用JS查找素数序列

Question

I have to get a sequence of prime numbers. But my code does not work. How is it possible to fix it?

var num1 = parseInt(prompt('Enter a number'));
var num2 = parseInt(prompt('Enter a number'));
var num3 = 0;
function primeSeq(num1, num2) {  
var b = 1;
var c = '';
if (num1 > num2) {
num3 = num2;
num2 = num1;
num1 = num3;
        }
for (var i = num1; i < num2; i++) {
for (var j = 2; j < i; j++) {
if (i % j == 0) {
b++;
}
if (b <= 1) {
c += i + ' ';               
}
}
}
return c;
}
alert(primeSeq(num1, num2));

Answer 1

I guess you wanted something like this

var num1 = parseInt(prompt('Enter a number'));
var num2 = parseInt(prompt('Enter a number'));
var num3 = 0;
if (num1 > num2) {
  num3 = num2;
  num2 = num1;
  num1 = num3;
}
function primeSeq(num1, num2) {  
  var b;
  var c = '';
  for (var i = num1; i < num2; i++) {
    b = 1;
    for (var j = 2; j < i; j++) {
      if (i % j === 0) {
        b++;
      }
    }     
    if (b === 1) {
      c += i + ' ';               
    }
  }
  return c;
}
alert(primeSeq(num1, num2));

So in short, b should reset to 1 on every new prime candidate ( i loop) and check of b should be outside of inner ( j ) loop.

Please note that there are more optimal algorithms.

Answer 2

The lot easier way is to use a sieve system, if a number is divisible by another prime number it is not a prime. You can write a function like this:

function primes(max) {
    let primes = [2];
    for (let i = 3; i <= max; i++) {
        let found = true;
        for (let j = 0; j < primes.length; j++) {
            if (i % primes[j] == 0) found = false;
        }
        if (found) primes.push(i);
    }
    return primes;
}

Explanation

What you know by default is that 2 is a prime, so you start at 3. You don't want to exeed the max, that is the i <= max statement. Assume it is a prime, then search in the array if it is divisible by primes you found before, if that is the case, set found to false. Now check if is was found, push is to the array and return the primes.

Answer 3

Here is bit more optimized algorithm:

function primeSeq(num1, num2) {
  var primes = [];
  var isPrime;
  var j;
  var results = [];

  for (var i = 2; i < num2; i++) {
    isPrime = true;
    j = 0;
    while (j < primes.length) {
      if (i % primes[j] === 0) {
        isPrime = false;
        break;
      }
      j++;
    }
    if (isPrime) {
      primes.push(i);
      if (i >= num1) {
        results.push(i);
      }
    }
  }
  return results.join(' ');
}

In order for number to be prime it must not be dividable with all the smaller primes, so we are generating an array of primes to check upon. There is a theory also that every prime bigger than 3 has a following form:

6k+1 or 6k-1

So this would simplify it bit more.

Answer 4

Try this one

<input ng-model="range" type="number" placeholder="Enter the range">
 <button ng-click="findPrimeNumber(range)">


    $scope.findPrimeNumber=function(range){
         var tempArray=[];
         for(var i=0;i<=range;i++){
            tempArray.push(i)
         }
         var primenumbers= tempArray.filter((number) => {
         for (var i = 2; i <= Math.sqrt($scope.range); i++) {
            if (number % i === 0) return false;
          }
          return true;
         });
         console.log(primenumbers);

     }