Check If Number Is Prime Number

Check Number prime in JavaScript

Time complexity: O(sqrt(n))

Space complexity: O(1)

const isPrime = num => {
    for(let i = 2, s = Math.sqrt(num); i <= s; i++)
        if(num % i === 0) return false; 
    return num > 1;
}

How to check if a number is prime in a more efficient manner?

The lowest hanging fruit here is your stopping conditional

i <= x/2

which can be replaced with

i * i <= x

having taken care to ensure you don't overflow an int.This is because you only need to go up to the square root of x, rather than half way. Perhaps i <= x / i is better still as that avoids the overflow; albeit at the expense of a division which can be relatively costly on some platforms.

Your algorithm is also defective for x == 2 as you have the incorrect return value. It would be better if you dropped that extra test, as the ensuing loop covers it.

How do I get an if statement to check if a user given number is a prime number?

This can help you:

static bool isPrime(int number)
    {
        for (int i = 2; i < number; i++)
        {
            if((number% i) == 0)
            {
                // Not Prime
                return false;
            }
        }

        // Just Prime!
        return true;
    }

A prime number is not divisible by any numbers between 2 and (itself - 1).

This way, if you call 'isPrime()' inside the if, this just works.

Hope this can help you!

How to check if a number is prime? and if is not, how to increment the number until the function returns the next prime number?

Looks like the site only allows you to have the function and nothing else. Specifically in this situation: no separate functions. So if I put your checkPrime function inside myFunction it will pass successfully.

function myFunction(number) {
  function checkPrime(number) {
    for (var i = 2; i < number; i++) {
      if (number % i === 0) {
        return false;
      }
    }
    return true;
  }
  if (checkPrime(number)) {
    return number;
  } else {
    while (checkPrime(number) === false) {
      number++;
    }
  }
  return number;
}

Validate if input number is prime

The sympy.isprime() is a built-in function under the SymPy module and can be utilized for checking of possible prime numbers. It is a direct function and returns True if the number to be checked is prime and False if the number is not prime.

>>> import simpy
  
>>> sympy.isprime(8)

False

>>> sympy.isprime(11)

True

or else define a function like this

>>> def isPrime(k):
    
    # 1 is not prime number
    if k==1:
        return False

    # 2, 3 are prime
    if k==2 or k==3: 
        return True

    # even numbers are not prime
    if k%2==0: 
        return False

    # check all numbers till square root of the number , 
    # if the division results in remainder 0
    # (skip 2 since we dont want to divide by even numbers)

    for i in range(3, int(k**0.5)+1, 2):
        if k%i==0:
            return False

    return True

>>> print(isPrime(13))

True

>>> print(isPrime(18))

False

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