union and structure

Though unions are similar to structure in so many ways, the difference between them is crucial to understand. This can be demonstrated by this example:

`#include <stdio.h>`
`union job {         //defining a union`
`   char name[32];`
`   float salary;`
`   int worker_no;`
`}u;`
`struct job1 {`
`   char name[32];`
`   float salary;`
`   int worker_no;`
`}s;`
`int main(){`
`   printf("size of union = %d",sizeof(u));`
`   printf("\nsize of structure = %d", sizeof(s));`
`   return 0;`
`}`

Output

`size of union = 32`
`size of structure = 40`

There is difference in memory allocation between union and structure as suggested in above example. The amount of memory required to store a structure variables is the sum of memory size of all members.

But, the memory required to store a union variable is the memory required for largest element of an union.

What difference does it make between structure and union?

As you know, all members of structure can be accessed at any time. But, only one member of union can be accessed at a time in case of union and other members will contain garbage value.

`#include <stdio.h>`
`union job {`
`   char name[32];`
`   float salary;`
`   int worker_no;`
`}u;`
`int main(){`
`   printf("Enter name:\n");`
`   scanf("%s",&u.name);`
`   printf("Enter salary: \n");`
`   scanf("%f",&u.salary);`
`   printf("Displaying\nName :%s\n",u.name);`
`   printf("Salary: %.1f",u.salary);`
`   return 0;`
`}`

Output

`Enter name `
`Hillary`
`Enter salary`
`1234.23`
`Displaying`
`Name: f%Bary   `
`Salary: 1234.2`

Note: You may get different garbage value of name.

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### Structure

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### Structure

Structure is the collection of variables of different types under a single name for better handling. For example: You want to store the information about person about his/her name, citizenship number and salary. You can create these information separately but, better approach will be collection of these information under single name because all these information are related to person.

#### Structure Definition in C

Keyword struct is used for creating a structure.

##### Syntax of structure
`struct structure_name `
`{`
`    data_type member1;`
`    data_type member2;`
`    .`
`    .`
`    data_type memeber;`
`};`

We can create the structure for a person as mentioned above as:

`struct person`
`{`
`    char name[50];`
`    int cit_no;`
`    float salary;`
`};`

This declaration above creates the derived data type struct person.

#### Structure variable declaration

When a structure is defined, it creates a user-defined type but, no storage is allocated. For the above structure of person, variable can be declared as:

`struct person`
`{`
`    char name[50];`
`    int cit_no;`
`    float salary;`
`};`
` `
`Inside main function:`
`struct person p1, p2, p[20];`

#### Accessing members of a structure

There are two types of operators used for accessing members of a structure.

1. Member operator(.)

2. Structure pointer operator(->) (will be discussed in structure and pointers chapter)

Any member of a structure can be accessed as: structure_variable_name.member_name

Suppose, we want to access salary for variable p2. Then, it can be accessed as:

`p2.salary`

##### Example of structure

Write a C program to add two distances entered by user. Measurement of distance should be in inch and feet.(Note: 12 inches = 1 foot)

` `
`#include <stdio.h>`
`struct Distance{`
`    int feet;`
`    float inch;`
`}d1,d2,sum;`
`int main(){`
`    printf("1st distance\n");`
`    printf("Enter feet: ");`
`    scanf("%d",&d1.feet);  /* input of feet for structure variable d1 */`
`    printf("Enter inch: ");`
`    scanf("%f",&d1.inch);  /* input of inch for structure variable d1 */`
`    printf("2nd distance\n");`
`    printf("Enter feet: ");`
`    scanf("%d",&d2.feet);  /* input of feet for structure variable d2 */`
`    printf("Enter inch: ");`
`    scanf("%f",&d2.inch);  /* input of inch for structure variable d2 */`
`    sum.feet=d1.feet+d2.feet;`
`    sum.inch=d1.inch+d2.inch;`
`    if (sum.inch>12){  //If inch is greater than 12, changing it to feet.`
`        ++sum.feet;`
`        sum.inch=sum.inch-12;`
`    }`
`    printf("Sum of distances=%d\'-%.1f\"",sum.feet,sum.inch); `
`/* printing sum of distance d1 and d2 */`
`    return 0;`
`}`

Output

`1st distance`
`Enter feet: 12`
`Enter inch: 7.9`
`2nd distance`
`Enter feet: 2`
`Enter inch: 9.8`
`Sum of distances= 15'-5.7"`

Structure and Pointer

Pointers can be accessed along with structures. A pointer variable of structure can be created as below:

struct name {

member1;

member2;

.

.

};

——– Insidefunction ——-

struct name *ptr;

Here, the pointer variable of type struct name is created.

Structure’s member through pointer can be used in two ways:

1. Referencing pointer to another address to access memory

2. Using dynamic memory allocation

Consider an example to access structure’s member through pointer.

`#include<stdio.h>`
`struct name{`
`int a;`
`float b;`
`};`
`int main(){`
`struct name *ptr,p;`
`ptr=&p;/* Referencing pointer to memory address of p */`
`printf("Enter integer: ");`
`scanf("%d",&(*ptr).a);`
`printf("Enter number: ");`
`scanf("%f",&(*ptr).b);`
`printf("Displaying: ");`
`printf("%d%f",(*ptr).a,(*ptr).b);`
`return0;`
`}`

In this example, the pointer variable of type struct name is referenced to the address of p. Then, only the structure member through pointer can canaccessed.

Structure pointer member can also be accessed using -> operator.

`(*ptr).a is same asptr->a`
`(*ptr).b is same asptr->b`
##### Accessing structure member through pointer using dynamic memory allocation

To access structure member using pointers, memory can be allocated dynamically using malloc() function defined under “stdlib.h” header file.

###### Syntax to use malloc()
`ptr=(cast-type*)malloc(byte-size)`

Example to use structure’s member through pointer using malloc() function.

`#include<stdio.h>`
`#include<stdlib.h>`
`struct name {`
`int a;`
`float b;`
`char c[30];`
`};`
`int main(){`
`struct name *ptr;`
`inti,n;`
`printf("Enter n: ");`
`scanf("%d",&n);`
`ptr=(struct name*)malloc(n*sizeof(struct name));`
`/* Above statement allocates the memory for n structures with pointer ptr pointing to base address */`
`for(i=0;i<n;++i){`
`printf("Enter string, integer and floating number  respectively:\n");`
`scanf("%s%d%f",&(ptr+i)->c,&(ptr+i)->a,&(ptr+i)->b);`
`}`
`printf("Displaying Infromation:\n");`
`for(i=0;i<n;++i)`
`printf("%s\t%d\t%.2f\n",(ptr+i)->c,(ptr+i)->a,(ptr+i)->b);`
`return0;`
`}`

Output

`Enter n:2`
`Enterstring, integer and floating number  respectively:`
`Programming`
`2`
`3.2`
`Enterstring, integer and floating number  respectively:`
`Structure`
`6`
`2.3`
`DisplayingInformation`
`Programming23.20`
`Structure62.30`

##### Passing structure by value

A structure variable can be passed to the function as an argument as normal variable. If structure is passed by value, change made in structure variable in function definition does not reflect in original structure variable in calling function.

Write a C program to create a structure student, containing name and roll. Ask user the name and roll of a student in main function. Pass this structure to a function and display the information in that function.

`#include <stdio.h>`
`struct student{`
`    char name[50];`
`    int roll;`
`};`
`void Display(struct student stu);`
`/* function prototype should be below to the structure declaration otherwise compiler shows error */`
`int main(){`
`    struct student s1;`
`    printf("Enter student's name: ");`
`    scanf("%s",&s1.name);`
`    printf("Enter roll number:");`
`    scanf("%d",&s1.roll);`
`    Display(s1);   // passing structure variable s1 as argument`
`    return 0;`
`}`
`void Display(struct student stu){`
`  printf("Output\nName: %s",stu.name);`
`  printf("\nRoll: %d",stu.roll);`
`}`

Output

`Enter student's name: Kevin Amla`
`Enter roll number: 149`
`Output`
`Name: Kevin Amla`
`Roll: 149`
##### Passing structure by reference

The address location of structure variable is passed to function while passing it by reference. If structure is passed by reference, change made in structure variable in function definition reflects in original structure variable in the calling function.

Write a C program to add two distances(feet-inch system) entered by user. To solve this program, make a structure. Pass two structure variable (containing distance in feet and inch) to add function by reference and display the result in main function without returning it.

`#include <stdio.h>`
`struct distance{`
`    int feet;`
`    float inch;`
`};`
`void Add(struct distance d1,struct distance d2, struct distance *d3); `
`int main()`
`{`
`    struct distance dist1, dist2, dist3;`
`    printf("First distance\n");`
`    printf("Enter feet: ");`
`    scanf("%d",&dist1.feet);`
`    printf("Enter inch: ");`
`    scanf("%f",&dist1.inch);`
`    printf("Second distance\n");`
`    printf("Enter feet: ");`
`    scanf("%d",&dist2.feet);`
`    printf("Enter inch: ");`
`    scanf("%f",&dist2.inch);`
`    Add(dist1, dist2, &dist3); `
` `
`/*passing structure variables dist1 and dist2 by value whereas passing structure variable dist3 by reference */`
`    printf("\nSum of distances = %d\'-%.1f\"",dist3.feet, dist3.inch);`
`    return 0;`
`}`
`void Add(struct distance d1,struct distance d2, struct distance *d3) `
`{`
`/* Adding distances d1 and d2 and storing it in d3 */`
`     d3->feet=d1.feet+d2.feet; `
`     d3->inch=d1.inch+d2.inch;`
`     if (d3->inch>=12) {     /* if inch is greater or equal to 12, converting it to feet. */`
`         d3->inch-=12;`
`         ++d3->feet;`
`    }`
`}`

Output

`First distance`
`Enter feet: 12`
`Enter inch: 6.8`
`Second distance`
`Enter feet: 5`
`Enter inch: 7.5`
` `
`Sum of distances = 18'-2.3"`

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Website -: http://www.vissicomp.com