while vs foreach when reading from STDIN [duplicate]

Question

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

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. But in the foreach loop, the line-input operator is being used in a list context (since foreach needs a list to iterate through). So it has to read all of the input before the loop can start running. That difference will become apparent when the input is coming from your 400 MB web server logfile! It's generally best to use code like the while loop's shortcut, which will process input a line at a time, whenever possible. Source-The Llama book

So should I use while every time? What can be the possible situations where foreach loop will dominate over while?

Answer 1

As you point out, iterating over a file with foreach slurps the file into memory before the loop starts.

While it's true that one of Perl's credos is "There Is More Than One Way To Do It", there exist ways to do things that have no real benefit over some other way. foreach is probably a poor alternative for looping over a file. If you're going to iterate over the file line by line, you generally use while(). If you're going to slurp the whole file into memory at once, you do that in one step and be done with it.

There are sometimes advantages to slurping the whole file. One of them, for example, is minimizing the time that the file must be opened. More often than not, it's for convenience in dealing with data that doesn't really fit the line by line or record by record model. foreach() slurps the file, but doesn't really give a good opportunity to close it immediately, so that benefit is gone. And looping over the file with foreach doesn't help you much if your data has to be treated as a chunk. So that reason is gone too. In the end, you get the negative of slurping a file, with none of the positives of slurping a file.

There will always be someone who comes up with a clever reason that others haven't considered. But I have yet to see it. (Usually when I say something like that someone posts "it", so it's always dangerous to suggest it absolutely has no merit.) Let's put it this way: Until you discover one of the extremely few good reasons for using foreach to iterate over a file, don't worry about it. Surely when you do make that discovery, you'll know it is time.

Answer 2

The general rule of thumb is to use while if the thing you're working with is changing while you're using it:

while(my $line = <>)
while(my ($k, $v) = each %h)

but for or foreach otherwise:

foreach my $line (@lines)
for my $k (keys %h)
for(my $i = 0; $i < $pancakes; ++$i)

In the case of a file, using while makes sense because you (probably) don't want to slurp the whole thing into memory (except, of course, when you do want to). But, if you already have your data in memory, then for/foreach would be a more natural choice to iterate over it.

Answer 3

You use foreach to iterate over each element of an array or each key of a hash when the data is already in memory. You use while when the data is not yet in memory (and you may not even need to keep it all in memory at once).

my %hash;
while (<>)
{
    chomp;
    my($key, $value) = split /,/;
    $hash{$key} = $value;
}

foreach my $key (sort keys %hash)
{
     print "$key => $hash{$key}\n";
}