Union basics - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a union named Data.

C

union [1] {
    int i;
    float f;
};

Drag options to blanks, or click blank then click option'

AData

BStruct

CVar

DType

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'struct' instead of 'union' keyword.

Forgetting to name the union.

Using variable names instead of the union name.

The keyword union is followed by the union name. Here, Data is the correct union name.

2fill in blank

medium

Complete the code to access the integer member of the union variable d.

C

union Data d;
d.[1] = 10;

Drag options to blanks, or click blank then click option'

Af

Bx

Ci

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using the float member name instead of the integer one.

Using undefined member names.

To access the integer member of the union, use the member name i.

3fill in blank

hard

Fix the error in the union initialization.

C

union Data d = [1];

Drag options to blanks, or click blank then click option'

A{3.14}

B{.i = 3}

C{.f = 3.14}

D{i = 3}

Attempts:

3 left

💡 Hint

Common Mistakes

Using assignment inside braces without dot notation.

Initializing with the wrong member name.

In C, to initialize a union member by name, use the designated initializer syntax like {.i = 3}.

4fill in blank

hard

Fill both blanks to create a union variable and assign a float value.

C

union Data [1];
[2].f = 5.5;

Drag options to blanks, or click blank then click option'

Ad

Bdata

Cvalue

Dnum

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for declaration and access.

Using undefined variable names.

The variable name must be consistent. Here, d is used both to declare and access the union variable.

5fill in blank

hard

Fill all three blanks to create a union, declare a variable, and assign an integer value.

C

union [1] {
    int i;
    float f;
} [2];

[3].i = 42;

Drag options to blanks, or click blank then click option'

AData

Bd

Cvalue

Dnum

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in declaration and assignment.

Confusing union name with variable name.

The union is named Data, the variable declared is d, and we assign to d.i.

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