如何从Java中的大型文本文件读取整数/双精度数

Question

I am making a Pi based RNG(Random Number Generator) for a research project. I am getting stumped at this point hence I cant seem to figure out how to read the digits form a rather large file (1GB). Here is the input:

........

File is ugly I know... its Pi to 1 Billionth decimal place. I am not going into details on why I am doing this but here is my goal. I want to be able to skip x number of decimal places before beginning printing output, I also need to be able to read out y number of consecutive digits at a time so like if it was 4 at a time output would look like:

1111\\n 2222\\n 3333\\n 4444\\n....

My base objective is to be able to print at least 1 number at a time hence after that I can piece them together how I want... So basic output is:

For input 3.1415.. I get.. 3,1,4,1,5....

I tried bunch of File Streams from Java API but it only prints bytes/bits... I have no idea on how to convert them to something meaningful.

Also, Reading line by line is not optimal hence I have to have my numbers be same length and I feel like reading line by line would cut them off in a funny way..

Answer 1

What you need is a character stream , basically a subclass of Reader , so you can read character by character, rather than byte by byte.

To achive what you need, you will have to:

• List item
• open a character stream to the file containing your input digits. Prefer a BufferedReader over a FileReader to speed up the I/O, since reading char by char can be very slow, especially with large files
• you will need to keep track of the previous character read (if any) and group strings of identical characters in an appropriate data strcuture (for instance a StringBuilder )
• if you need to skip the first n characters, use Reader.skip(n); at the start

The following code does exactly what I understand of your requirements:

public class Test {
  public static void main(String[] args) {
    final char decimalSeparator = ',';
    try (Reader reader = new BufferedReader(new FileReader("pi.txt"))) {
      int prevC = -1; // previous character read from the stream
      int c; // latest character read from the stream
      StringBuilder sb = new StringBuilder();
      while ((c = reader.read()) != -1) {
        // if first digit or same as previous digit
        if ((prevC == -1) || (c == prevC)) {
          sb.append((char) c);
        } else {
          // print the group of digits and reset sb
          if (sb.length() > 0) {
            System.out.println(sb.toString());
            sb = new StringBuilder();
          }
          sb.append((char) c);
        }
        prevC = c;
      }
      // print the last digits group
      if (sb.length() > 0) {
        System.out.println(sb.toString());
      }
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}

Answer 2

Okay I have spoken to a CS professor and it seems that I have forgotten my basic Java training. 1Byte = 1 char. In this case BufferedInputReader spits out ASCII values for said chars. Here is simple solution:

FileInputStream ifs = new FileInputStream(pi); //Input File containing 1 billion digits
BufferedInputStream bis = new BufferedInputStream(ifs);
System.out.println((char)bis.read()); //Build strings or parse chars how you want

..Rinse and repeat. Sorry for wasting time... but I hope this will set someone one the right track down the road.