I am trying to teach myself node.js (no javascript or real programming experience) and have hit a road block on one of the problems I am trying to solve. My goal is to write the first 100 prime numbers to txt file. Below is my code so far.

var fs = require('fs');
var outfile = "test.txt";
var primality = function () {
    var arr = [];
    for (var n = 2; n <= 542; n++) {
        var primeTrue = true;
        for (var i = 2; i <= Math.sqrt(n); i++) {
            if (n % i === 0) {
                primeTrue = false;
            }
        }
        if (primeTrue) {
            arr.push(n);
        }
    }
    return arr;
}
fs.writeFileSync(outfile, arr);

I was using the codecademy javascript lab to test my loop and conditions and this code does seem to work. (It is also likely not the best way to do this since I had to set my counter to stop at 542 rather than have the program stop at the 100th prime number). In any event, when I added

var outfile = "test.txt"

and

fs.writeFileSync(outfile, arr);

this did not output the 100 prime numbers to the txt file as I thought it would. I'm still on the ground floor of my learning so I greatly appreciate any help you can provide.

Thank you in advance.

Kevin

I am trying to teach myself node.js (no javascript or real programming experience) and have hit a road block on one of the problems I am trying to solve. My goal is to write the first 100 prime numbers to txt file. Below is my code so far.

var fs = require('fs');
var outfile = "test.txt";
var primality = function () {
    var arr = [];
    for (var n = 2; n <= 542; n++) {
        var primeTrue = true;
        for (var i = 2; i <= Math.sqrt(n); i++) {
            if (n % i === 0) {
                primeTrue = false;
            }
        }
        if (primeTrue) {
            arr.push(n);
        }
    }
    return arr;
}
fs.writeFileSync(outfile, arr);

I was using the codecademy javascript lab to test my loop and conditions and this code does seem to work. (It is also likely not the best way to do this since I had to set my counter to stop at 542 rather than have the program stop at the 100th prime number). In any event, when I added

var outfile = "test.txt"

and

fs.writeFileSync(outfile, arr);

this did not output the 100 prime numbers to the txt file as I thought it would. I'm still on the ground floor of my learning so I greatly appreciate any help you can provide.

Thank you in advance.

Kevin

• javascript
• node.js

Share Improve this question Follow edited Jun 29, 2013 at 17:42 Felix Kling 817k181181 gold badges1.1k1.1k silver badges1.2k1.2k bronze badges asked Jun 29, 2013 at 17:33 Kevin DarkKevin Dark 2,14955 gold badges2222 silver badges2323 bronze badges 6

• is this because you are trying to learn node.js?? – HIRA THAKUR Commented Jun 29, 2013 at 17:33
• Your plan is to indent the code only once it works to save time? – 6502 Commented Jun 29, 2013 at 17:42
• 3 You are never calling primality to create the array of numbers. arr is only defined inside the function. You might want fs.writeFileSync(outfile, primality());. I remend to read eloquentjavascript/chapter3.html. – Felix Kling Commented Jun 29, 2013 at 17:42
• @MESSIAH You are correct, I am trying to learn node.js using a couple books, going through codecademy's javascript track and taking a class that includes programming problems such as the one above. In general I am just trying to learn to program and have tried a couple other languages with limited sucees. – Kevin Dark Commented Jun 29, 2013 at 17:47
• 1 @KDark11, you agreed with the Coursera terms, so you have to make your homework yourself :) Don't take it seriously, I'm just kidding, IMO not points or certificates are important but your knowledge – sasha.sochka Commented Jun 29, 2013 at 21:12

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2 Answers 2

Sorted by: Reset to default 12

You're doing a lot in one function. The code may be a bit easier to follow if you break it up into two functions, one to make the list of primes and another to test if a specific number is prime:

function listPrimes( nPrimes ) {
    var primes = [];
    for( var n = 2;  nPrimes > 0;  n++ ) {
        if( isPrime(n) ) {
            primes.push( n );
            --nPrimes;
        }
    }
    return primes;
}

function isPrime( n ) {
    var max = Math.sqrt(n);
    for( var i = 2;  i <= max;  i++ ) {
        if( n % i === 0 )
            return false;
    }
    return true;
}

Now you can run it in Node:

var fs = require('fs');
fs.writeFileSync( 'test.txt', listPrimes(100) );

or directly in the browser console:

listPrimes( 100 );

(I didn't test the code in Node, only in the browser.)

A couple of related notes:

1. The sqrt() calculation is moved outside the loop in isPrime(), so it doesn't have to be recalculated for each number you're testing.
2. The nPrimes variable lets you generate the exact number of primes you want without the 542 hack.

Having written this simple version, it's interesting to look at possible optimizations. One is to check for divisibility only on the previously generated primes, instead of checking all integers up to the square root. You could do that like this:

function listPrimes( nPrimes ) {
    var primes = [];
    for( var n = 2;  nPrimes > 0;  n++ ) {
        if( isPrime( n, primes ) ) {
            primes.push( n );
            --nPrimes;
        }
    }
    return primes;
}

function isPrime( n, primes ) {
    var max = Math.sqrt(n);
    for( var i = 0;  i < primes.length  &&  primes[i] <= max;  i++ ) {
        if( n % primes[i] === 0 )
            return false;
    }
    return true;
}

That may be faster if you're generating a large number of primes, although for 100 of them it hardly matters and I'd be inclined to stick with the simpler code.

Of course if you're talking about optimization, it's always worth considering a different algorithm. The Sieve of Eratosthenes is a fun one because it's fast and fairly simple and easy to understand. That Wikipedia article has a great illustration of how it works. In JavaScript it might look something like this:

function listPrimes( max ) {
    // Start with an empty list of primes
    var primes = [];
    // Initialize the sieve - each number is prime unless proven otherwise
    var sieve = new Array( max );
    for( var i = 1;  i <= max;  i++ ) {
        sieve[i] = true;
    }
    // Now check each number from 2 through max
    for( var p = 2;  p <= max;  p++ ) {
        if( sieve[p] ) {
            // p is prime, save it in the output list
            primes.push( p );
            // Mark p * 2, p * 3, p * 4, etc. as non-prime
            for( var t = p * 2;  t <= max;  t += p ) {
                sieve[t] = false;
            }
        }
    }
    return primes;
}

Yes, after remending splitting the code into two functions, I'm now back to one function. :-)

One difference about the Sieve is that you can't really say, "please give me the first N primes"; instead you ask it, "please give me all the primes less than N". But if N is a large number, it is much faster than the other approach.

This works even beter if you preinitialize the list and skip testing the primality of multiples of 2

var primes = [2];
--nPrimes
for( var n = 3;  nPrimes > 0;  n += 2 )

I just finished very similar code for the Startup Engineering course homework @Coursera ;)