Printing an array within double quotes

Question

my @a = (1,2,3,4,5);

print @a;      #output: 12345

print "\n";

print "@a";    #output: 1 2 3 4 5

Printing an array by putting its name withing double quotes puts a space between each index's value at output. How does it do it? Why doesn't print @a; prints the same? What's the need of both types? I mean when will you use print @a; instead of print "@a"; and vice versa.

Answer 1

Even better question: Why doesn't it print something like array{0x1232ef}. Print is suppose to print a string output and @a isn't a scalar.

Heck, even better: This is a scalar context, so why not print 5 which is the number of elements in the array. This is how:

print scalar @a;

would print.

Instead, the print command is taking some liberties to try to do what you intended and not what you said you want.

Let's take a look at this little program:

@a = qw(a b c d e);

print "@a";         #prints "a b c d e"
print "\n";

print @a;           #prints "abcde"
print "\n";

print @a . "\n";    #prints "5"

print scalar @a;    #prints "5"

Notice that print @a prints abcde, but if I add a \n on the end, it then prints @a in a scalar context.

Take a look at the Perldoc on print (try the command perldoc -f print. On most systems, the entire Perl documentation is available via perldoc)

* print LIST
* print

Prints a string or a list of strings. Returns true if successful[...]

Ah! If given a list, it'll print a list of strings.

The current value of $, (if any) is printed between each LIST item. The current value of $\ (if any) is printed after the entire LIST has been printed. Because print takes a LIST, anything in the LIST is evaluated in list context, including any subroutines whose return lists you pass to print.

Let's try a new program:

@a = qw(a b c d e);

$, = "--";
print "@a";         #prints "a b c d e"
print "\n";

print @a;           #prints "a--b--c--d--e"
print "\n";

print @a . "\n";    #prints "5"

print scalar @a;    #prints "5"

Hmmm... The $, added the double dashes between the list elements, but it didn't affect the @a in quotes. And, if $, is mentioned in the perldoc, why is everyone prattling about $"?

Let's take a look at perldoc perlvar

* $LIST_SEPARATOR
* $"

When an array or an array slice is interpolated into a double-quoted string or a
similar context such as /.../ , its elements are separated by this value. Default
is a space. For example, this:

print "The array is: @array\n";

is equivalent to this:

print "The array is: " . join($", @array) . "\n";

Mnemonic: works in double-quoted context.

So, that explains everything!

The default of $" is a single space, and the default of $, is null. That's why we got what we got!

One more program...

@a = qw(a b c d e);

$, = "--";
$" = "++";
print "@a";         #prints "a++b++c++d++e"
print "\n";

print @a;           #prints "a--b--c--d--e"
print "\n";

print @a . "\n";    #prints "5"
print scalar @a;    #prints "5"