Scalar Value Constructions

Perl Data Types – Part 3

Perl Course

Foreword: In this part of the series, I talk about scalar value constructions.

By: Chrysanthus Date Published: 22 May 2015

Introduction

This is part 3 of my series, Perl Data Types. In this part of the series, I talk about scalar value constructions. All data in Perl is a scalar, an array of scalars, or a hash of scalars. A scalar contains one single value in any of three different flavors: a number, a string, or a reference. A scalar variable is preceded by $. You should have read the previous parts of the series before reaching here, as this is a continuation.

Number
An integer is a whole number. A number can be an integer, as the following statement shows:

    $num = 54;

A number with a decimal point is a floating-point number. A number in Perl can be a float as the following statement shows:

    $num = 23.48;

In Perl, if a number is long, you do not use commas to group the digits in threes; you use but the underscore, as in the following statement:

    my $num = 4_294_967_296;

This is the number, 4294967296, written on paper as 4,294,967,296, but written in Perl as 4_294_967_296 or 4294967296 (without any separation).

You can write a number in Perl in what is known as Perl’s Exponential Form. The number 256.39 can be written in Perl’s exponential form as,

    2.5639E+2

To code a number this way, you begin with a digit. This is followed by a decimal point. After that you have the rest of the digits without any other decimal point. Then you have the letter, E. This is followed by + or - .You type + if the decimal point has to be moved to the right to reach its original position. You type – if the decimal point has to be moved to the left to reach its original position. Then you type a number for the number of digits the decimal point has to be carried over to reach its original position.

String
In Perl, a normal string literal is text in double quotes or single quotes. For a double quoted string, a variable in the string is substituted by its value, and an escape sequence in the string takes its effect. For a single quoted string, a variable is not substituted by its value and an escape sequence does not take effect, except for \' and \\ .

So for the code segment,

    my $quantity = 50;
    my $str = "I have $quantity of them.\nYou have nothing.";
    print $str;

The string would be printed as:

    I have 50 of them.
    You have nothing.

At the output, $quantity has been replaced by 50 and the \n has had its effect creating a new line. Neither $quantity nor \n could have caused any change if the string was within single quotes.

Try the following code where the string is delimited by single quotes:

use strict;

    my $quantity = 50;
    my $str = 'I have $quantity of them.\nThe \\ and \' take effect';
    print $str;

The output is:

    I have $quantity of them.\nThe \ and ' take effect

Since the string is within single quotes, $quantity has not been expanded (replaced by its value); \n has not had its effect. The single quote exceptions of \\ and \' have had their effects: \\ is the escape sequence for \ and \ alone has been displayed; \' is the escape sequence for ' and only ' has been displayed.

Special Literals
Special literals are: __FILE__, __LINE__, __PACKAGE__, __SUB__, __END__ and __DATA__ . Here I will only talk about __FILE__ and __LINE__ . I will talk about the rest later as we go along in the course. Each of these literals has double underscores, __.

__FILE__ represents the current filename. I tried

    print __FILE__ ;

in my computer and I had

    C:\sample.pl

where C:\ is the directory path and sample.pl is the name of the file.

__LINE__ represents the current line number in the file. I tried

    print __LINE__ ;

in my computer and I had

    10

because the print statement was typed at line 10 in the file, counting from the top. Note: line counting begins from 1 and not zero.

Reference
A reference is the equivalent of the starting address of an entity in memory. Try the following code:

use strict;

    my $myVar = "We are the world.";
    my $item  = \$myVar;
    print $item, "\n";

In memory, you have the entity, "We are the world.", which is identified by the variable, $myVar. To obtain a reference to this scalar entity, precede the $myVar variable with \ . The return value of that is held by the scalar variable, $item in the code. This is not holding a normal value (string or number); it is holding a reference, which is a kind of scalar. In my computer the printing of $item gives,

    SCALAR(0xe2fb8c)

which is of the starting address of the string entity.

Bareword
A bareword can be considered as a stray word in code, not preceded by $ or @ or % or &. Its presence is an error.

That is it for this part of the series. We stop here and continue in the next part.

Chrys

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