Perl scalar Function

The Perl scalar function enables you to evaluate an expression in a scalar context and returns the value of that expression.

This short tutorial has the meaning to point out a few cases where and how you can use this function.

The syntax form of the Perl scalar function is as follows:

scalar EXPR

that will return the EXPR in scalar context.

One of the powers of the Perl language is the fact that it provides the variable context. A lot of functions and operators can operate either in scalar or list context.

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The context feature distinguishes the Perl language from other programming languages and helps you write more accurate code, closer to the human language.

And here comes the Perl scalar function that allows you to determine the context in which you want to evaluate an expression. You don’t need to explicitly use the scalar function anytime you invoke a scalar context, but it’s usually a good thing. For example, see the following code:

my @array1 = qw(one two);
my @array2 = qw(three four five);

# implicit scalar context (caused by the + operator)
print @array1 + @array2, "\n";

# explicit scalar context
print scalar @array1 + scalar @array2, "\n";

Both print function will display the number 5 which is the total number of elements of the two arrays (see also Get the size of an array example).

Generally, there’s no equivalent function to invoke a list context, it suffices to put an item enclosed in round parentheses, and you have a list context.

The Perl scalar function can be also assimilated with a unary operator.

Be cautious if you use the expression as a parenthesized list because this will behave as a scalar comma expression (in scalar context the comma ',' operator evaluates its left argument, throws that value away, then evaluates its second argument and so on, finishing by evaluating the last element in scalar context and returning this last element). For example, in the next code snippet:

my ($one, $two, $three) = (1, 2, 3);
print scalar ($one, $two, $three), "\n";

because of the comma operator used in scalar context, the print function will display the value of the last element of the list included in the parentheses, i.e. the value 3.

If you want to see the Perl scalar function in action, the following examples will show you some specific ways to use it.

Get the size of an array

Let’s suppose that you have the @colors array and you need to find the number of the array elements. One way to do this is to use the more cryptic notation $#colors which means the subscript, or index of the last element of the array. So, because the first element of an array has the index 0, the size of the array (or the number of its elements) is $#colors+1.

An alternative way is to use the Perl scalar function with @colors as argument; this will force the @colors to be interpreted in scalar context and it will return the size of the array.

Look at the below code for more details:

# initialize an array
my @colors = qw(cyan magenta white brown red);

print "The \@colors array has: ", $#colors+1, " elements\n";
# it prints: The @colors array has: 5 elements

print "The \@colors array has: ", scalar @colors, " elements\n";
# it prints: The @colors array has: 5 elements

Because scalar (@colors) – 1 is the index of the last element of the array, you can use this construct to traverse an array using foreach:

my @colors = qw(cyan magenta white brown red);
foreach my $color (0..scalar @colors - 1) {
  print $color + 1 . " $colors[$color]\n";

The output after running this code is as follows:

1 cyan
2 magenta
3 white
4 brown
5 red

Use the scalar function in conjunction with the <> operator

As you know, you can use the diamond operator <> to read from a file; if you don’t supply any handle to the file and any argument to the script in command line, the input operator will read from <STDIN> - either from the keyboard or from a redirected file.

You can use it like this:

my $str = <>;

In this case, because of the scalar context forced by the $str, the <> operator will read only one line from the keyboard.

If you have an array on the left side of the <> operator, Perl will read as many lines until the EOF is reached. Each line is stored as an independent element into the array:

my @array = <>;

But if you precede the <> operator with the Perl scalar function like in the following line of code:

my @array = scalar <>;

the Perl interpreter will read only one line and it will store it into the @array variable; this array will have only one element – the content of the line read from <STDIN>.

Use the scalar function in conjunction with the localtime function

Invoked in a list context, the localtime function returns a list. But if you invoke it in a scalar context, you’ll get a system formatted string. You can achieve this by using the Perl scalar function. If you invoke the localtime function without using the Perl scalar function, you’ll get a list of elements:

($sec, $min, $hour, $mday, $mon, $year, wday, $yday, $isdst)

  • $sec, $min, $hour are the seconds, minutes and hours of the specified time
  • $mday is the day of the month
  • $mon is the month in the range 0..11
  • $year is the number of years since 1900
  • $wday is the day of the week with 0 indicating Sunday
  • $yday is the day of the year in the range 0..364 (or 0..365 in leap years)
  • $isdst is true if the specified time occurs during Daylight Saving Time, false otherwise
If you use the Perl scalar function, you’ll get a readable time format. See the following lines of code:

my @time = localtime(time);
print "@time\n";

print scalar localtime(time);

A sample output could be as follows:

47 19 0 21 1 110 0 51 0
Sun Feb 21 00:19:47 2010

Please click here to download the Perl scalar script with all the above examples included.

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Table of Contents:

A Perl Script
Install Perl
Running Perl
Perl Data Types
Perl Variables
Perl Operators
Perl Lists
Perl Arrays
    Array Size
    Array Length
Perl Hashes
Perl Statements
    Perl if
    Perl unless
    Perl switch
    Perl while
    Perl do-while
    Perl until
    Perl do-until
    Perl for
    Perl foreach
Built-in Perl Functions
    Functions by Category
        String Functions
        Regular Expressions and Pattern Matching
        List Functions
        Array Functions
        Hash Functions
        Miscellaneous Functions
    Functions in alphabetical order
        scalar (more)

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