How are values ​​printed in a foreach statement?

Question

I am very new to perl programming.

When reading about loops for a foreach loop, I got two examples.

One example:

foreach ('hickory','dickory','doc') { print $_; print "\n"; } 

Output: -

 hickory dickory doc 

The $ _ variable contains each element. So he prints.

In another example, they said they did not specify the $ _ variable in the print statement. Only an empty print statement. How he prints foreach arguments.

 foreach ('hickory','dickory','doc') { print; print "\n"; } 

Output: -

 hickory dickory doc 

For this, too, the same conclusion. How he prints values. In this book, they did not give any explanation for this. They searched me on the Internet. But I can’t find anything.

Answer 1

Your question about print in foreach will be answered, here is a bit more on $_ .

From General Variables in perlvar

Here are the places where Perl will accept $_ , even if you are not using it:

• The following functions use $_ as the default argument:

  • abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf, quotemeta, readlink, readpipe, ref, require, reverse (only in a scalar context), rmdir, say, sin, split (for the second argument), sqrt, stat, study, uc, ucfirst, unlink, unpack.

• All file checks ( -f , -d ), except for -t , which defaults to STDIN. See -X

• Pattern matching operations m// , s/// and tr/// (aka y/// ) when used without the operator =~ .

• The default iterator variable in the foreach , unless another variable is specified.

• The implicit iterator variable in grep() and map() .

• Implicit variable given() .

• The default location for placing the next value or input record when the result of the <FH> , readline , readdir or each operation is checked by itself as the only criterion for the while test. Outside of the while tag, this will not happen.

• $_ is a global variable by default.

As you can see, it is available almost everywhere, and it is really used a lot. Note that the perlvar page describes a lot more similar variables, many of which are well known.

Here is an example. Think that we are reading lines from a file, we want to discard those that have only spaces, or begin with # (comments), and for others, separate them with spaces into words.

 open my $fh, '<', $file or die "Can't open $file: $!"; while (<$fh>) { next if not /\S/; next if /^\s*#/; my @words = split; # do something with @words ... } 

Let's see how many uses of $_ are in the above example. Here is the equivalent program

 while (my $line = <$fh>) { next if not $line =~ m/\S/; # if not matching any non-space character next if $line =~ m/^\s*#/; # if matching # after only (possible) spaces my @words = split ' ', $line; # split $line by ' ' (any white space) # do something with @words ... } 

Compare these two

• the file identifier read <$fh> in while assigns the $_ condition available in the loop.

• The regex matching operator defaults to $_ . m itself can be discarded.

• split by default splits $_ . We also use a different default value, because the pattern breaks the string into ' ' (any amount of any space).

• as soon as we do $line = <$fh> deal with $_ off (it is undefined in the loop), and we should use $line everywhere. So either do this or do while (<$fh>) and use $_ .

To illustrate all this a bit further, we find the longest headword on each line

 use List::Util 'max'; my $longest_cap = max map { length } grep { /^[AZ]/ } @words; 

grep takes the list in @words and applies a block to each element. Each element is assigned $_ and, therefore, is available for code inside the block as $_ . This is what regex uses by default. Those that satisfy the condition are passed to map , which also iterates over, assigning them $_ , which of course is the default value for length . Finally, max from List :: Util selects the largest.

Note that $_ never written and no temporary variable is required.

Here are some of the relevant documents. Perlop 's I / O statements discuss while (<$fh>) and all sorts of related things. Part of the regular expression is in the Regexp Quote-Like Operators in perlop and in perlretut . Also see split .

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The default values ​​are used regularly and for reading code written by others, you should understand them. When you write your own code, you can choose whether to use $_ or not, since you can always enter a lexical variable instead.

So, when to use $_ by default (which does not need to be written), and if not?

The proper use of the default values, $_ in particular, can lead to clearer and more understandable code. Which generally means better code. But it is possible to push it too far and end with a dark, complex and fragile code. Good taste is needed.

Another case is that some parts of the code benefit from $_ for their default values, while in other places you need to explicitly use $_ . I would say that if $_ visible more than once or twice in a section of code, it means that there should be a properly named variable instead.

In general, if in doubt, just name everything.

Answer 2

If you do not declare any variable in the foreach , it sets $_ by default

From perldoc about foreach :

The foreach keyword is actually synonymous with the for keyword, so you can use either. If VAR is omitted, $ _ is set for each value.

So this explains the first cycle.

The second loop, as you already know, now that $_ is specified with each element from your array, it will work because you omit $ var .

You can use the foreach loop with an explicit variable as follows:

 foreach my $item ( @list ) { print "My item is: $item\n"; } 

Or you can omit, just like you, and print will still work like @Dada, because:

If FILEHANDLE is omitted, the output handle is printed to the last selected (see selection). If LIST is omitted, prints $ _ to the currently selected output descriptor.

Answer 3

I will explain why you get the same results with different syntax:

If you omit the control variable from the beginning of the foreach loop, Perl uses its favorite default variable, $ _. This (basically), like any other scalar variable, except for its unusual name. For instance:

 foreach ('hickory','dickory','doc') { print $_; print "\n"; } 

Output:

 hickory dickory doc 

Although this is not Perls, only the default is a long snapshot, its pearls are the most common by default. You will see many other cases where Perl will automatically use $ _ if you do not tell it to use any other variable or value, thereby relieving the programmer of the hard work of having to invent and enter a new variable name. In order not to keep you busy, one of these cases is print, which will print $ _ unless another argument is specified:

 foreach ('hickory','dickory','doc') { print; # prints $_ by default print "\n"; } 

Output: -

 hickory dickory doc