Javascript查找素数

Question

I am writing a little script to find and print out all the prime numbers from X thru Y. Here is what I have written:

var numX = prompt('Enter a number greater than 0:','');
var numY = prompt('Enter a number greater than ' + numX + ':','');

while (numX <= numY) {
    if (numX == 1 || numX == 2 || numX == 3) {
    document.write(numX + '</br>');
    } else if (numX % 2 === 0 || numX % 3 === 0 || numX % 5 === 0 || numX % 7 === 0){
    document.write();
    } else {
    document.write(numX + '</br>');
    }
    numX++;
};

Now, this works just fine so long as the first number is 1. If, however, the first number is anything greater than 1 it does not print out anything. I am not sure if this is the right forum for this question (perhaps a math forum?), but I thought I would ask here on the off chance someone could help me out. I also know that a sieve is the better way to go about this, but I wanted to try and figure this out as a while loop first. Any and all help is appreciated!

Answer 1

While I understand what you are trying to do, I highly recommend taking a look at the Sieve of Eratosthenes . You really want to get the hang of knowing different algorithms to compute these things in case you decide to deal with really large numbers. While the way you go about it now might work in smaller ranges, bigger ranges are going to go crazy.

Also I believe this Stackoverflow question is very similar to this one and the answer for it is very well made:

finding sum of prime numbers under 250

Answer 2

您可以在这里尝试任何一种选择： http : //www.javascripter.net/faq/numberisprime.htm

Answer 3

Hi i have added bit change to ur code (added condition for 5 and 7 prime numbers) and its working...

var numX = prompt('Enter a number greater than 0:','');
var numY = prompt('Enter a number greater than ' + numX + ':','');

while (numX <= numY) {
    if (numX == 1 || numX == 2 || numX == 3 || numX == 5 || numX == 7) {
    document.write(numX + '</br>');
    } else if (numX % 2 === 0 || numX % 3 === 0 || numX % 5 === 0 || numX % 7 === 0){
    document.write();
    } else {
    document.write(numX + '</br>');
    }
    numX++;
};

Check the demo here

Answer 4

OK, turns out that I jumped the gun on asking this question. I was more concerned with getting the else if statement working that I failed to even note that my formula was seriously flawed!

The issue possibly could be with the second variable. If the first variable is 1 then the second variable can be any number. However, if the first variable is greater than 1 then the second variable has to be less than 100 or it will not work.