Question

I am writing this java program to find all the prime numbers up to num using the Sieve of Eratosthenes, but when I try to compile, it says I can’t use a long var as an array index, and it expects an int var in its place. But I’ll be working with large numbers, so I can’t use int. What can I do?

import java.util.*; import java.lang.*;  public class t3{     public static void main(String[] args){         long num = 100;          //declaring list and filling it with numbers         ArrayList<Long> numlist = new ArrayList<Long>();         for(long x=2 ; x<num ; x++){             numlist.add(new Long(x));         }          //sieve or eratosthenes         for(long x=0 ; x<Math.sqrt(num) ; x++){             for(long y=x+1 ; y<numlist.size() ; y++){                 if(numlist[y]%numlist[x] == 0){                     numlist.remove(y);                 }             }         }          //print list         for(Object item : numlist){             System.out.println((Long)item);         }     } } 

Answer 1

I’m not sure why your code would compile to begin with.

You’re not supposed to use [] in an array list to access members. An arraylist is merely a list that is internally stored in an array. You have to use the list get operation (which would still be O(1)). Writing numlist[index] means that you have an array of objects in numlist. You cannot override the [] operation as in C++.

In addition, an int is 32 bits in Java. Having an array of length greater than 2^32 (so you would need long indices) is unlikely and I’m not even sure the specification allows it.