Perl foreach file

Here's some sample Perl code where I show how to loop through all the elements of a hash in Perl, using the Perl foreach operator. I've put a few comments in the code to help explain it:. I just added the extra line to make this Perl hash print example a little more clear. Here's a very similar example, where I again loop through all the elements of a Perl hash, this time using the each and while operators to traverse the hash:. I haven't tested the speed or memory use of these two approaches, but I have read that this Perl while loop approach is preferred for larger hashes.

However, as always, your mileage may vary, and if you're concerned about performance you should test this on your own system and your own Perl hash.

In Perl, foreach is defined as a looping statement which can traverse or iterate through a given list or set of elements one at a time and is the only known for traversing the given set of items when we want to iterate over the items of the set instead of iterating with the entire range given which is done automatically by a foreach loop.

In general, we can define foreach for iterating through the set of elements where the variables containing values of these elements of the given set one at a time are sometimes referred to as for loop in Perl and other programming languages. In this article, we will see foreach loop in Perl, which is mainly used whenever we want to traverse through a given array or list or set of items which means it iterates or traverses the set of values of elements and will further set the variables to each element in the list or set of elements which would be easy for accessing each element one at a time using this foreach loop.

In Perl, for and foreach are interchangeable. From the above syntax, we can see that a list is passed where the values of elements in this list are iterated one by one, so the foreach loop in Perl returns each value of the elements in the list and prints the values one by one on the output screen.

In Perl, another function is provided where it can flatten multiple lists into a single big list that can be specified within the parenthesis separated by a comma with each list names. In the above program, we can see we have declared two lists, one having string and the other having numerical. Now we will see another simple example where we can rewrite the above code; instead of calling foreach twice, we can just specify the list names. Change Language. Related Articles.

Control Flow. OOP Concepts. Regular Expressions. File Handling. CGI Programming. Table of Contents. Improve Article. Save Article. Like Article. Last Updated : 16 Jun, Perl program to illustrate. It does not use the next item in the list though. After getting a blank line when it tries to read the second line, it tries the second line again:. A better style might be to read lines with while to the point that lines is large enough:. The work with labels and nested loops.

You can read more about them in perlsyn or Learning Perl. Since foreach goes through each element of a list, some people reach for it when they want to go through each line in a file:. This is usually not a good idea. The foreach needs to have to entire list all at once.

This means that the foreach reads in all of standard input before it does anything. Or worse, it tries to completely read terabytes of data from that filehandle.

Memory is cheap, but not that cheap. A suitable replacement is the while idiom that reads and processes one line at a time:. This is really a shortcut for an assignment in scalar context.

That reads only one line from the filehandle:.