Union

Union

AU: May-19, Marks 13

Definition: Union is a user defined data structure which is just similar to structure. It is used to store members of different data types.

'C' allows another type of structure, the union, which allows the variables to be interpreted in several different ways.

For example: Consider a structure giving the details of student. Such as his name, roll number and standard. The categorisation of the standard will be primary or pre-primary. The primary standard will be denoted by 1, 2 till 4 standards. The pre-primary standard will be denoted by "Play group", "Jr. K.G.", and "Sr. K.G." respectively.

Let us see, how it can be denoted in 'C'.

struct student

{

char name[20];

int roll_no;

union {

int primary;

char pre_pri [10];

}std;

};

For example : If a record of student having name - Rama, roll number as 5 and he is in 3rd standard. It will be stored in the following manner.

Similarly a record having name Shyam roll number 2 and standard as Jr. K.G will be mapped as

The structure and the union both are very similar data structures. Both are meant for storing the various member variables of different data types. But the only difference between the two is that various members of the structure stored at various is memory locations. And in case of unions, all the members are stored at the same memory location and that is why only one member can be accessed at a time by the unions variable. You just observe the above given example, the union is defined for the standards primary and pre-primary. Note that the same location is utilized to store the data of different data types. But at a time you can access only one data type of union. In case of above example, the memory location for std will hold either primary data or pre-primary data. The dot operators (exactly like the structure) are used to access the members of the union.

 

Ex. 2.2.1 Define union having array of characters of size 2, one integer and one float as its elements.

Sol. :

union {

char a[2];

int i;

float j;

};

 

1. Difference between Structure a

 

Ex. 2.2.2 Illustrate the representation of structure and union for an employee record having empid, emp-name, DOB, basic pay, allowances, deductions, grosspay and netpay. Examine their memory allocation.

AU May-19, Marks 13

Sol. : Structure allocates different memory locations for all its members while union allocates common memory location for all its members. The memory occupied by a union will be large enough to hold the largest member of the union.

For example - Consider following C program with union w

#include<stdio.h>

union Employee

{

int empid;

char empName[30];

char dob[20];

float basic_pay;

float allowances;

float deduction;

float grosspay;

float netpay;

};

int main( )

{

union Employee E;

printf("\nEnter Employee Id : ");

scanf("%d", &E.empid);

 

printf("\nEnter Employee Name: ");

scanf("%s", &E.empName);

 

printf("\nEnter Employee dob: ");

scanf("%s", &E.dob);

 

printf("\nEnter Employee basic_pay: ");

scanf("%f",&E.basic_pay);

 

printf("\nEnter allowances: ");

scanf("%f",&E.allowances);

 

printf("\nEnter deductions: ");

scanf("%f",&E.deduction);

E.grosspay = (E.basic_pay+E.allowances);

E.netpay = (E.basic_pay+E.allowances);

- E.deduction;

printf("\n\nEmployee Id : %d",E.empid);

printf("\nEmployee Name: %s",E.empName);

printf("\nEmployee DOB: %s",E.dob);

printf("\nEmployee gross Salary: %f",E.grosspay);

printf("\nEmployee Net Salary: %f",E.netpay);

return 0;

}

Output

Enter Employee Id: 101

Enter Employee Name: AAA

Enter Employee dob: 12-10-01

Enter Employee basic_pay : 10000

Enter allowances : 2000

Enter deductions : 500

 

Employee Id: 1148846080

Employee Name :

Employee DOB:

Employee gross Salary: 10000.000000

Employee Net Salary: 0.000000

 

Note that only one memory location will be used by union at a time, rest of the locations are treated as garbage. But in structure it is not so, for each member the structure will allocate the separate memory location

Consider following C program with structure

#include<stdio.h>

struct Employee

{

int empid;

char empName[30];

char dob[20];

float basic_pay;

float allowances;

float deduction;

float grosspay;

float netpay;

};

int main( )

{

struct Employee E;

printf("\nEnter Employee Id : ");

scanf("%d", &E.empid);

 

printf("\nEnter Employee Name : ");

scanf("%s", &E.empName);

 

printf("\nEnter Employee dob: ");

scanf("%s", &E.dob);

 

printf("\nEnter Employee basic_pay: ");

scanf("%f",&E.basic_pay);

 

printf("\nEnter allowances: ");

scanf("%f",&E.allowances);

 

printf("\nEnter deductions: ");

scanf("%f",&E.deduction);

E.grosspay (E.basic_pay+E.allowances);

E.netpay = (E.basic_pay +E.allowances)

- E.deduction;

printf("\n\nEmployee Id : %d", E.empid);

printf("\nEmployee Name: %s",E.empName);

printf("\nEmployee DOB: %s",E.dob);

printf("\nEmployee gross Salary: %f",E.grosspay);

printf("\nEmployee Net Salary: %f",E.netpay);

return 0;

}

Output

Enter Employee Id : 101

Enter Employee Name: AAA

Enter Employee dob: 12-10-01

Enter Employee basic_pay : 10000

Enter allowances : 2000

 

Enter deductions : 500

Employee Id: 101.

Employee Name : AAA

Employee DOB: 12-10-01

Employee gross Salary: 12000.000000

Employee Net Salary: 11500.000000