Positional patterns in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to match a point with coordinates (0, 0).

C Sharp (C#)

if (point is ([1], 0))
{
    Console.WriteLine("Point is on the Y axis.");
}

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

A1

B0

Cx

Dy

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name instead of a literal 0.

Using 1 instead of 0.

The pattern (0, 0) matches a point at the origin. Here, the first coordinate must be 0.

2fill in blank

medium

Complete the code to extract the X coordinate from a point using positional patterns.

C Sharp (C#)

if (point is ([1], int y))
{
    Console.WriteLine($"X coordinate is {x}");
}

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

Ax

Bvar x

C0

Dint x

Attempts:

3 left

💡 Hint

Common Mistakes

Using just 'x' without type.

Using 'var x' which is invalid in positional patterns.

To extract the X coordinate, declare it as int x in the pattern.

3fill in blank

hard

Fix the error in the positional pattern to correctly match a point with X > 0.

C Sharp (C#)

if (point is (int x, int y) && x [1] 0)
{
    Console.WriteLine("X is positive.");
}

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

A>

B<=

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' which checks for negative values.

Using '==' which checks for equality.

The condition x > 0 checks if X is positive.

4fill in blank

hard

Fill both blanks to match a point where X equals Y.

C Sharp (C#)

if (point is ([1] x, [2] y) && x == y)
{
    Console.WriteLine("X equals Y.");
}

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

Aint

Bvar

Cdouble

Dfloat

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing types like int and double.

Using 'var' which is not allowed in positional patterns.

Both X and Y should be declared as int to match integer coordinates.

5fill in blank

hard

Fill all three blanks to create a method that uses positional patterns to check if a point is at the origin.

C Sharp (C#)

bool IsOrigin((int x, int y) point) => point is ([1], [2]);

// Usage
var p = (0, 0);
Console.WriteLine(IsOrigin(p)); // [3]

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

A0

Btrue

Cfalse

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-zero values in the pattern.

Returning false instead of true for the origin.

The pattern (0, 0) matches the origin. The method returns true for point (0,0).

Practice

1.

What is the main purpose of positional patterns in C#?

record Point(int X, int Y);
Using case Point(var x, var y): inside a switch statement helps to:

easy

A. Extract values of X and Y from a Point object easily

B. Create a new Point object with given X and Y

C. Change the values of X and Y in the Point object

D. Call a method named Deconstruct on the Point object

Solution

1. Step 1: Understand positional pattern usage

   Positional patterns allow matching an object and extracting its parts in one step, like case Point(var x, var y):.

2. Step 2: Identify what the pattern does with Point

   It extracts the X and Y values from the Point object into variables x and y for use inside the case block.

3. Final Answer:

   Extract values of X and Y from a Point object easily -> Option A

4. Quick Check:

   Positional pattern extracts values [OK]

Hint: Positional patterns extract parts of an object [OK]

Common Mistakes:

• Thinking positional patterns create or modify objects
• Confusing Deconstruct method call with pattern matching
• Assuming positional patterns only check types without extracting

2.

Which of the following is the correct syntax for using a positional pattern with a record Person(string Name, int Age) inside a switch expression?

switch(person) {
    case ???:
        // code
        break;
}

easy

A. Person(var name, var age)

B. Person[name, age]

C. Person{name, age}

D. Person(name: name, age: age)

Solution

1. Step 1: Recall positional pattern syntax

   Positional patterns use parentheses with variables: Type(var1, var2).

2. Step 2: Match syntax to options

   Person(var name, var age) uses parentheses and variables correctly: Person(var name, var age). Others use invalid syntax for positional patterns.

3. Final Answer:

   Person(var name, var age) -> Option A

4. Quick Check:

   Positional pattern syntax uses parentheses [OK]

Hint: Positional patterns always use parentheses, not brackets or braces [OK]

Common Mistakes:

• Using square brackets or curly braces instead of parentheses
• Trying to use named arguments syntax inside patterns
• Omitting var keyword or variable names

3.

What is the output of the following code?

record Point(int X, int Y);

var p = new Point(3, 4);
switch (p)
{
    case Point(0, 0):
        Console.WriteLine("Origin");
        break;
    case Point(var x, var y) when x == y:
        Console.WriteLine("On diagonal");
        break;
    case Point(var x, var y):
        Console.WriteLine($"Point at ({x}, {y})");
        break;
}

medium

A. Origin

B. Point at (3, 4)

C. On diagonal

D. No output

Solution

1. Step 1: Check first case Point(0, 0)

   p is (3,4), so it does not match (0,0).

2. Step 2: Check second case with condition x == y

   x=3, y=4, so x != y; condition fails.

3. Step 3: Third case matches any Point and prints coordinates

   It matches and prints "Point at (3, 4)".

4. Final Answer:

   Point at (3, 4) -> Option B

5. Quick Check:

   Pattern matches last case with output [OK]

Hint: Check cases top to bottom; first match runs [OK]

Common Mistakes:

• Assuming (3,4) matches (0,0)
• Ignoring the when condition in second case
• Thinking no case matches and no output

4.

Identify the error in this code snippet using positional patterns:

record Rectangle(int Width, int Height);

var rect = new Rectangle(5, 10);
switch(rect)
{
    case Rectangle(var w, var h):
        Console.WriteLine($"Width: {w}, Height: {h}");
        break;
    case Rectangle(w, h):
        Console.WriteLine("Invalid pattern");
        break;
}

medium

A. Switch expression cannot use records

B. First case should use named arguments instead of positional pattern

C. Second case uses variables without var keyword, causing a syntax error

D. No error, code runs fine

Solution

1. Step 1: Review positional pattern variable declaration

   Variables in positional patterns must be declared with var or a type.

2. Step 2: Identify error in second case

   Second case uses Rectangle(w, h) without var, which is invalid syntax.

3. Final Answer:

   Second case uses variables without var keyword, causing a syntax error -> Option C

4. Quick Check:

   Variables need var keyword in patterns [OK]

Hint: Always declare variables with var in positional patterns [OK]

Common Mistakes:

• Omitting var keyword for variables in patterns
• Thinking records can't be used in switch
• Confusing named arguments with pattern variables

5.

Given the record Person(string Name, int Age), write a switch expression using positional patterns that returns:

• "Child" if Age < 13
• "Teen" if Age is between 13 and 19 inclusive
• "Adult" otherwise

Which of the following switch expressions correctly implements this?

hard

A. person switch {
Person(_, var age) when age < 13 => "Child",
Person(_, var age) when age > 13 && age <= 19 => "Teen",
Person(_, _) => "Adult"
};

B. person switch {
Person(var name, var age) when age < 13 => "Child",
Person(var name, var age) when age > 13 && age < 19 => "Teen",
_ => "Adult"
};

C. person switch {
Person(var name, var age) when age <= 13 => "Child",
Person(var name, var age) when age >= 13 && age <= 19 => "Teen",
Person(var name, var age) => "Adult"
};

D. person switch {
Person(var name, var age) when age < 13 => "Child",
Person(var name, var age) when age >= 13 && age <= 19 => "Teen",
Person(var name, var age) => "Adult"
};

Solution

1. Step 1: Check age ranges and conditions

   Child: age < 13, Teen: 13 ≤ age ≤ 19, Adult: otherwise.

2. Step 2: Analyze each option's conditions

   person switch { Person(var name, var age) when age < 13 => "Child", Person(var name, var age) when age >= 13 && age <= 19 => "Teen", Person(var name, var age) => "Adult" }; correctly uses age < 13 for Child, age >= 13 && age <= 19 for Teen, and default Adult case. person switch { Person(_, var age) when age < 13 => "Child", Person(_, var age) when age > 13 && age <= 19 => "Teen", Person(_, _) => "Adult" }; misses age=13 (falls to Adult) because Teen condition uses > 13. person switch { Person(var name, var age) when age < 13 => "Child", Person(var name, var age) when age > 13 && age < 19 => "Teen", _ => "Adult" }; excludes age == 13 and 19 for Teen. person switch { Person(var name, var age) when age <= 13 => "Child", Person(var name, var age) when age >= 13 && age <= 19 => "Teen", Person(var name, var age) => "Adult" }; overlaps Child and Teen at age 13.

3. Step 3: Confirm the correct implementation

   person switch { Person(var name, var age) when age < 13 => "Child", Person(var name, var age) when age >= 13 && age <= 19 => "Teen", Person(var name, var age) => "Adult" }; matches all conditions correctly and clearly.

4. Final Answer:

   Option D code correctly implements the age groups -> Option D

5. Quick Check:

   Check age ranges carefully in when clauses [OK]

Hint: Check boundary conditions carefully in when clauses [OK]

Common Mistakes:

• Overlapping or missing age boundaries
• Using _ wildcard instead of variables when needed
• Incorrect logical operators in conditions