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    Unions in C: Usage and Examples

    Rumman Ansari July 05, 2024 15,365 views
    Unions in C: Usage and Examples

    A union is a special data type available in C that allows storing 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 multiple-purpose.

    Unions are quite similar to structures in C. Like structures, unions are also derived types.

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

    Defining a union is as easy as replacing the keyword struct with the keyword union.

    How to create union variables?

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

    union car
{
  char name[50];
  int price;
} car1, car2, *car3;

    OR

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

int main()
{
  union car car1, car2, *car3;
  return 0;
}

    In both cases, union variables car1, car2 and union  pointer  variable car3 of type union car is created.

    Accessing members of a union

    Again, the member of unions can be accessed in a similar manner as structures.

    In the above example, suppose you want to access price for union variable car1, it can be accessed as:

    car1.price

    Likewise, if you want to access price for the union pointer variable car3, it can be accessed as:

    (*car3).price
or;
car3->price

    More memory is allocated to structures than union

    As seen in the above example, there is a difference in memory allocation between union and structure.

    The amount of memory required to store a structure variable is the sum of memory size of all members.

    Memory allocation of structure variable in C programming

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

    Memory allocation of union variable in C programming

    Program

    #include
#include
 
union student 
{
            char name[20];
            char subject[20];
            float percentage;
};
 
int main() 
{
    union student record1;
    union student record2;
 
    // assigning values to record1 union variable
       strcpy(record1.name, "Raju");
       strcpy(record1.subject, "Maths");
       record1.percentage = 86.50;
 
       printf("Union record1 values example\n");
       printf(" Name       : %s \n", record1.name);
       printf(" Subject    : %s \n", record1.subject);
       printf(" Percentage : %f \n\n", record1.percentage);
 
    // assigning values to record2 union variable
       printf("Union record2 values example\n");
       strcpy(record2.name, "Mani");
       printf(" Name       : %s \n", record2.name);
 
       strcpy(record2.subject, "Physics");
       printf(" Subject    : %s \n", record2.subject);
 
       record2.percentage = 99.50;
       printf(" Percentage : %f \n", record2.percentage);
       return 0;
}

    Output

    Union record1 values example
 Name       :
 Subject    :
 Percentage : 86.500000

Union record2 values example
 Name       : Mani
 Subject    : Physics
 Percentage : 99.500000
Press any key to continue . . .

    Another Way of Declaration

    Program

    #include
#include
 
union student 
{
            char name[20];
            char subject[20];
            float percentage;
}record;
 
int main() 
{
 
 strcpy(record.name, "Rambo Azmi");
 strcpy(record.subject, "Maths");
 record.percentage = 86.50;
 
 printf(" Name       : %s \n", record.name);
 printf(" Subject    : %s \n", record.subject);
 printf(" Percentage : %f \n", record.percentage);
 return 0;
}

    Output

    Name       :
 Subject    :
 Percentage : 86.500000
Press any key to continue . . .

    Difference between union and structure

    Though unions are similar to structure in so many ways, the difference between them is crucial to understand.

    The primary difference can be demonstrated by this example:

    Program

    #include 
union unionJob
{
   //defining a union
   char name[32];
   float salary;
   int workerNo;
} uJob;

struct structJob
{
   char name[32];
   float salary;
   int workerNo;
} sJob;

int main()
{
   printf("size of union = %d", sizeof(uJob));
   printf("\nsize of structure = %d \n", sizeof(sJob));
   return 0;
}

    Output

    size of union = 32
size of structure = 40
Press any key to continue . . .

    Difference Between Structure and Union

    C Structure
    C Union

    Structure allocates storage space for all its members separately.

    		 Union allocates one common storage space for all its members.
    Union finds that which of its member needs high storage space over other members and allocates that much space
    Structure occupies higher memory space. Union occupies lower memory space over structure.
    We can access all members of structure at a time. We can access only one member of union at a time.
    Structure example:
    struct student
    {
    int mark;
    char name[6];
    double average;
    };    		 Union example:
    union student
    {
    int mark;
    char name[6];
    double average;
    };
    For above structure, memory allocation will be like below.
    int mark – 2B
    char name[6] – 6B
    double average – 8B 
    Total memory allocation = 2+6+8 = 16 Bytes    		 For above union, only 8 bytes of memory will be allocated since double data type will occupy maximum space of memory over other data types. 
    Total memo

    MCQ Section

    MCQ 1
    What is the similarity between a structure, union and enumeration?
    MCQ 2
    Size of a union is determined by size of the.
    MCQ 3
    Comment on the following union declaration? 
    
    #include <stdio.h>
    union temp
    {
        int a;
        float b;
        char c;
    };
    union temp s = {1,2.5,'A'}; //REF LINE Which member of the union will be active after REF LINE?
    MCQ 4
    What would be the size of the following union declaration? 
    
    #include <stdio.h>
    union uTemp
    {
        double a;
        int b[10];
        char c;
    }u;
    (Assuming size of double = 8, size of int = 4, size of char = 1)
    MCQ 5
    What type of data is holded by variable u int this C code? 
    
    #include <stdio.h>
    union u_tag
    {
        int ival;
        float fval;
        char *sval;
    } u;
    The variable u here
    MCQ 6
    Members of a union are accessed as________________
    MCQ 7
    What is the output of this C code? 
        #include "stdio.h"
    struct
    {
        char *name;
        union
        {
            char *sval;
        } u;
    } symtab[10];
    the first character of the string sval by either of
    MCQ 8
    What is the output of this C code(size of int and float is 4)? 
        #include "stdio.h"
    union
    {
        int ival;
        float fval;
    } u;
    void main()
    {
        printf("%d", sizeof(u));
    }
    MCQ 9
    What is the output of this C code? 
        #include "stdio.h"
    union stu
    {
        int ival;
        float fval;
    };
    void main()
    {
        union stu r;
        r.ival = 5;
        printf("%d", r.ival);
    }
    MCQ 10
    What will be the output of the program? 
    #include"stdio.h"

int main()
{
    union a
    {
        int i;
        char ch[2];
    };
    union a u;
    u.ch[0]=3;
    u.ch[1]=2;
    printf("%d, %d, %d\n", u.ch[0], u.ch[1], u.i);
    return 0;
}
    MCQ 11
    What will be the output of the program? 
    #include"stdio.h"

int main()
{
    union var
    {
        int a, b;
    };
    union var v;
    v.a=10;
    v.b=20;
    printf("%d\n", v.a);
    return 0;
}
    MCQ 12
    What will be the output of the program? 
    #include"stdio.h"

int main()
{
    struct value
    {
        int bit1:1;
        int bit3:4;
        int bit4:4;
    }bit={1, 2, 13};

    printf("%d, %d, %d\n", bit.bit1, bit.bit3, bit.bit4);
    return 0;
}
    MCQ 13
    What will be the output of the program in 16 bit platform (Turbo C under DOS)? 
    #include"stdio.h"

int main()
{
    struct value
    {
        int bit1:1;
        int bit3:4;
        int bit4:4;
    }bit;
    printf("%d\n", sizeof(bit));
    return 0;
}
    MCQ 14
    What will be the output of the program? 
    #include"stdio.h"

int main()
{
    enum days {MON=-1, TUE, WED=6, THU, FRI, SAT};
    printf("%d, %d, %d, %d, %d, %d\n", MON, TUE, WED, THU, FRI, SAT);
    return 0;
}
    MCQ 15
    What will be the output of the program? 
    
#include<stdio.h>

int main()
{
    enum status {pass, fail, absent};
    enum status stud1, stud2, stud3;
    stud1 = pass;
    stud2 = absent;
    stud3 = fail;
    printf("%d %d %d\n", stud1, stud2, stud3);
    return 0;
}
    MCQ 16
    What will be the output of the program in Turbo C (under DOS)? 
    
#include<stdio.h>

int main()
{
    struct emp
    {
        char *n;
        int age;
    };
    struct emp e1 = {"Dravid", 23};
    struct emp e2 = e1;
    strupr(e2.n);
    printf("%s\n", e1.n);
    return 0;
}
    MCQ 17
    What will be the output of the program in 16-bit platform (under DOS)? 
    
#include<stdio.h>

int main()
{
    struct node
    {
        int data;
        struct node *link;
    };
    struct node *p, *q;
    p = (struct node *) malloc(sizeof(struct node));
    q = (struct node *) malloc(sizeof(struct node));
    printf("%d, %d\n", sizeof(p), sizeof(q));
    return 0;
}
    MCQ 18
    What will be the output of the program? 
    
#include<stdio.h>

int main()
{
    struct byte
    {
        int one:1;
    };
    struct byte var = {1};
    printf("%d\n", var.one);
    return 0;
}
    MCQ 19
    What will be the output of the program? 
    
#include<stdio.h>

int main()
{
    enum days {MON=-1, TUE, WED=6, THU, FRI, SAT};
    printf("%d, %d, %d, %d, %d, %d\n", ++MON, TUE, WED, THU, FRI, SAT);
    return 0;
}
    MCQ 20
    What will be the output of the program? 
    
#include<stdio.h>

    struct course
    {
        int courseno;
        char coursename[25];
    };
int main()
{
    struct course c[] = { {102, "Java"}, 
                          {103, "PHP"}, 
                          {104, "DotNet"}     };

    printf("%d ", c[1].courseno);
    printf("%s\n", (*(c+2)).coursename);
    return 0;
}
    MCQ 21
    What will be the output of the program given below in 16-bit platform? 
    
#include<stdio.h>

int main()
{
    enum value{VAL1=0, VAL2, VAL3, VAL4, VAL5} var;
    printf("%d\n", sizeof(var));
    return 0;
}
    MCQ 22
    What is the output of this C code? 
    
    #include <stdio.h>
    union
    {
        int x;
        char y;
    }p;
    int main()
    {
        p.x = 10;
        printf("%d\n", sizeof(p));
    }
    MCQ 23
    What is the output of this C code? 
    
    #include <stdio.h>
    union
    {
        int x;
        char y;
    }p;
    int main()
    {
        p.y = 60;
        printf("%d\n", sizeof(p));
    }
    MCQ 24
    What is the output of this C code? 
    
    #include <stdio.h>
    union p
    {
        int x;
        char y;
    };
    int main()
    {
        union p p, b;
        p.y = 60;
        b.x = 12;
        printf("%d\n", p.y);
    }
    MCQ 25
    What is the output of this C code? 
    
    #include <stdio.h>
    union p
    {
        int x;
        char y;
    }k = {1, 97};
    int main()
    {
        printf("%d\n", k.y);
    }
    MCQ 26
    What is the output of this C code? 
    
    #include <stdio.h>
    union p
    {
        int x;
        char y;
    }k = {.y = 97};
    int main()
    {
        printf("%d\n", k.y);
    }
    MCQ 27
    What is the output of this C code? 
    
    #include <stdio.h>
    union p
    {
        int x;
        float y;
    };
    int main()
    {
        union p p, b;
        p.x = 10;
        printf("%f\n", p.y);
    }
    MCQ 28
    Which of the following share a similarity in syntax?
    1. Union, 2. Structure, 3. Arrays and 4. Pointers
    MCQ 29
    What is the output of this C code? 
    
    #include <stdio.h>
    union utemp
    {
        int a;
        double b;
        char c;
    }u;
    int main()
    {
        u.c = 'A';
        u.a = 1;
        printf("%d", sizeof(u));
    }
    The output will be: (Assuming size of char = 1, int = 4, double = 8)
    MCQ 30
    What value will be printed for data.c? 
    #include 
#include 
void main()
{
union Data{
	int i;
	unsigned char c;
}data;

data.c = 'C';
data.i = 89;
printf("%c\n", data.c);

}