Union basics - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of union member assignment

What is the output of this C program?

C

#include <stdio.h>

union Data {
    int i;
    float f;
    char str[20];
};

int main() {
    union Data data;
    data.i = 10;
    data.f = 220.5f;
    printf("%d\n", data.i);
    return 0;
}

A Some unpredictable integer value

B 220

C 10

D Compilation error

💡 Hint

Remember that all members of a union share the same memory space.

In a union, all members share the same memory. Assigning to data.f overwrites data.i. Printing data.i after assigning data.f will show an unpredictable integer value representing the float bits.

❓ Predict Output

intermediate

Size of union vs struct

What will be the output of this program?

C

#include <stdio.h>

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

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

int main() {
    printf("%zu %zu\n", sizeof(union U), sizeof(struct S));
    return 0;
}

A 16 24

B 16 16

C 8 16

D 24 24

💡 Hint

Union size is the size of its largest member; struct size is sum plus padding.

The union size equals the largest member size (double = 8 or 16 bytes depending on platform, here 16). The struct size is sum of members plus padding, here 24 bytes.

🔧 Debug

advanced

Why does this union code cause unexpected output?

Consider this code snippet. Why does printing data.str after assigning data.i show unexpected characters?

C

#include <stdio.h>
#include <string.h>

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

int main() {
    union Data data;
    data.i = 0x41424344; // Hex for 'ABCD'
    printf("%s\n", data.str);
    return 0;
}

A Because the integer value is too large to fit in the union

B Because the string is not null-terminated, printf reads beyond the array causing garbage output

C Because union members cannot be accessed after assignment to another member

D Because printf cannot print char arrays inside unions

💡 Hint

Think about how strings are represented in C and what printf expects.

The char array in the union is not null-terminated after assigning the integer. printf expects a null-terminated string, so it reads past the array causing garbage output.

🧠 Conceptual

advanced

Union member alignment and padding

Which statement about union member alignment and padding is true?

A Union size is always the sum of all member sizes including padding

B Union members are stored in separate memory locations to avoid padding

C Union size is the size of its largest member, aligned to the strictest member's alignment requirement

D Union size is always equal to the smallest member size

💡 Hint

Think about how unions share memory and how alignment affects size.

Unions share the same memory for all members. The size is the largest member size, rounded up to meet the strictest alignment requirement among members.

❓ Predict Output

expert

Output of union with bit fields and overlapping members

What is the output of this program?

C

#include <stdio.h>

union U {
    struct {
        unsigned int a:4;
        unsigned int b:4;
    } bits;
    unsigned char byte;
};

int main() {
    union U u;
    u.byte = 0;
    u.bits.a = 9;  // 1001 in binary
    u.bits.b = 6;  // 0110 in binary
    printf("%u\n", u.byte);
    return 0;
}

A Compilation error

B 150

C 90

D 105

💡 Hint

Calculate the bits: a in lower 4 bits, b in upper 4 bits.

Bits a=9 (1001) in lower 4 bits, b=6 (0110) in upper 4 bits. Combined byte = 01101001 binary = 0x69 = 105 decimal.

Practice

1. What is the main characteristic of a union in C?

easy

A. Unions can only store integers

B. Each member has its own separate memory

C. All members share the same memory location

D. Unions automatically initialize all members

Solution

1. Step 1: Understand union memory sharing

   A union allocates one shared memory block for all its members, so they overlap.

2. Step 2: Compare with struct memory allocation

   Unlike structs, unions do not allocate separate memory for each member.

3. Final Answer:

   All members share the same memory location -> Option C

4. Quick Check:

   Union memory sharing = All members share the same memory location [OK]

Hint: Remember: union members overlap in memory [OK]

Common Mistakes:

• Thinking each member has separate memory
• Assuming unions initialize all members
• Believing unions only store one data type

2. Which of the following is the correct way 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', the name, and braces with members inside.

2. Step 2: Check each option

   union Data { int i; float f; }; uses correct syntax with braces and semicolons. Options A, B, and C have syntax errors or wrong keywords.

3. Final Answer:

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

4. Quick Check:

   Correct union syntax = union Data { int i; float f; }; [OK]

Hint: Use 'union Name { members; };' syntax [OK]

Common Mistakes:

• Using struct instead of union
• Using parentheses instead of braces
• Assigning values in the definition

3. What will be the output of the following C code?

union Number {
  int i;
  float f;
};

int main() {
  union Number num;
  num.i = 65;
  printf("%c", num.f);
  return 0;
}

medium

A. A

B. 65

C. Compilation error

D. Undefined behavior or garbage character

Solution

1. Step 1: Understand union member sharing

   Setting num.i = 65 stores integer bits in union memory.

2. Step 2: Printing num.f as char

   Printing num.f with %c treats float bits as a char, causing unpredictable output.

3. Final Answer:

   Undefined behavior or garbage character -> Option D

4. Quick Check:

   Accessing different union member types = Undefined behavior or garbage character [OK]

Hint: Accessing different union members can cause garbage output [OK]

Common Mistakes:

• Expecting integer 65 to print as 'A'
• Assuming union members convert automatically
• Ignoring type mismatch in printf

4. Identify the error in this union usage:

union Data {
  int i;
  float f;
};

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

medium

A. Using %d to print a union member is invalid

B. Assigning to d.f overwrites d.i value

C. Cannot assign to multiple union members sequentially

D. Union must be initialized before use

Solution

1. Step 1: Analyze assignments to union members

   Assigning d.i then d.f means d.f overwrites the same memory used by d.i.

2. Step 2: Understand printf output

   Printing d.i after assigning d.f prints the overwritten value, not 10.

3. Final Answer:

   Assigning to d.f overwrites d.i value -> Option B

4. Quick Check:

   Union members share memory, last assignment wins = Assigning to d.f overwrites d.i value [OK]

Hint: Last assigned union member overwrites previous values [OK]

Common Mistakes:

• Thinking union members hold values independently
• Believing %d cannot print union members
• Assuming union needs initialization

5. You want to store either an int, a float, or a char in the same memory space and print the stored value correctly. Which approach using a union and an additional variable is best?

hard

A. Use a union for data and an enum to track the active member type

B. Use a union and print all members every time

C. Use separate variables for each type without union

D. Use a union and always print the int member

Solution

1. Step 1: Understand union limitation

   Union stores one member at a time, so we must know which member is active.

2. Step 2: Use enum to track active member

   An enum variable can track which type is stored, so printing uses correct format.

3. Final Answer:

   Use a union for data and an enum to track the active member type -> Option A

4. Quick Check:

   Union + enum for type tracking = Use a union for data and an enum to track the active member type [OK]

Hint: Track active union member with enum for correct printing [OK]

Common Mistakes:

• Printing all union members blindly
• Not tracking which member is active
• Using separate variables wastes memory