function crossOff(flags, prime){

    for (var i = prime * prime; i < flags.length; i += prime){
        flags[i] = false;
    }
}

function getNextPrime(flags, prime){

    var next = prime + 1;
    while (next < flags.length && !flags[next]){
        next ++;
    }
    return next;
}

function init(flags){

    flags[0] = false;
    flags[1] = false;
    for (var i = 2; i < flags.length; i ++){
        flags[i] = true
    }
}

//sieveOfEratosthenes method
function getPrimes(){

    var max = parseInt(document.getElementById('to_range').value, 10);
    var min = parseInt(document.getElementById('from_range').value, 10);

    if (max < min){
        tempo.innerHTML = '';
        var errorMsg = document.createElement("p");
        errorMsg.style.color = 'red';
        tempo.appendChild(errorMsg).appendChild(
                document.createTextNode('Invalid Range'));
        return;
    }

    var flags = new Array(max+1);
    var count = 0;

    init(flags);
    var prime = 2;

    while (prime <= Math.sqrt(max)){

        crossOff(flags, prime);
        prime = getNextPrime (flags, prime);
    }

    return flags;
}