Structure vs union comparison - Visual Side-by-Side Comparison

Concept Flow - Structure vs union comparison

Declare struct with multiple members

↓

Memory allocated = sum of all members' sizes

↓

Each member has its own memory space

↓

Declare union with multiple members

↓

Memory allocated = size of largest member

↓

All members share the same memory space

↓

Assign value to one union member

↓

Reading other union members shows overlapping data

Structures allocate separate memory for each member, unions share the same memory for all members.

Execution Sample

C

struct Example {
  int a;
  float b;
};

union ExampleU {
  int a;
  float b;
};

Defines a struct with two members and a union with the same two members.

Execution Table

Step	Action	Memory Allocation	Member Values	Notes
1	Declare struct Example	Size = sizeof(int) + sizeof(float)	a = uninitialized, b = uninitialized	Struct allocates separate memory for each member
2	Assign struct.a = 10	Same as above	a = 10, b = uninitialized	Value stored in struct.a's memory
3	Assign struct.b = 3.14	Same as above	a = 10, b = 3.14	Value stored in struct.b's memory
4	Declare union ExampleU	Size = max(sizeof(int), sizeof(float))	a = uninitialized, b = uninitialized	Union allocates shared memory for all members
5	Assign union.a = 10	Same as above	a = 10, b = overlapping bits of 10	Value stored in shared memory
6	Read union.b	Same as above	a = 10, b = value from same memory bits as a	Reading b shows overlapping data, may be garbage or interpreted bits
7	Assign union.b = 3.14	Same as above	a = bits of 3.14, b = 3.14	Value stored in shared memory, overwriting a
8	Read union.a	Same as above	a = value from bits of 3.14, b = 3.14	Reading a shows overlapping data, may be garbage or interpreted bits
9	End	-	-	Execution ends

💡 Program ends after demonstrating memory differences between struct and union

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	After Step 6	After Step 7	After Step 8	Final
struct.a	uninitialized	10	10	-	-	-	-	10
struct.b	uninitialized	uninitialized	3.14	-	-	-	-	3.14
union.a	uninitialized	-	-	10	10	bits of 3.14	bits of 3.14	bits of 3.14
union.b	uninitialized	-	-	overlapping bits of 10	overlapping bits of 10	3.14	3.14	3.14

Key Moments - 3 Insights

Why does assigning to union.a affect union.b's value?

Because union members share the same memory, changing one member overwrites the shared space, affecting all members (see steps 5 and 6 in execution_table).

Why can struct.a and struct.b hold different values at the same time?

Struct allocates separate memory for each member, so each can store independent values simultaneously (see steps 2 and 3 in execution_table).

What happens when you read a union member different from the last assigned one?

You read the same memory bits interpreted as that member's type, which may produce unexpected or garbage values (see steps 6 and 8 in execution_table).

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what are the values of struct.a and struct.b?

A a = 10, b = 3.14

B a = 3.14, b = 10

C a = uninitialized, b = 3.14

D a = 10, b = uninitialized

💡 Hint

Check the 'Member Values' column at step 3 in execution_table

At which step does union.b first get a meaningful assigned value?

A Step 5

B Step 7

C Step 6

D Step 8

💡 Hint

Look for when union.b is explicitly assigned a value in execution_table

If you assign union.b = 5.5 at step 7 instead of 3.14, what happens to union.a at step 8?

A union.a holds 5.5 as int

B union.a remains unchanged

C union.a holds bits of 5.5 interpreted as int

D union.a becomes zero

💡 Hint

Recall union members share memory; see variable_tracker for union.a after step 7

Concept Snapshot

Structure vs Union in C:
- struct allocates separate memory for each member.
- union allocates shared memory equal to largest member.
- struct members hold independent values simultaneously.
- union members share memory; last write affects all.
- Reading different union member than last written may show unexpected data.

Full Transcript

This visual execution compares structures and unions in C. Structures allocate separate memory for each member, so each can hold independent values at the same time. Unions allocate shared memory equal to the largest member, so all members share the same space. Assigning a value to one union member overwrites the shared memory, affecting all members. Reading a union member different from the last assigned one may produce unexpected values because the bits are interpreted differently. The execution table shows step-by-step how values are assigned and how memory is allocated differently for struct and union. The variable tracker follows the changes in member values after each step. Key moments clarify common confusions about memory sharing and value overwriting in unions versus independent storage in structures.

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