Scalar Interpolation

Recall that when we discussed double-quotes strings (see Double-quoted Strings), we noted that we had to backslash the $ character (e.g., "\$"). Now, we discuss the reason that this was necessary. Any scalar variable, when included in a double-quoted string interpolates.

Interpolation of scalar variables allows us to insert the value of a scalar variable right into a double-quoted string. In addition, since Perl largely does all data conversion necessary, we can often use variables that have integer and float values and interpolate them right into strings without worry. In most cases, Perl will do the right thing.

Consider the following sample code:

     use strict;
     my $friend = 'Joe';
     my $greeting = "Howdy, $friend!";
                 # $greeting contains "Howdy, Joe!"
     my $cost = 20.52;
     my $statement = "Please pay \$$cost.\n";
              # $statement contains "Please pay $20.52.\n"
     my $debt = "$greeting  $statement";
              # $debt contains "Howdy, Joe!  Please pay $20.52.\n"
     

As you can see from this sample code, you can build up strings by placing scalars inside double-quotes strings. When the double-quoted strings are evaluated, any scalar variables embedded within them are replaced with the value that each variable holds.

Note in our example that there was no problem interpolating $cost, which held a numeric scalar value. As we have discussed, Perl tries to do the right thing when converting strings to numbers and numbers to strings. In this case, it simply converted the numeric value of 20.52 into the string value '20.52' to interpolate $cost into the double-quoted string.

Interpolation is not only used when assigning to other scalar variables. You can use a double-quoted string and interpolate it in any context where a scalar expression is appropriate. For example, we could use it as part of the print statement.

     #!/usr/bin/perl
     
     use strict;
     use warnings;
     
     my $owner  = 'Elizabeth';
     my $dog    = 'Rex';
     my $amount = 12.5;
     my $what   = 'dog food';
     
     print "${owner}'s dog, $dog, ate $amount pounds of $what.\n";
     

     Elizabeth's dog, Rex, ate 12.5 pounds of dog food.
     

Notice how we are able to build up a large string using four variables, some text, and a newline character, all contained within one interpolated double-quoted string. We needed only to pass one argument to print! Recall that previously (see Printing Numeric Literals) we had to separate a number of scalar arguments by commas to pass them to print. Thus, using interpolation, it is very easy to build up smaller scalars into larger, combined strings. This is a very convenient and frequently used feature of Perl.

You may have noticed by now that we did something very odd with $owner in the example above. Instead of using $owner, we used ${owner}. We were forced to do this because following a scalar variable with the character ' would confuse Perl. ¹ To make it clear to Perl that we wanted to use the scalar with name owner, we needed to enclose owner in curly braces ({owner}).

In many cases when using interpolation, Perl requires us to do this. Certain characters that follow scalar variables mean something special to Perl. When in doubt, however, you can wrap the name of the scalar in curly braces (as in ${owner}) to make it clear to Perl what you want.

Note that this can also be a problem when an interpolated scalar variable is followed by alpha-numeric text or an underscore. This is because Perl cannot tell where the name of the scalar variable ends and where the literal text you want in the string begins. In this case, you also need to use the curly braces to make things clear. Consider:

     use strict;
     
     my $this_data = "Something";
     my $that_data = "Something Else ";
     
     print "_$this_data_, or $that_datawill do\n"; # INVALID: actually refers
                                                   # to the scalars $this_data_
                                                   # and $that_datawill
     
     print "_${this_data}_, or ${that_data}will do\n";
                # CORRECT: refers to $this_data and $that_data,
                #          using curly braces to make it clear