Structure vs union comparison - Quick Revision & Key Differences

Recall & Review

beginner

What is a structure in C?

A structure is a user-defined data type in C that groups different variables under one name. Each member has its own memory location.

beginner

What is a union in C?

A union is a user-defined data type where all members share the same memory location. Only one member can hold a value at a time.

intermediate

How does memory allocation differ between structures and unions?

Structures allocate separate memory for each member, so total size is sum of all members (plus possible padding). Unions allocate memory equal to the largest member only.

intermediate

Can you access multiple members of a union at the same time?

No, because all members share the same memory, changing one member overwrites others. Only one member's value is valid at a time.

beginner

When should you use a structure vs a union?

Use a structure when you need to store multiple values simultaneously. Use a union to save memory when only one value is needed at a time.

Which of the following is true about structures in C?

A Each member has its own memory location.

B All members share the same memory location.

C Structures can only have members of the same type.

D Structures cannot contain other structures.

Structures allocate separate memory for each member, allowing multiple members to hold values simultaneously.

What is the size of a union in C?

A Sum of sizes of all members.

B Size of the largest member.

C Size of the smallest member.

D Always 4 bytes.

A union's size equals the size of its largest member because all members share the same memory.

If you assign a value to one member of a union, what happens to other members?

A They keep their previous values.

B They cause a compile error.

C They automatically update to the same value.

D They become invalid or overwritten.

Since all members share memory, assigning one member overwrites the others' data.

Which is a good reason to use a union instead of a structure?

A When you want to save memory and only use one value at a time.

B When you need to store multiple values at once.

C When you want to group unrelated variables.

D When you want to create a new data type with fixed size.

Unions save memory by sharing space among members, useful when only one member is used at a time.

Can a structure contain a union as a member in C?

A No, structures cannot contain unions.

B Only if the union has one member.

C Yes, structures can contain unions.

D Only in C++ but not in C.

Structures can contain unions as members, allowing flexible data grouping.

Explain the main differences between a structure and a union in C.

Think about how memory is used and when you want to store multiple values or just one.

Describe a scenario where using a union is better than a structure.

Imagine you have a variable that can be an int or a float but never both at once.

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