#!/usr/bin/perl -w


###################################
# This Document Explains perl Input/output
# facilities.
# AAK: Last Modification:
# Tue Sep 11 21:52:39 PDT 2012
# #################################

# -w option in first line will turning on the warnings

# if no argument is entered, it uses standard input

print "@ARGV \n";

# @ARGV is an array that stores the argument passed to the script

# as explained before, you can read from standard input via:

# $inp = <STDIN>;



#"say": is the same as print but with \n at the end:
#to use say, you need to load features of perl v5.1:

use 5.010;
say "Hello!";

#you can format the output using printf:

printf "Hello, %s; did you know 2 + 3 = %d !\n", Arman, 5;
# %s for string, %d for integer, %g for flexible floating-pint, integer exponential:
printf "%g %g %g \n", 7/13, 28**43, 1/(2.34**73.232);

printf "%10.30f \n", sqrt(2);
printf "%10d \n", 287321;

#perl uses STDIN, STDOUT, STDERR, DATA ARGV, ARGVOUT
#for file handling, user can also define "file handlers" using open operator:

open OUTPUT, ">output.dat";
open LOG, ">>log";
open STH, "STH";

# > >> are linux standard output redirectings
# you can check if the opening was sucessfull:

if ( ! open INPUTDATA, "mydata.dat" ) {
   die "opening input data failed: $!"
}

# die commands exits the program with none-zero status,
# $! is human readable error that happend

#using diamont operator <> you can read from files:
#following will read from all files named in the argument
#and for each line will do sth:

#to close a filehandle:

close STH;

#perl will close the filehanldes when it exits, but 
#it is always good idea to be tidy

# you can use warn operator instead of die, just to give

warn "this is warning usage";

#you can use the file handle just like build-in ones:

while (<INPUTDATA>) {
   print $_;
}

print LOG "this is log file created by perl \n";
printf LOG "%d %s \n", 2, line;

#or you can change the default output filehandle by select:
select LOG;
print "another line in log\n";

select OUTPUT;
print "this will go to output.dat";

select STDOUT;
say "Hello again!";

#one more example, to send standard error to somewhere else:

if ( ! open STDERR, ">>./.error_log" ) {
   die "Can't open error log : $!";
}

warn "this warning will not be displayed on the terminal";
warn "warnings are going to .error_log file";

close STDERR;
warn "this is gonna go nowhere!";

while (defined($line=<>)) {
   print $line;
}