Structure vs union comparison - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of structure size and member values

What is the output of this C program?

C

#include <stdio.h>

struct Data {
    int i;
    char c;
    double d;
};

int main() {
    struct Data s = {5, 'A', 3.14};
    printf("%d %c %.2f\n", s.i, s.c, s.d);
    printf("Size of struct Data: %zu\n", sizeof(s));
    return 0;
}

A

5 A 3.14
Size of struct Data: 12

B

5 A 3.14
Size of struct Data: 24

C

5 A 3.14
Size of struct Data: 16

D

5 A 3.14
Size of struct Data: 13

💡 Hint

Think about padding and alignment in structures.

The structure contains an int (4 bytes), a char (1 byte), and a double (8 bytes). Due to alignment, padding bytes are added so the double starts at an 8-byte boundary. This makes the total size 16 bytes on most systems.

❓ Predict Output

intermediate

Output of union member assignment

What will be printed by this C program?

C

#include <stdio.h>

union Value {
    int i;
    float f;
};

int main() {
    union Value v;
    v.i = 1065353216;
    printf("%d %.2f\n", v.i, v.f);
    return 0;
}

A 1065353216 1.00

B 1065353216 0.00

C 0 1.00

D 1065353216 1065353216.00

💡 Hint

Remember that union members share the same memory.

The integer 1065353216 has the same bit pattern as the float 1.0 in IEEE 754 format. So printing the float member shows 1.00.

🔧 Debug

advanced

Why does this union code print unexpected values?

Consider this code snippet. Why does printing both union members after assignment give unexpected results?

C

#include <stdio.h>

union Data {
    int i;
    char str[4];
};

int main() {
    union Data d;
    d.i = 0x41424344;
    printf("%d %c %c %c %c\n", d.i, d.str[0], d.str[1], d.str[2], d.str[3]);
    return 0;
}

A Because the char array is not initialized, it contains garbage values unrelated to the int.

B Because the union stores all members in the same memory, the char array shows the byte representation of the int, which depends on endianness.

C Because the int and char array have different sizes, accessing str causes a buffer overflow.

D Because the printf format specifiers are incorrect for union members.

💡 Hint

Think about how data is stored in memory and how endianness affects byte order.

Unions share the same memory for all members. Assigning to the int sets the bytes, which the char array then interprets as characters. The order depends on whether the system is little-endian or big-endian.

📝 Syntax

advanced

Which code snippet correctly declares and initializes a union?

Which of the following code snippets correctly declares a union and initializes its member?

A union Number n; n.i = 10; union Number { int i; float f; };

B union Number { int i; float f; } n; n = {10};

C union Number { int i; float f; } n = (int)10;

D union Number { int i; float f; } n = { .i = 10 };

💡 Hint

Look for correct syntax for union declaration and initialization.

Option D uses designated initializer syntax to initialize the union member correctly at declaration. Other options have syntax errors or invalid assignments.

🚀 Application

expert

Memory size comparison between struct and union

Given the following definitions, what is the total size in bytes of struct S and union U on a typical 64-bit system?

C

struct S {
    char c;
    int i;
    double d;
};

union U {
    char c;
    int i;
    double d;
};

A struct S: 16 bytes, union U: 8 bytes

B struct S: 16 bytes, union U: 24 bytes

C struct S: 24 bytes, union U: 8 bytes

D struct S: 24 bytes, union U: 24 bytes

💡 Hint

Remember struct size includes all members plus padding; union size equals largest member size.

The struct has a char (1 byte), then padding (3 bytes), then int (4 bytes), then double (8 bytes), totaling 16 bytes due to alignment. The union size equals the largest member size, which is double (8 bytes).

Practice

1. What is the main difference between a struct and a union in C?

easy

A. Both struct and union share the same memory for all members.

B. A struct stores all members separately, while a union shares the same memory for all members.

C. Both struct and union store members in separate memory locations.

D. A union stores all members separately, while a struct shares the same memory for all members.

Solution

1. Step 1: Understand struct memory allocation

   A struct allocates separate memory for each member, so all members exist independently.

2. Step 2: Understand union memory allocation

   A union uses the same memory location for all members, so only one member can hold a value at a time.

3. Final Answer:

   A struct stores all members separately, while a union shares the same memory for all members. -> Option B

4. Quick Check:

   Struct = separate memory, Union = shared memory [OK]

Hint: Struct = all data stored; Union = one data stored at a time [OK]

Common Mistakes:

• Thinking union stores all members separately
• Confusing struct and union memory sharing
• Assuming both behave the same in memory

2. Which of the following is the correct syntax to define a union named Data with an int and a float member?

easy

A. union Data = { int i; float f; };

B. struct Data { int i; float f; };

C. union Data ( int i; float f; );

D. union Data { int i; float f; };

Solution

1. Step 1: Recall union syntax

   The correct syntax uses the keyword union, followed by the name and braces containing members separated by semicolons.

2. Step 2: Check each option

   union Data { int i; float f; }; matches the correct syntax. struct Data { int i; float f; }; uses struct instead of union. union Data ( int i; float f; ); uses parentheses instead of braces. union Data = { int i; float f; }; uses an equals sign incorrectly.

3. Final Answer:

   union Data { int i; float f; }; -> Option D

4. Quick Check:

   union syntax = keyword + name + braces + members [OK]

Hint: Use braces {} and keyword union for definition [OK]

Common Mistakes:

• Using struct keyword instead of union
• Using parentheses instead of braces
• Adding equals sign in definition

3. Consider the following code:

union Data {
  int i;
  float f;
};

int main() {
  union Data d;
  d.i = 10;
  d.f = 3.14;
  printf("%d", d.i);
  return 0;
}

What will be the output?

medium

A. Undefined or garbage value

B. 10

C. 3

D. 3.14

Solution

1. Step 1: Understand union member assignment

   Assigning d.i = 10; stores 10 in the union memory. Then d.f = 3.14; overwrites the same memory with the float value.

2. Step 2: Check the printf output

   Printing d.i after assigning d.f reads the memory as int, but the memory holds float bits, causing undefined or garbage output.

3. Final Answer:

   Undefined or garbage value -> Option A

4. Quick Check:

   Union stores one value; last assignment wins [OK]

Hint: Last union member assigned overwrites memory [OK]

Common Mistakes:

• Expecting both values to be stored
• Printing float as int correctly
• Ignoring memory overwrite in union

4. Identify the error in this code snippet:

struct Point {
  int x;
  int y;
};

union Point {
  int x;
  int y;
};

int main() {
  struct Point p1 = {1, 2};
  union Point p2;
  p2.x = 3;
  p2.y = 4;
  printf("%d %d", p1.x, p1.y);
  printf("%d %d", p2.x, p2.y);
  return 0;
}

medium

A. Cannot define struct and union with the same name in the same scope.

B. Union members cannot be assigned values.

C. Struct members must be initialized separately, not with braces.

D. printf cannot print multiple variables.

Solution

1. Step 1: Check struct and union naming rules

   In C, struct and union tags share the same namespace. Defining both with the same name Point causes a redefinition error.

2. Step 2: Verify other statements

   Union members can be assigned values. Struct initialization with braces is valid. printf can print multiple variables with format specifiers.

3. Final Answer:

   Cannot define struct and union with the same name in the same scope. -> Option A

4. Quick Check:

   Struct and union tags must have unique names [OK]

Hint: Struct and union names share namespace; avoid duplicates [OK]

Common Mistakes:

• Thinking union members can't be assigned
• Believing struct brace initialization is invalid
• Assuming printf can't print multiple values

5. You want to store either an integer ID or a floating-point score in a variable but never both at the same time to save memory. Which is the best choice and why?

hard

A. Use a struct because it stores both values separately.

B. Use two separate variables to avoid confusion.

C. Use a union because it shares memory and stores only one value at a time.

D. Use an array to store both values.

Solution

1. Step 1: Analyze memory needs

   Since only one value (ID or score) is needed at a time, saving memory is important.

2. Step 2: Choose between struct and union

   A struct stores both values separately, using more memory. A union shares memory for all members, storing only one value at a time, saving memory.

3. Final Answer:

   Use a union because it shares memory and stores only one value at a time. -> Option C

4. Quick Check:

   One value at a time + memory saving = union [OK]

Hint: Union saves memory when storing one of many values [OK]

Common Mistakes:

• Choosing struct and wasting memory
• Using separate variables unnecessarily
• Using array which doesn't fit this need