What's the difference between iterating over a file with foreach or while in Perl?

Question

I have a filehandle FILE in Perl, and I want to iterate over all the lines in the file. Is there a difference between the following?

while (<FILE>) {     # do something } 

and

foreach (<FILE>) {     # do something } 

Answer 1

For most purposes, you probably won't notice a difference. However, foreach reads each line into a list (not an array) before going through it line by line, whereas while reads one line at a time. As foreach will use more memory and require processing time upfront, it is generally recommended to use while to iterate through lines of a file.

EDIT (via Schwern): The foreach loop is equivalent to this:

my @lines = <$fh>; for my $line (@lines) {     ... } 

It's unfortunate that Perl doesn't optimize this special case as it does with the range operator (1..10).

For example, if I read /usr/share/dict/words with a for loop and a while loop and have them sleep when they're done I can use ps to see how much memory the process is consuming. As a control I've included a program that opens the file but does nothing with it.

USER       PID %CPU %MEM      VSZ    RSS   TT  STAT STARTED      TIME COMMAND schwern  73019   0.0  1.6   625552  33688 s000  S     2:47PM   0:00.24 perl -wle open my $fh, shift; for(<$fh>) { 1 } print "Done";  sleep 999 /usr/share/dict/words schwern  73018   0.0  0.1   601096   1236 s000  S     2:46PM   0:00.09 perl -wle open my $fh, shift; while(<$fh>) { 1 } print "Done";  sleep 999 /usr/share/dict/words schwern  73081   0.0  0.1   601096   1168 s000  S     2:55PM   0:00.00 perl -wle open my $fh, shift; print "Done";  sleep 999 /usr/share/dict/words 

The for program is consuming almost 32 megs of real memory (the RSS column) to store the contents of my 2.4 meg /usr/share/dict/words. The while loop only stores one line at a time consuming just 70k for line buffering.

Answer 2

In scalar context (i.e. while) <FILE> returns each line in turn.

In list context (i.e. foreach) <FILE> returns a list consisting of each line from the file.

You should use the while construct.

See perlop - I/O Operators for more.

Edit: j_random_hacker rightly says that

while (<FILE>) { … } 

tramples on $_ while foreach does not (foreach localises $_ first). Surely this is the most important behavioural difference!