7. User-defined Data Types

 

Structure

• A structure is a user-defined data type that allows you to group together variables of different data types under a single name.
• It is defined using the "struct" keyword, followed by the name of the structure and the list of variables within braces.
• The variables within a structure are called members, and they can be accessed using the dot notation.

1. What is the output of the following C code?
#include <stdio.h>


struct demo
{
    char a;
    char b;
};


int main()
{
    struct demo obj;
    obj.a = 'A';
    printf("%d\n", sizeof(struct demo));
    printf("%d\n", sizeof(obj));
    printf("%d\n", sizeof(obj.a));
    printf("%d\n", sizeof(obj.b));
    printf("%c\n", obj.b);
    printf("%c", obj.a);
    return 0;
}

2
2
1
1

A
Explanation: Size of two char variables is 2 bytes and total size of a structure is total size of all variables. If we don't assign a variable, then it will not be initialsed by any value.

2. What is the output of the following C code?
#include <stdio.h>


struct demo
{
    char a;
    int b;
};


int main()
{
    struct demo obj;
    printf("%d\n", sizeof(struct demo));
    printf("%d\n", sizeof(obj.a));
    printf("%d", sizeof(obj.b));
    return 0;
}

8
1
4
Explanation: The size of a struct in C is not always the sum of the sizes of its members. In some cases, there may be padding added by the compiler in order to align the members to memory boundaries. This is done for performance reasons, as accessing aligned memory can be faster on some architectures.

3. What is the compilation status of the following code in C and C++?
#include <stdio.h>


struct emp
{
    char name[20];
    int age;
};
int main()
{
    emp xx;
    return 0;
}

In C
error: unknown type name 'emp'; use 'struct' keyword to refer to the type
emp xx;
In C++, it will compile successfully.
Explanation: In C++, the struct keyword is optional before declaration of a variable, but in C, it is mandatory.

4. What is the output of the following C code?
#include <stdio.h>


struct num
{
    float f;
};


int main()
{
    struct num n1, n2, n3;
    n1.f = 4;
    n2.f = 3;
    n3 = n1 + n2;
    printf("%f", n2.f);
}

error: invalid operands to binary + (have 'struct num' and 'struct num')
n3 = n1 + n2;
The C structure does not allow the struct data type to be treated like build-in data types. We cannot use operators like +, - etc. on structure variables.

5. What is the output of the following C code?
#include <stdio.h>


struct num
{
    float f;
};


int main()
{
    struct num n1, n2, n3;
    n1.f = 4;
    n2.f = 3;
    printf("%f", n2);
}

0.000000
Explanation: n2 is the object of struct num, and we are initialising the variable of n2 object.

6. What is the output of the following C code?
#include <stdio.h>


struct emp
{
    char name[] = "Deepak";
    int age = 20;
};
int main()
{
    struct emp *ptr;
    printf("%s\n", ptr->name);
    printf("%d", ptr->age);
    return 0;
}

error: expected ':' , ',' , ';' , '}' or '__attribute__' before '=' token
char name[] = "Deepak";
error: 'struct emp' has no member named 'name'
printf("%s\n", ptr->name);
error: 'struct emp' has no member named 'age'
printf("%d", ptr->age);
Explanation: When we declare a structure, we actually declare a new data type suitable for our purpose. So we cannot initialize values as it is not a variable declaration but a data type declaration.

Union

• A union is also a user-defined data type that allows you to store different types of data in the same memory location.
• Unlike a structure, a union can only hold the value of one member at a time.
• A union is defined using the "union" keyword, followed by the name of the union and the list of members within braces.
• A union can only hold one value at a time, accessing one member will overwrite the value of any other member that was previously stored in the union.

1. What is the output of the following C code?
#include <stdio.h>


union demo
{
    char a;
    int b;
};


int main()
{
    union demo obj;
    obj.a = 'A';
    printf("%d\n", sizeof(obj));
    printf("%c\n", obj.a);
    printf("%d", obj.b);
}

4
A
65
Explanation: A union can only hold one value at a time, accessing one member will overwrite the value of any other member that was previously stored in the union.

2. What is the output of the following C code?
#include <stdio.h>


union demo
{
    int i;
    char ch[2];
};


int main()
{
    union demo obj;
    obj.ch[0] = 3;
    obj.ch[1] = 2;
    printf("%d %d %d\n", obj.ch[0], obj.ch[1], obj.i);
    obj.i = 0;
    printf("%d %d %d", obj.ch[0], obj.ch[1], obj.i);
}

3 2 515
0 0 0
Explanation: The memory will be of 4 bytes. Initialization happens from right to left of memory blocks. So 0023 will be stored in binary form, which can be written as 00000000 00000000 00000010 00000011 which can be written as 515 in decimal form.

Enumerator

• Enumeration (or enum) is a user-defined data type in C. It is mainly used to assign names to integral constants, the names make a program easy to read and maintain.
• Two or more enum names can have the same value.
• If we do not explicitly assign values to enum, the compiler by default assigns values starting from 0.
• We can assign values to some name in any order. All unassigned names get value as value of previous name plus one.
• The value assigned to enum names must be some integral constant, i.e., the value must be in range from minimum possible integer value to maximum possible integer value.
• All enum constants must be unique in their scope.

1. What is the output of the following C code?
#include <stdio.h>
#include <math.h>


int main()
{
    float f = 5, g = 10;
    enum
    {
        i = 10,
        j = 20,
        k = 50
    };
    printf("%d", ++k);
    printf("%d", f << 2);
    printf("%d", f % g);
    printf("%lf", fmod(f, g));
    return 0;
}

error: lvalue required as increment operand
printf("%d", ++k);
error: invalid operands to binary << (have 'float' and 'int')
printf("%d", f << 2);
error: invalid operands to binary % (have 'float' and 'float')
printf("%d", f % g);
Explanation: Here i, j and k are inside enum and therefore, they are like constants. In other words, if want to use 10 anywhere in the program ,we can use i instead. In the first printf(), the value of k is being modified which is not allowed because it's enum constant. In the second printf(), left-shift operator is being applied on a float which is also not allowed. Similarly in the third printf(), modulus operator is being applied on float f and g which is also not allowed, we can use fmod() for remainder among floats.

2. What is the output of the following C code?
#include <stdio.h>


enum week
{
    Mon,
    Tue,
    Wed,
    Thu,
    Fri,
    Sat,
    Sun
};


int main()
{
    int day;
    for (day = Mon; day <= Sun; day++)
        printf("%d ", day);
    return 0;
}

0 1 2 3 4 5 6

3. What is the output of the following C code?
#include <stdio.h>


enum week
{
    Tubelight = 2,
    Fan = 2,
    AC = 5
};


int main()
{
    printf("%d %d %d", Tubelight, Fan, AC);
    return 0;
}

2 2 5
Explanation: Two or more enum names can have the same value.

4. What is the output of the following C code?
#include <stdio.h>


enum week
{
    Mon = 1,
    Tue,
    Wed = 5,
    Thu,
    Fri = 10,
    Sat,
    Sun
};


int main()
{
    printf("%d %d %d %d %d %d %d", Mon, Tue, Wed, Thu, Fri, Sat, Sun);
    return 0;
}

1 2 5 6 10 11 12

5. What is the output of the following C code?
#include <stdio.h>


enum state
{
    high = 1,
    low = 0,
    high = 9
};


int main()
{
    printf("%d", high);
    return 0;
}

error: redeclaration of enumerator 'high'
high = 9
Explanation: All enum constants must be unique in their scope.

6. What is the output of the following C code?
#include <stdio.h>


enum CSK
{
    mahi = 7,
    faf = 13
};


enum RCB
{
    faf = 13,
    virat = 18
};


int main()
{
    printf("%d", faf);
    return 0;
}

error: redeclaration of enumerator 'faf'
faf = 13
Explanation: All enum constants must be unique in their scope.

Enum VS Macro

• We can also use macros to define names constants.
#define PI 3.14
• There are multiple advantages of using enum over macro when many related named constants have integral values:
• Enum follow scope rules.
• Enum variables are automatically assigned values.
• Also we can assign float constants with macros, but in enum we can only assign integer constants.

Typedef

• typedef gives us a opportunity to rename a data type.
• It can be used with built-in data types as well as user-defined data types.

1. What is the output of the following C code?
#include <stdio.h>


int main()
{
    typedef int i;
    i num = 10;
    printf("%d", num);
    return 0;
}

10
Explanation: int is renamed as i using typedef.