CODEWITHSUNDEEP
perl
Breaking Bioinformatics
Breaking Bioinformatics

Reputation: 388

So what exactly does <FILE> do?

So, I've used <FILE> a large number of times. A simple example would be: 

open (FILE, '<', "someFile.txt");
while (my $line = <FILE>){
print $line;
}

So, I had thought that using <FILE> would take a a part of a file at a time (a line specifically) and use it, and when it was called on again, it would go to the next line. And indeed, whenever I set <FILE> to a scalar, that's exactly what it would do. But, when I told the computer a line like this one:

print <FILE>;

it printed the entire file, newlines and all. So my question is, what does the computer think when it's passed <FILE>, exactly?

Upvotes: 6

Views: 207

Answers (3)

mpapec
mpapec

Reputation: 50657

Diamond operator <> used to read from file is actually built-in readline function.

From perldoc -f readline

Reads from the filehandle whose typeglob is contained in EXPR (or from *ARGV if EXPR is not provided). In scalar context, each call reads and returns the next line until end-of-file is reached, whereupon the subsequent call returns undef. In list context, reads until end-of-file is reached and returns a list of lines.

If you would like to check particular context in perl,

sub context { return wantarray ? "LIST" : "SCALAR" }

print my $line  = context(), "\n";
print my @array = context(), "\n";
print context(), "\n";

output

SCALAR
LIST
LIST

Upvotes: 12

RobEarl
RobEarl

Reputation: 7912

The behaviour is different depending on what context it is being evaluated in:

my $scalar = <FILE>; # Read one line from FILE into $scalar
my @array = <FILE>; # Read all lines from FILE into @array

As print takes a list argument, <FILE> is evaluated in list context and behaves in the latter way.

Upvotes: 2

Toto
Toto

Reputation: 91498

It depends if it's used in a scalar context or a list context.

In scalar context: my $line = <file> it reads one line at a tie.
In list context: my @lines = <FILE> it reads the whole file.

When you say print <FILE>; it's list context.

Upvotes: 7

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