Structure vs union comparison - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a structure named Person.

C

struct Person {
    char name[50];
    int age;
    float height;
};

struct [1] person1;

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

AStruct

BUnion

CData

DPerson

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Union' or 'Struct' as a type name instead of the declared structure name.

Forgetting to use the structure name after 'struct'.

The keyword Person is the name of the structure type declared earlier. We use it to declare a variable of that type.

2fill in blank

medium

Complete the code to declare a union named Data.

C

union [1] {
    int i;
    float f;
    char str[20];
};

union Data data1;

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

APerson

BData

CUnion

DStruct

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Union' or 'Struct' as the union name.

Using the wrong name that does not match the union declaration.

The union is named Data. We use this name to declare variables of this union type.

3fill in blank

hard

Fix the error in accessing the union member to assign a value.

C

union Data data1;
data1.[1] = 10;

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

Ai

Bheight

Cname

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to access members that belong to a structure but not the union.

Using member names that do not exist in the union.

The union Data has a member named i for integer values. We assign 10 to data1.i.

4fill in blank

hard

Fill both blanks to create a structure and a union variable.

C

struct [1] person2;
union [2] data2;

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

APerson

BData

CEmployee

DInfo

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up structure and union names.

Using names not declared earlier.

The structure is named Person and the union is named Data. We use these names to declare variables.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension-like code in C style (simulate with comments) comparing sizes of structure and union.

C

/* Size of struct Person: sizeof(struct [1]) */
/* Size of union Data: sizeof(union [2]) */
/* The size difference is because [3] */

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

APerson

BData

Cunion shares memory among members

Dstruct shares memory among members

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing structure and union names.

Incorrect explanation about memory sharing.

The size of a structure is the sum of all members, so we use Person. The union size is the size of its largest member, so Data. The union shares memory among members, which is why its size is smaller.

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