Methods that operate on state in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a method that changes the value of the field.

C Sharp (C#)

class Counter {
    private int count = 0;
    public void [1]() {
        count++;
    }
}

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

AIncrement

BGetCount

CReset

DDisplay

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name that does not describe changing the state, like GetCount.

The method name Increment clearly shows it increases the count field by one.

2fill in blank

medium

Complete the method to reset the count field to zero.

C Sharp (C#)

class Counter {
    private int count = 0;
    public void Reset() {
        count [1] 0;
    }
}

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

A=

B==

C+=

D++

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '=' causes a syntax error.

Using '+=' or '++' changes the value incorrectly.

The assignment operator = sets the count field to zero.

3fill in blank

hard

Fix the error in the method that returns the current count.

C Sharp (C#)

class Counter {
    private int count = 0;
    public int GetCount() {
        return [1];
    }
}

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

ACount

Bthis.count()

CgetCount

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase 'Count' which is undefined.

Trying to call count as a method with parentheses.

The field name is count with lowercase 'c'. It must be returned exactly as declared.

4fill in blank

hard

Fill both blanks to create a method that adds a given number to the count.

C Sharp (C#)

class Counter {
    private int count = 0;
    public void Add([1] number) {
        count [2] number;
    }
}

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

Aint

B+=

C-=

Dfloat

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' operator instead of '+='.

Using a float parameter when count is int.

The method parameter must be an int to match the count type, and += adds the number to count.

5fill in blank

hard

Fill all three blanks to create a method that multiplies count by a factor and returns the result.

C Sharp (C#)

class Counter {
    private int count = 1;
    public int MultiplyBy([1] factor) {
        count [2] factor;
        return [3];
    }
}

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

Aint

B*=

Ccount

Ddouble

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the factor instead of count.

Using double type when count is int.

The parameter type is int, *= multiplies and assigns, and the method returns the updated count.

Practice

1. What is the main purpose of methods that operate on state in a C# class?

easy

A. To perform calculations without changing any data

B. To allow objects to keep and change their own data safely

C. To handle user input from the console

D. To create new classes from existing ones

Solution

1. Step 1: Understand what 'state' means in programming

   State refers to the data stored inside an object that can change over time.

2. Step 2: Identify the role of methods operating on state

   These methods allow the object to update or read its own data safely, keeping control inside the object.

3. Final Answer:

   To allow objects to keep and change their own data safely -> Option B

4. Quick Check:

   Methods on state = safe data change inside object [OK]

Hint: Think: methods change or read object's own data [OK]

Common Mistakes:

• Confusing methods on state with inheritance
• Thinking methods only perform calculations
• Believing methods handle external input only

2. Which of the following is the correct syntax for a method that changes an object's state in C#?

easy

A. public void UpdateName(string newName) { name = newName; }

B. void UpdateName(string newName) name = newName;

C. public UpdateName(string newName) { name = newName; }

D. public void UpdateName(string newName) => return name = newName;

Solution

1. Step 1: Check method declaration syntax

   In C#, methods must specify access modifier, return type, name, and parameters inside parentheses, with body in braces.

2. Step 2: Verify the method body updates the state correctly

   public void UpdateName(string newName) { name = newName; } correctly assigns newName to the field name inside braces.

3. Final Answer:

   public void UpdateName(string newName) { name = newName; } -> Option A

4. Quick Check:

   Correct method syntax = public void UpdateName(string newName) { name = newName; } [OK]

Hint: Remember method syntax: access + return type + name(params) { body } [OK]

Common Mistakes:

• Missing braces around method body
• Omitting return type
• Using return with void methods incorrectly

3. What will be the output of this C# code?

class Counter {
  private int count = 0;
  public void Increment() { count++; }
  public int GetCount() { return count; }
}

var c = new Counter();
c.Increment();
c.Increment();
Console.WriteLine(c.GetCount());

medium

A. 3

B. 1

C. 0

D. 2

Solution

1. Step 1: Trace the Increment method calls

   Each call to Increment increases count by 1. Two calls increase count from 0 to 2.

2. Step 2: Check the GetCount method output

   GetCount returns the current count, which is 2 after two increments.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   2 increments = count 2 [OK]

Hint: Count increments twice, so output is 2 [OK]

Common Mistakes:

• Forgetting that count starts at 0
• Assuming Increment adds more than 1
• Confusing method names or outputs

4. Identify the error in this method that tries to update an object's state:

public void SetAge(int age) {
  int age = age;
}

medium

A. The method redeclares 'age' variable causing a conflict

B. The method is missing a return statement

C. The method should be static to update state

D. The method should not have parameters

Solution

1. Step 1: Analyze variable declarations inside the method

   The method declares a new local variable 'int age', which conflicts with the parameter 'age'.

2. Step 2: Understand how to update the object's field

   To update the object's state, assign the parameter to the field, e.g., this.age = age; without redeclaring.

3. Final Answer:

   The method redeclares 'age' variable causing a conflict -> Option A

4. Quick Check:

   Variable redeclaration error = The method redeclares 'age' variable causing a conflict [OK]

Hint: Don't redeclare parameter names inside method [OK]

Common Mistakes:

• Thinking missing return causes error in void method
• Assuming static needed to update instance state
• Believing parameters should be removed

5. You have a class BankAccount with a private field balance. You want to add a method Withdraw that subtracts an amount only if there is enough balance. Which method implementation correctly operates on the state safely?

hard

A. public decimal Withdraw(decimal amount) { return balance - amount; }

B. public void Withdraw(decimal amount) { balance -= amount; }

C. public void Withdraw(decimal amount) { if (amount <= balance) balance -= amount; else Console.WriteLine("Insufficient funds"); }

D. public void Withdraw(decimal amount) { if (amount < 0) balance += amount; }

Solution

1. Step 1: Check for safe state update conditions

   Method should only subtract amount if balance is enough to avoid negative balance.

2. Step 2: Verify method behavior on insufficient funds

   public void Withdraw(decimal amount) { if (amount <= balance) balance -= amount; else Console.WriteLine("Insufficient funds"); } checks amount <= balance and prints a message if not enough, preventing invalid state.

3. Final Answer:

   public void Withdraw(decimal amount) { if (amount <= balance) balance -= amount; else Console.WriteLine("Insufficient funds"); } -> Option C

4. Quick Check:

   Safe state update with condition = public void Withdraw(decimal amount) { if (amount <= balance) balance -= amount; else Console.WriteLine("Insufficient funds"); } [OK]

Hint: Check balance before subtracting to avoid negative state [OK]

Common Mistakes:

• Subtracting without checking balance
• Returning new value without updating state
• Adding amount when negative instead of subtracting