function prime_number()
{ 
            var num, i, c, flag = true;
            num = parseInt(document.getElementById("one").value);
            for(i = 2; i <= num - 1; i  )
            {
                if(num % i === 0)
                    {
                        flag = false;
                        break;
                    }
                
                if (flag === true)
                    var c = "It is a prime number";
                else 
                    var c = "It is not a prime number";
            }
    document.getElementById("two").value = c;
}

THIS IS THE CODING FOR FORM

 <form>
        <input type="number" id="one" name="one" placeholder="Enter a positive number"><br>
        <input type="text" id="two" name="two"><br>
        <input type="button" id="submit" name="submit" value="CHECK" onclick="prime_number()"><br>
    </form>

I've tried the ALERT method to show the text "THIS IS A PRIME NUMBER" or "THIS IS NOT A PRIME NUMBER" and that worked.

CodePudding user response：

Because your assignments of c are withing the for loop, when the loop breaks your assigmnents don't get executed. So, take em out of the for loop, secondly don't redefine c with var.

function prime_number() { 
  var num, i, c, flag = true;
  num = parseInt(document.getElementById("one").value);
  for(i = 2; i <= num - 1; i  ) {
    if(num % i === 0)
    {
      flag = false;
      break;
    }
  }
  console.log(c);
    if (flag === true)
       c = "It is a prime number";
    else 
       c = "It is not a prime number";
  document.getElementById("two").value = c;
}

<form>
        <input type="number" id="one" name="one" placeholder="Enter a positive number"><br>
        <input type="text" id="two" name="two"><br>
        <input type="button" id="submit" name="submit" value="CHECK" onclick="prime_number()"><br>
    </form>

P.S: You can make a more efficient check in the for loop as:
for(i = 2; i <= num/2; i )

A number cannot be divisible by more than it's half, so you don't need to test those scenarios, i.e.
x % (x/2 epsilon) is never 0.

CodePudding user response：

Two points needs to pay attention:

1. use c = xxx instead of var c = xxx when you assign value to it
2. need to assign value to c outside for loop

function prime_number()
{ 
  var num, i, c, flag = true;
  num = parseInt(document.getElementById("one").value);
  for(i = 2; i <= num - 1; i  )
  {
      if(num % i === 0)
          {
              flag = false;
              break;
          }
  }
  
  if (flag){
     c = "It is a prime number";
   }else{
      c = "It is not a prime number";
   }
  document.getElementById("two").value = c;
}

<form>
        <input type="number" id="one" name="one" placeholder="Enter a positive number"><br>
        <input type="text" id="two" name="two"><br>
        <input type="button" id="submit" name="submit" value="CHECK" onclick="prime_number()"><br>
    </form>

CodePudding user response：

You should use same c = "It is a prime number" etc. because when you use var c again you declare variable c again, but it already exists.