CODEWITHSUNDEEP
javascript
nonono
nonono

Reputation: 601

Javascript prime number check

I've been trying to pass a challenge for so long, and despite reading up multiple solutions and advice online I can't seem to apply it correctly.

The instructions are "Define a function isPrime that takes one integer argument and returns true or false depending on if the integer is a prime."

I've tried a lot of things but this is the furthest I've been able to get: 

function isPrime(num) {
  if (num <= 1) return false;
  if (num === 2) return true;

  for (var i = 2; i < num; i++) 
    if (num % i === 0) return false;
    else return true;
}

But then I'm told: "9 is not a prime number."

Any help would be appreciated and thanks!

Upvotes: 4

Views: 10609

Answers (4)

Serkan KONAKCI
Serkan KONAKCI

Reputation: 1360

I have try to calculate using RxJS operators. I hope you have enjoyed.

import { range, filter, take } from 'rxjs';

/**
 * It returns true if the number is prime, false otherwise
 * @param {number} x - number - the number to check if it's prime
 * @returns A function that takes a number and returns a boolean.
 */
const isPrime = (x: number): boolean => {
  let state = true;

  range(2, x - 2 > 0 ? x - 2 : 0)
    .pipe(
      filter((loop: number): boolean => x % loop === 0),
      take(1),
    )
    .subscribe(() => {
      state = false;
    });

  return x > 1 && state;
};

const theNumber = 44;

console.log(theNumber, isPrime(theNumber) ? ' is' : ' is not', ' prime number');

Bonus:

Below code gives the prime numbers between 5 and 100. You can also subscribe to obs[1] for rest numbers.

const start = 5;
const end = 100;

const obs = partition(
  generate(
    start,
    (x: number) => x < end,
    (x: number) => x + 1,
  ),
  (x) => isPrime(x),
);

obs[0].pipe(toArray()).subscribe((x) => console.log('Prime numbers: ', x));
obs[1].pipe(toArray()).subscribe((x) => console.log('Rest numbers: ', x));

Upvotes: 0

Ashish Kumar
Ashish Kumar

Reputation: 113

function primeNo(){
//"number1" is "ID" of HTML input block.
var num2 = document.getElementById("number1").value;
var dev = num2 / 2;
var i;
 for(i=2; i < dev ; i++){
    if(num2%i == 0){
        //"ans1" is "ID of <p> where ans needs to display"
        document.getElementById("ans1").innerHTML = "Not a Prime No";
        break;
    }
    else{
        document.getElementById("ans1").innerHTML = "Prime No";
    }
 }

}

Upvotes: 0

Pranav C Balan
Pranav C Balan

Reputation: 115282

You should avoid the else case and return only after for loop completion. Although for loop count can be reduced by updating condition to i <= Math.sqrt(num) ( as @PatrickRoberts suggested ).

function isPrime(num) {
  if (num <= 1) return false;
  if (num === 2) return true;

  // storing the calculated value would be much 
  // better than calculating in each iteration
  var sqrt = Math.sqrt(num);

  for (var i = 2; i <= sqrt; i++) 
    if (num % i === 0) return false;
  return true;
}

FYI : In your code on the first iteration of the loop num % i === 0(9 % 2) would be false and it would return the else statement(true).

Upvotes: 4

Aaron
Aaron

Reputation: 24822

The problem is with the else return true; in your for loop.

At the first iteration, where i is 2, just after you checked that 9 % i wasn't 0, you execute the else block and return true.

You shouldn't return true before all the iterations failed to return false.

Upvotes: 4

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