Check if Number is Prime in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if a number is less than 2.

DSA Typescript

function isPrime(num: number): boolean {
  if (num [1] 2) {
    return false;
  }
  return true;
}

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

A<

B>

C===

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' causes wrong logic.

Using '===' checks equality, not less than.

The function returns false if the number is less than 2 because numbers less than 2 are not prime.

2fill in blank

medium

Complete the for loop to check divisors from 2 up to the square root of the number.

DSA Typescript

for (let i = 2; i [1] Math.sqrt(num); i++) {
  if (num % i === 0) {
    return false;
  }
}

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

A<

B>=

C>

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>=' or '>' causes the loop to not run correctly.

Using '<' misses checking the square root for perfect squares.

The loop should run while i is less than or equal to the square root of num to check all possible divisors.

3fill in blank

hard

Fix the error in the modulus check to correctly detect divisibility.

DSA Typescript

if (num [1] i === 0) {
  return false;
}

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

A/

B%

C*

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '/' returns quotient, not remainder.

Using '*' or '-' are arithmetic operations unrelated to divisibility.

The modulus operator '%' returns the remainder. If remainder is 0, num is divisible by i.

4fill in blank

hard

Fill both blanks to complete the function that returns true if no divisors are found.

DSA Typescript

function isPrime(num: number): boolean {
  if (num [1] 2) {
    return false;
  }
  for (let i = 2; i [2] Math.sqrt(num); i++) {
    if (num % i === 0) {
      return false;
    }
  }
  return true;
}

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

A<

B<=

C>

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' in first blank causes wrong logic.

Using '<' instead of '<=' in second blank misses checking the square root.

The first blank checks if num is less than 2 (not prime). The second blank uses '<=' to include the square root in the loop.

5fill in blank

hard

Fill all three blanks to create a complete prime checking function with proper loop and condition.

DSA Typescript

function isPrime(num: number): boolean {
  if (num [1] 2) {
    return false;
  }
  for (let i = 2; i [2] Math.sqrt(num); i++) {
    if (num [3] i === 0) {
      return false;
    }
  }
  return true;
}

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

A<

B<=

C%

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' in first blank.

Using '<' instead of '<=' in second blank.

Using '/' instead of '%' in third blank.

The function returns false if num is less than 2. The loop runs while i is less than or equal to the square root of num. The modulus operator '%' checks divisibility.

Practice

1. What does it mean for a number to be prime?

easy

A. It is divisible by 2 only.

B. It has more than two divisors.

C. It is an even number greater than 2.

D. It has exactly two distinct positive divisors: 1 and itself.

Solution

1. Step 1: Understand the definition of prime number

   A prime number is a number greater than 1 that has no divisors other than 1 and itself.

2. Step 2: Compare options with the definition

   Only It has exactly two distinct positive divisors: 1 and itself. correctly states this definition.

3. Final Answer:

   It has exactly two distinct positive divisors: 1 and itself. -> Option D

4. Quick Check:

   Prime number = two divisors [OK]

Hint: Prime means only divisible by 1 and itself [OK]

Common Mistakes:

• Thinking even numbers are always prime
• Confusing prime with odd numbers
• Believing prime numbers have multiple divisors

2. Which of the following TypeScript function signatures correctly checks if a number is prime?

easy

A. function isPrime(n: boolean): string

B. function isPrime(n: string): number

C. function isPrime(n: number): boolean

D. function isPrime(): void

Solution

1. Step 1: Identify correct parameter and return types

   The function should take a number and return a boolean indicating prime or not.

2. Step 2: Match options with correct signature

   function isPrime(n: number): boolean matches: input number, output boolean.

3. Final Answer:

   function isPrime(n: number): boolean -> Option C

4. Quick Check:

   Input number, output boolean [OK]

Hint: Input number, output true/false for prime [OK]

Common Mistakes:

• Using wrong input types like string or boolean
• Returning wrong types like number or void
• Missing function parameters

3. What will be the output of the following TypeScript code?

function isPrime(n: number): boolean {
  if (n <= 1) return false;
  for (let i = 2; i <= Math.sqrt(n); i++) {
    if (n % i === 0) return false;
  }
  return true;
}
console.log(isPrime(11));

medium

A. true

B. false

C. undefined

D. SyntaxError

Solution

1. Step 1: Check input and initial condition

   Input is 11, which is greater than 1, so continue checking.

2. Step 2: Loop from 2 to sqrt(11) ≈ 3.3

   Check divisibility by 2 and 3. 11 % 2 != 0 and 11 % 3 != 0, so no divisor found.

3. Final Answer:

   true -> Option A

4. Quick Check:

   11 is prime = true [OK]

Hint: Check divisors up to square root only [OK]

Common Mistakes:

• Checking divisors beyond sqrt(n)
• Returning true too early
• Not handling n <= 1 correctly

4. Identify the error in this TypeScript function to check prime numbers:

function isPrime(n: number): boolean {
  if (n < 2) return false;
  for (let i = 2; i < n / 2; i++) {
    if (n % i === 0) return false;
  }
  return true;
}

medium

A. Loop should run until i <= Math.sqrt(n) for efficiency

B. Return type should be number, not boolean

C. Loop should run until i <= n / 2, not i < n / 2

D. Condition if (n < 2) should be if (n <= 1)

Solution

1. Step 1: Identify the logic error in loop condition

   The loop uses i < n / 2. For n=4, n/2=2, i=2 < 2 false; loop skips check for 2, returns true (but 4 not prime).

2. Step 2: Correct the loop bound

   For efficiency and correctness, the loop should run until i <= Math.sqrt(n) instead of n/2.

3. Final Answer:

   Loop should run until i <= Math.sqrt(n) for efficiency -> Option A

4. Quick Check:

   Loop bound to sqrt(n) is optimal and correct [OK]

Hint: Loop to i <= sqrt(n) for efficient prime check [OK]

Common Mistakes:

• Using i < n/2 instead of i <= n/2 in loop
• Wrong return type
• Incorrect boundary condition for n

5. You want to find all prime numbers between 10 and 20 using this function:

function isPrime(n: number): boolean {
  if (n <= 1) return false;
  for (let i = 2; i <= Math.sqrt(n); i++) {
    if (n % i === 0) return false;
  }
  return true;
}

Which of the following arrays correctly lists these primes?

medium

A. [10, 12, 14, 16, 18, 20]

B. [11, 13, 17, 19]

C. [11, 12, 13, 14, 15, 16, 17, 18, 19, 20]

D. [9, 11, 13, 15, 17, 19]

Solution

1. Step 1: Identify prime numbers between 10 and 20

   Check each number: 11 (prime), 12 (not), 13 (prime), 14 (not), 15 (not), 16 (not), 17 (prime), 18 (not), 19 (prime), 20 (not).

2. Step 2: List only primes in the array

   The primes are 11, 13, 17, and 19.

3. Final Answer:

   [11, 13, 17, 19] -> Option B

4. Quick Check:

   Primes 10-20 = [11,13,17,19] [OK]

Hint: Check each number with isPrime, collect true results [OK]

Common Mistakes:

• Including non-prime even numbers
• Including numbers less than 10 or greater than 20
• Confusing primes with odd numbers