Union basics - Mini Project: Build & Apply

Union basics

📖 Scenario: Imagine you want to store either an integer or a floating-point number in the same place in memory, but never both at the same time. This is useful when you want to save memory and know that only one type will be used at a time.

🎯 Goal: You will create a union in C that can hold either an int or a float. Then you will assign values and print them to see how unions work.

📋 What You'll Learn

Create a union named Number with two members: int i and float f.

Create a variable num of type union Number.

Assign an integer value to num.i and print it.

Assign a float value to num.f and print it.

Observe how the union shares memory for both members.

💡 Why This Matters

🌍 Real World

Unions are used in low-level programming, such as embedded systems, where memory is limited and you want to store different types of data in the same space.

💼 Career

Understanding unions is important for systems programming, device driver development, and working with hardware interfaces where efficient memory use is critical.

1

Create the union and variable

Write a union named Number with two members: int i and float f. Then create a variable called num of type union Number.

C

union Number {
    int i;
    float f;
};

union Number num;
// Your code here

Hint

Use the union keyword to define the union. Inside, declare int i; and float f;. Then declare a variable num of this union type.

2

Assign an integer value

Assign the integer value 42 to the member num.i.

C

union Number {
    int i;
    float f;
};

union Number num;

num.i = 42;  // Your code here

Hint

Use the dot operator to assign 42 to num.i.

3

Assign a float value

Assign the float value 3.14f to the member num.f.

C

union Number {
    int i;
    float f;
};

union Number num;

num.i = 42;

num.f = 3.14f;  // Your code here

Hint

Use the dot operator to assign 3.14f to num.f.

4

Print the values

Print the values of num.i and num.f using printf. Use %d for the integer and %f for the float. Print num.i first, then num.f on the next line.

C

union Number {
    int i;
    float f;
};

union Number num;

num.i = 42;

num.f = 3.14f;

printf("%d\n", num.i);  // Your code here
printf("%f\n", num.f);  // Your code here

Hint

Use printf("%d\n", num.i); to print the integer and printf("%f\n", num.f); to print the float. Remember to include stdio.h and write a main function.

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