Structure vs union comparison

Introduction

Structures and unions help group different data types together. They let you store related information in one place.

When you want to store multiple related values together, like a person's name, age, and height.

When you need to save memory by sharing the same space for different data types, but only one at a time.

When you want to organize data clearly for easier reading and maintenance.

When you want to pass multiple values as one unit to a function.

When you want to handle different types of data in the same memory location depending on the situation.

Syntax

C

struct StructureName {
    dataType1 member1;
    dataType2 member2;
    ...
};

union UnionName {
    dataType1 member1;
    dataType2 member2;
    ...
};

A structure stores all members separately, each with its own memory.

A union shares the same memory space for all members, so only one member can hold a value at a time.

Examples

This structure stores a person's name, age, and height all at once.

C

struct Person {
    char name[20];
    int age;
    float height;
};

This union can hold an int, a float, or a string, but only one at a time.

C

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

Sample Program

This program shows how a structure keeps all values separately, while a union shares memory for all members. Changing one union member overwrites the others.

C

#include <stdio.h>
#include <string.h>

struct Person {
    char name[20];
    int age;
    float height;
};

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

int main() {
    struct Person p1;
    strcpy(p1.name, "Alice");
    p1.age = 30;
    p1.height = 5.5f;

    printf("Structure Person:\n");
    printf("Name: %s\n", p1.name);
    printf("Age: %d\n", p1.age);
    printf("Height: %.1f\n", p1.height);

    union Data d;
    d.i = 10;
    printf("\nUnion Data after setting int: %d\n", d.i);
    d.f = 220.5f;
    printf("Union Data after setting float: %.1f\n", d.f);
    strcpy(d.str, "Hello");
    printf("Union Data after setting string: %s\n", d.str);

    printf("\nNote: After setting string, int and float values are overwritten.\n");

    return 0;
}

Important Notes

Structures use more memory because they store all members separately.

Unions save memory by sharing space but only one member can be used at a time.

Accessing a union member that was not last set can give unexpected results.

Summary

Structures store all members separately and keep all data at once.

Unions share the same memory for all members, so only one value is stored at a time.

Use structures when you need to keep all data together, and unions when you want to save memory and only use one value at a time.

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