OReilly_Learning Perl 5th Chapter 5 Input & Output

1>: Input from Standard Input

chomp($line = );

(Since the line-input operator will return undef when you reach end-of-file, this is handy

for dropping out of loops)

while (defined($line = )) {

print "I saw $line";

}

(Use the defined function, which returns false for undef, and true for everything else)

while () {

print "I saw $_"; # same as above

}

In the while loop, Perl reads a line of input, puts it into a variable, and runs the body of the loop. 

Then, it goes back to find another line of input.

foreach () {

print "I saw $_";

}

In the foreach loop, the line-input operator is being used in a list context

So it has to read all of the input before the loop can start running.

2> : Input from the Diamond Operator

$ ./my_program fred barney betty

while (defined($line = <>)) {

chomp($line);

print "It was $line that I saw!\n";

}

while (<>) {

chomp; # This works like the loop above, but with less typing

print "It was $_ that I saw!\n";

}

(The Invocation Arguments)

@ARGV is already stuffed full of the list of invocation arguments

@ARGV = qw# larry moe curly #; # force these three files to be read

while (<>) {

chomp;

print "It was $_ that I saw in some stooge-like file!\n";

}

3>: Output to Standard Output

@array=qw/ fred barney betty /;

print @array; # fredbarneybetty

print "@array"; # fred barney betty

$result = print("hello world!\n"); # result get 1,indicating the success of the print

print (2+3)*4; # print 5

(print (2+3))*4; # Oops!

4>: Formatted Output with printf

printf "Hello, %s; your password expires in %d days!\n", $user, $days_to_die;

%d：means a decimal integer

 %f：means a floating-point

 %g：(General)  automatically chooses floating-point,integer,or even exponential notation

 %s：means a string

%%：to print a real percent sign

printf "%g %g %g\n", 5/2, 51/17, 51 ** 17;

printf "in %d days!\n", 17.85; # in 17 days (Note that this is truncated, not rounded)

printf "m\n", 42; # output like ````42  (the ` symbol stands for a space)

printf "-\n", 2e3 + 1.95; # 2001

printf "s\n", "wilma"; # (right-justified) looks like `````wilma

printf "%-15s\n", "flintstone"; # (left-justified) looks like flintstone`````

printf "f\n", 6 * 7 + 2/3; # looks like ```42.666667

printf ".3f\n", 6 * 7 + 2/3; # looks like ``````42.667

printf ".0f\n", 6 * 7 + 2/3; # looks like ``````````43

printf "Monthly interest rate: %.2f%%\n", 5.25/12; # the value looks like "0.44%"

5>: Arrays and printf

my @items = qw( wilma dino pebbles );

printf "The items are: \n" . ("s\n" x @items), @items;

(Note that here we have @items being used once in a scalar context, to get its length, and

once in a list context, to get its contents. Context is important.)

6>: Filehandle

(the recommendation from Larry is that you

use all uppercase letters in the name of your filehandle)

STDIN, STDOUT, STDERR, DATA, ARGV, and ARGVOUT