Question

Say there is a file too big to be put to memory. How can I get a random line from it? Thanks.

Update:
I want to the probabilities of getting each line to be equal.

Answer 1

Here’s a solution. Take a look at the choose() method which does the real thing (the main() method repeatedly exercises choose(), to show that the distribution is indeed quite uniform).

The idea is simple: when you read the first line it has a 100% chance of being chosen as the result. When you read the 2nd line it has a 50% chance of replacing the first line as the result. When you read the 3rd line it has a 33% chance of becoming the result. The fourth line has a 25%, and so on….

import java.io.*;
import java.util.*;

public class B {

  public static void main(String[] args) throws FileNotFoundException {
     Map<String,Integer> map = new HashMap<String,Integer>();
     for(int i = 0; i < 1000; ++i)
     {
        String s = choose(new File("g:/temp/a.txt"));
        if(!map.containsKey(s))
           map.put(s, 0);
        map.put(s, map.get(s) + 1);
     }

     System.out.println(map);
  }

  public static String choose(File f) throws FileNotFoundException
  {
     String result = null;
     Random rand = new Random();
     int n = 0;
     for(Scanner sc = new Scanner(f); sc.hasNext(); )
     {
        ++n;
        String line = sc.nextLine();
        if(rand.nextInt(n) == 0)
           result = line;         
     }

     return result;      
  }
}