Predicate delegate type in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a Predicate delegate that checks if a number is positive.

C Sharp (C#)

Predicate<int> isPositive = [1];

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

Ax => x > 0

Bx => x == 0

Cx => x < 0

Dx => x != 0

Attempts:

3 left

💡 Hint

Common Mistakes

Using a lambda that checks for zero or negative numbers instead of positive.

Not returning a boolean value.

The Predicate delegate requires a method or lambda that returns true or false. Here, x => x > 0 correctly checks if the number is positive.

2fill in blank

medium

Complete the code to use the Predicate delegate to find if any number in the array is even.

C Sharp (C#)

int[] numbers = {1, 3, 5, 6};
bool hasEven = Array.Exists(numbers, [1]);

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

Ax => x % 2 == 1

Bx => x % 2 == 0

Cx => x > 5

Dx => x < 0

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for odd numbers instead of even.

Using conditions unrelated to evenness.

The Array.Exists method uses a Predicate to check if any element matches the condition. x => x % 2 == 0 checks for even numbers.

3fill in blank

hard

Fix the error in the Predicate delegate that checks if a string is empty.

C Sharp (C#)

Predicate<string> isEmpty = s => s.[1] == 0;

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

AIsEmpty

BEquals

CEmpty

DLength

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-existent method IsEmpty.

Trying to call Length as a method instead of a property.

Strings do not have an IsEmpty() method. Instead, checking s.Length == 0 tells if the string is empty. Here, the lambda uses s.Length property.

4fill in blank

hard

Fill both blanks to create a Predicate that checks if a number is between 10 and 20 (inclusive).

C Sharp (C#)

Predicate<int> isBetween = x => x [1] 10 && x [2] 20;

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

A>=

B<

C<=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong comparison operators like < or > only.

Mixing up the order of operators.

The predicate checks if x is greater than or equal to 10 and less than or equal to 20. So the correct operators are >= and <=.

5fill in blank

hard

Fill all three blanks to create a Predicate that checks if a string starts with 'A' and has length greater than 3.

C Sharp (C#)

Predicate<string> check = s => s.[1]("A") && s.[2] [3] 3;

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

AStartsWith

BLength

C>

DContains

Attempts:

3 left

💡 Hint

Common Mistakes

Using Contains instead of StartsWith.

Using incorrect comparison operators for length.

The predicate uses StartsWith("A") to check the first letter, then checks if Length > 3 to ensure the string is longer than 3 characters.

Practice

1.

What does a Predicate<T> delegate represent in C#?

easy

A. A method that takes a parameter of type T and returns a boolean indicating a condition.

B. A method that takes no parameters and returns void.

C. A method that takes two parameters and returns an integer.

D. A method that returns a string representation of an object.

Solution

1. Step 1: Understand the Predicate delegate signature

   A Predicate delegate always takes one parameter of type T and returns a boolean value.

2. Step 2: Match the description to the options

   A method that takes a parameter of type T and returns a boolean indicating a condition correctly describes this signature as a method taking T and returning bool.

3. Final Answer:

   A method that takes a parameter of type T and returns a boolean indicating a condition. -> Option A

4. Quick Check:

   Predicate<T> = method(T) returns bool [OK]

Hint: Predicate always returns true or false for a given input [OK]

Common Mistakes:

• Confusing Predicate with Action or Func delegates
• Thinking Predicate returns void or other types
• Assuming Predicate takes multiple parameters

2.

Which of the following is the correct syntax to declare a Predicate delegate for integers?

?

easy

A. Predicate<int> isPositive = x => x > 0;

B. Predicate<int> isPositive = (int x) => { return x; };

C. Predicate<int> isPositive = x => x + 1;

D. Predicate<int> isPositive = () => true;

Solution

1. Step 1: Check the lambda expression return type

   Predicate<int> expects a method returning bool. Predicate<int> isPositive = x => x > 0; returns x > 0, which is bool.

2. Step 2: Verify parameter and return correctness

   Predicate<int> isPositive = (int x) => { return x; }; returns x (int), not bool. Predicate<int> isPositive = x => x + 1; returns x + 1 (int), not bool. Predicate<int> isPositive = () => true; has no parameter but Predicate requires one.

3. Final Answer:

   Predicate<int> isPositive = x => x > 0; -> Option A

4. Quick Check:

   Predicate<int> = int input => bool output [OK]

Hint: Predicate must return bool, not int or void [OK]

Common Mistakes:

• Returning int instead of bool
• Missing parameter in lambda
• Using incorrect lambda syntax

3.

What will be the output of the following C# code?

using System;
class Program {
    static void Main() {
        Predicate<string> isLong = s => s.Length > 5;
        string[] words = {"apple", "banana", "pear"};
        int count = 0;
        foreach (var word in words) {
            if (isLong(word)) count++;
        }
        Console.WriteLine(count);
    }
}

medium

A. 3

B. 2

C. 1

D. 0

Solution

1. Step 1: Understand the Predicate condition

   The predicate checks if the string length is greater than 5.

2. Step 2: Count words with length > 5

   "apple" length 5 (false), "banana" length 6 (true), "pear" length 4 (false). Only "banana" passes.

3. Final Answer:

   2 -> Option B

4. Quick Check:

   Words with length > 5 are "banana" only, count = 1 [Incorrect]

Hint: Count items where predicate returns true [OK]

Common Mistakes:

• Counting words with length equal to 5
• Misreading predicate condition
• Assuming all words pass the test

4.

Identify the error in this code using a Predicate delegate:

Predicate<int> isEven = n => n % 2 == 0;
int[] numbers = {1, 2, 3, 4};
int evenCount = 0;
foreach (int num in numbers) {
    if (isEven) evenCount++;
}
Console.WriteLine(evenCount);

medium

A. The Predicate delegate is not assigned correctly.

B. The foreach loop variable should be declared as var.

C. The if condition should call the delegate with an argument.

D. The Console.WriteLine statement is missing parentheses.

Solution

1. Step 1: Check how Predicate is used in if condition

   The code uses if (isEven) which checks the delegate itself, not its result.

2. Step 2: Correct usage requires calling the delegate with an argument

   It should be if (isEven(num)) to evaluate the condition for each number.

3. Final Answer:

   The if condition should call the delegate with an argument. -> Option C

4. Quick Check:

   Call Predicate with input: isEven(num) [OK]

Hint: Always call Predicate with an argument inside if [OK]

Common Mistakes:

• Using delegate name without parentheses
• Confusing delegate variable with method call
• Ignoring parameter requirement

5.

You want to filter a list of integers to keep only positive even numbers using a Predicate delegate. Which of the following implementations correctly combines these conditions?

hard

A. Predicate<int> isPositiveEven = x => x % 2 == 0;

B. Predicate<int> isPositiveEven = x => x > 0 || x % 2 == 0;

C. Predicate<int> isPositiveEven = x => x < 0 && x % 2 == 0;

D. Predicate<int> isPositiveEven = x => x > 0 && x % 2 == 0;

Solution

1. Step 1: Understand the filtering requirement

   We want numbers that are both positive and even.

2. Step 2: Analyze each option's condition

   Predicate<int> isPositiveEven = x => x > 0 && x % 2 == 0; uses AND (&&) to require both conditions true. Predicate<int> isPositiveEven = x => x > 0 || x % 2 == 0; uses OR (||), allowing positives or evens, which is incorrect. Predicate<int> isPositiveEven = x => x < 0 && x % 2 == 0; checks for negative and even, wrong sign. Predicate<int> isPositiveEven = x => x % 2 == 0; checks only even, missing positivity.

3. Final Answer:

   Predicate<int> isPositiveEven = x => x > 0 && x % 2 == 0; -> Option D

4. Quick Check:

   Use AND to combine conditions for filtering [OK]

Hint: Use && to combine multiple conditions in Predicate [OK]

Common Mistakes:

• Using || instead of && for combined conditions
• Checking wrong sign for positivity
• Ignoring one of the required conditions