STDIN in scalar and list context
We have learned about scalar and list context in Perl, and we have seen how localtime behaves in scalar and list context. We also saw what an array returns in scalar context. We also saw how to read from a file in SCALAR and LIST context.
This time we'll look at reading from STDIN using <STDIN> in scalar and list context.
STDIN in scalar context
use strict; use warnings; print "Please type in your name: "; my $name = <STDIN>; chomp $name; print "Hello $name, how are you?\n";
The relevant part is that we read from STDIN and assign to a scalar variable $name.
If we run this script, it will print "Please type in your name: " and then it will wait till we press ENTER. Then it will take everything we type, including the ENTER at the end, and assign it to the $name variable. chomp will eliminate the trailing newlines.
STDIN in list context
If we assign the <STDIN> readline operator to an array it means we put the readline operator in LIST context. Let's see this example:
use strict; use warnings; print "Please type in the names of the programming languages you know: "; my @names = <STDIN>; chomp @names; print "Hello, I see you know " . scalar(@names) . "\n";
This script prints some text too and then waits for your input. Pressing ENTER after you typed in some text will leave Perl waiting for the next line. In order to tell Perl that you have finished typing, you need to press ENTER (to get to the next empty line) and then send an EOF a.k.a. End of File signal. This depends on the Operating system. On Linux/Unix system pressing Ctrl-D (also written as ^D sometimes) will send this signal. On MS Windows you have to type Ctrl-Z (sometimes written as ^Z) and then also press ENTER.
In both cases you have to send the signal on a new line, that is, after pressing ENTER.
Once you send that signal, Perl will assign everything you typed to the @names array. Every line you typed in, every string that you finished by pressing ENTER will be in a separate element of the array. So, if you type in:
Please type in the names of the programming languages you know: JavaENTER PerlENTER PythonENTER ^D
The array will have 3 elements like these:
@names = ( "Java\n", "Perl\n", "Python\n", );
Then we apply chomp to all of them at once removing all the trailing newlines and getting:
@names = ( "Java", "Perl", "Python", );
On the last line we print out the number of elements in the array.
You almost always need to read from STDIN in scalar context, that is, to assign the result to a scalar variable:
$input = <STDIN>;
Published on 2015-03-18