Sieve of Eratosthenes Find All Primes in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the array that marks prime numbers.

DSA C

int prime[[1]];

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An+1

B100

Cn-1

Dn*2

Attempts:

3 left

💡 Hint

Common Mistakes

Using size n instead of n+1 causes out-of-bound errors.

Using a fixed size like 100 limits the range.

We need an array of size n+1 to mark numbers from 0 to n.

2fill in blank

medium

Complete the code to initialize all entries as true (1) in the prime array.

DSA C

for (int i = 0; i <= n; i++) {
    prime[i] = [1];
}

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A-1

Bn

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Setting to 0 marks all as not prime initially.

Using negative values is incorrect for boolean flags.

We mark all numbers as prime initially by setting to 1 (true).

3fill in blank

hard

Fix the error in the loop condition to run only up to the square root of n.

DSA C

for (int p = 2; p [1]= sqrt(n); p++) {
    if (prime[p] == 1) {
        for (int i = p * p; i <= n; i += p) {
            prime[i] = 0;
        }
    }
}

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A<=

B<

C>

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' misses markings when sqrt(n) is integer (perfect square n).

Using '>=' or '>' reverses the loop and causes no iterations.

We loop while p is less than or equal to the square root of n.

4fill in blank

hard

Fill both blanks to print all prime numbers after sieving.

DSA C

for (int i = [1]; i <= n; i++) {
    if (prime[i] == [2]) {
        printf("%d ", i);
    }
}

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A2

B1

C0

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Starting from 0 or 1 prints non-prime numbers.

Checking for prime[i] == 0 prints non-primes.

We start from 2 because 0 and 1 are not prime, and print numbers marked as 1 (prime).

5fill in blank

hard

Fill all three blanks to complete the sieve function header and variable declaration.

DSA C

[1] sieve(int [2]) {
    int [3][n+1];
    // rest of the code
}

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Avoid

Bn

Cprime

Dint

Attempts:

3 left

💡 Hint

Common Mistakes

Using int as return type without return statement.

Wrong parameter name or type.

Incorrect array name.

The function returns nothing (void), takes int n as input, and declares int prime array.