Accessing structure members - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a structure in C?

A structure in C is a user-defined data type that groups different variables under one name, allowing you to store related data together.

beginner

How do you access a member of a structure variable in C?

You use the dot operator (.) to access a member of a structure variable. For example, if person is a structure variable and age is a member, you write person.age.

beginner

How do you access a member of a structure through a pointer in C?

You use the arrow operator (->) to access a member of a structure through a pointer. For example, if p is a pointer to a structure and name is a member, you write p->name.

intermediate

What is the difference between the dot (.) and arrow (->) operators in C structures?

The dot operator (.) is used to access members of a structure variable directly, while the arrow operator (->) is used to access members through a pointer to a structure.

beginner

Given struct Point { int x; int y; } p;, how do you set the x coordinate to 10?

You write p.x = 10; to set the x coordinate of the structure variable p to 10.

Which operator is used to access a member of a structure variable in C?

A . (dot)

B -> (arrow)

C * (asterisk)

D & (ampersand)

The dot operator (.) is used to access members of a structure variable directly.

How do you access a structure member through a pointer in C?

A Using the asterisk (*)

B Using the dot operator (.)

C Using the arrow operator (->)

D Using the ampersand (&)

The arrow operator (->) is used to access members through a pointer to a structure.

Given struct Person { int age; } *ptr;, how do you access the age member?

A ptr.age

B &ptr.age

C *ptr.age

D ptr->age

You use ptr->age to access the age member through the pointer.

What will happen if you try to use the dot operator on a pointer to a structure?

A It works fine

B It causes a compilation error

C It accesses the member correctly

D It dereferences the pointer automatically

Using the dot operator on a pointer to a structure causes a compilation error because the pointer must be dereferenced first or use the arrow operator.

If p is a structure variable, which of these is correct to assign 5 to member val?

A p.val = 5;

B *p.val = 5;

C p->val = 5;

D &p.val = 5;

You use the dot operator to assign a value to a member of a structure variable.

Explain how to access members of a structure variable and a pointer to a structure in C.

Think about how you access properties of an object directly versus through a pointer.

Describe the difference between the dot (.) and arrow (->) operators when working with structures in C.

Consider what happens when you have a pointer versus a normal variable.

Practice

1. Which operator is used to access members of a structure variable in C?

easy

A. Colon operator (:)

B. Arrow operator (->)

C. Double colon (::)

D. Dot operator (.)

Solution

1. Step 1: Understand structure member access

   In C, when you have a structure variable, you use the dot operator (.) to access its members.

2. Step 2: Differentiate from pointer access

   The arrow operator (->) is used when you have a pointer to a structure, not the structure variable itself.

3. Final Answer:

   Dot operator (.) -> Option D

4. Quick Check:

   Structure variable member access = dot operator (.) [OK]

Hint: Use dot (.) for struct variables, arrow (->) for pointers [OK]

Common Mistakes:

• Confusing dot (.) with arrow (->) operator
• Using colon (:) which is invalid in C
• Using double colon (::) which is from other languages

2. Given the structure struct Point { int x; int y; } p;, which is the correct way to assign 10 to member x?

easy

A. p->x = 10;

B. p:x = 10;

C. p.x = 10;

D. p[x] = 10;

Solution

1. Step 1: Identify the variable type

   Here, p is a structure variable, not a pointer.

2. Step 2: Use correct member access syntax

   For structure variables, use the dot operator (.) to access members, so p.x = 10; is correct.

3. Final Answer:

   p.x = 10; -> Option C

4. Quick Check:

   Structure variable assignment uses dot (.) [OK]

Hint: Use dot (.) for struct variables, arrow (->) only for pointers [OK]

Common Mistakes:

• Using arrow (->) with non-pointer variable
• Using colon (:) which is invalid syntax
• Using array-like syntax on struct members

3. What is the output of this code?

struct Person {
  char name[10];
  int age;
};

int main() {
  struct Person p = {"Amy", 25};
  printf("%s %d", p.name, p.age);
  return 0;
}

medium

A. Amy 25

B. Amy 0

C. Error: cannot assign string to array

D. Random garbage values

Solution

1. Step 1: Understand structure initialization

   The structure p is initialized with "Amy" for name and 25 for age.

2. Step 2: Print structure members

   The printf prints p.name as a string and p.age as an integer, so output is Amy 25.

3. Final Answer:

   Amy 25 -> Option A

4. Quick Check:

   Struct initialized correctly, prints members [OK]

Hint: Initialize structs with braces; access members with dot [OK]

Common Mistakes:

• Thinking string assignment to array is invalid here
• Confusing pointer and array initialization
• Expecting syntax error for string in struct

4. Identify the error in this code snippet:

struct Data {
  int id;
  float value;
};

int main() {
  struct Data d;
  d->id = 5;
  d.value = 3.14;
  return 0;
}

medium

A. Using arrow operator (->) with structure variable

B. Missing semicolon after struct definition

C. Incorrect float assignment

D. No error, code is correct

Solution

1. Step 1: Check member access operators

   The variable d is a structure variable, not a pointer, so d->id is incorrect.

2. Step 2: Correct operator usage

   Use dot operator (.) for structure variables: d.id = 5; is correct.

3. Final Answer:

   Using arrow operator (->) with structure variable -> Option A

4. Quick Check:

   Arrow (->) only for pointers, dot (.) for variables [OK]

Hint: Use dot (.) for struct variables, arrow (->) only for pointers [OK]

Common Mistakes:

• Using arrow (->) on non-pointer struct variable
• Ignoring dot operator for member access
• Assuming no error when mixing operators

5. Consider this code:

struct Employee {
  char name[20];
  int id;
};

int main() {
  struct Employee emp1 = {"John", 101};
  struct Employee *ptr = &emp1;
  ptr->id = 202;
  printf("%s %d", emp1.name, emp1.id);
  return 0;
}

What will be the output?

hard

A. John 101

B. John 202

C. Error: cannot use arrow operator here

D. John 0

Solution

1. Step 1: Understand pointer to structure

   ptr points to emp1, so ptr->id changes emp1.id.

2. Step 2: Update and print values

   After ptr->id = 202;, emp1.id becomes 202. Printing emp1.name and emp1.id outputs John 202.

3. Final Answer:

   John 202 -> Option B

4. Quick Check:

   Pointer access updates struct member correctly [OK]

Hint: Use arrow (->) with pointers to structs to access members [OK]

Common Mistakes:

• Thinking arrow operator causes error with pointers
• Not realizing pointer changes original struct
• Confusing dot and arrow operators with pointers