Project Euler 7 Scala Problem
- by Nishu
I was trying to solve Project Euler problem number 7 using scala 2.8
First solution implemented by me takes ~8 seconds
def problem_7:Int = {
var num = 17;
var primes = new ArrayBuffer[Int]();
primes += 2
primes += 3
primes += 5
primes += 7
primes += 11
primes += 13
while (primes.size < 10001){
if (isPrime(num, primes)) primes += num
if (isPrime(num+2, primes)) primes += num+2
num += 6
}
return primes.last;
}
def isPrime(num:Int, primes:ArrayBuffer[Int]):Boolean = {
// if n == 2 return false;
// if n == 3 return false;
var r = Math.sqrt(num)
for (i <- primes){
if(i <= r ){
if (num % i == 0) return false;
}
}
return true;
}
Later I tried the same problem without storing prime numbers in array buffer. This take .118 seconds.
def problem_7_alt:Int = {
var limit = 10001;
var count = 6;
var num:Int = 17;
while(count < limit){
if (isPrime2(num)) count += 1;
if (isPrime2(num+2)) count += 1;
num += 6;
}
return num;
}
def isPrime2(n:Int):Boolean = {
// if n == 2 return false;
// if n == 3 return false;
var r = Math.sqrt(n)
var f = 5;
while (f <= r){
if (n % f == 0) {
return false;
} else if (n % (f+2) == 0) {
return false;
}
f += 6;
}
return true;
}
I tried using various mutable array/list implementations in Scala but was not able to make solution one faster. I do not think that storing Int in a array of size 10001 can make program slow. Is there some better way to use lists/arrays in scala?
