getting java to read from a file.

[philthee]

Hi there, i want to know how to get java to read a file that i will call from unix command line, do something with it and the n output the results.

the furthest i have got in what you do is to put the class

import java.io.*;

in at the beginning.

I want to read in a file, assign each column in the line to a variable

i.e if i was using perl it would be

while (my $line = <>) {   // while there is input from file
chomp $line;              // take off trailing space
($a, $b, $c, $d) = split ("\t", $line);  // 4 things are split by tabs on each line, put them into these variables 

print "$a\t$c\n";   // print first and third col                      

}

[Belisarius]

You can do this a couple of different ways. I suggest you make friends with the API, as it contains all the objects you'll use to make this work.

The simplest way is to take the standard input (System.in), and wrap it around a BufferedReader. That way you can pipe in the contents of the file (java MyProgram < input.txt). You can then use a StringTokenizer to parse the contents of each line.

Here's a bit of code you can use to get yourself started:

BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
String line = br.readLine();
while(line != null){
  StringTokenizer toks = new StringTokenizer(line);
  // Do stuff
  line = br.readLine();
}

[philthee]

thanks a lot,
i am away from my unix machine and java stuff for the weekend but i will crack on with that on Monday

Cheers

[philthee]

i have tried putting in the code you have given me but the IDE does not seem to like it at all.
I am using Netbeans and when i inserted that bit of code it just underlines it with squiggly red line.

i have put it here in the main loop
public static void main(String[] args) {

your code

}

why does it not like that?

[Belisarius]

Well, that code is pretty much verbatim from a quick Java file I coded up in pico. Netbeans can be a bit finicky at times (I use it myself), and if you had a typo earlier in the file it could throw the whole thing off.

[Belisarius]

Oh, for future reference, StringTokenizer is in java.util, I believe, and that's probably why you had the red error lines.

[philthee]

cheers i got that