Perl Variables: Comparing Scalars

Size counts, often, when you must weigh one value against another. Returning to our currency conversion example, you may wish to determine whether the Canadian dollar figure is a greater or lesser number than its American dollar equivalent. When you compare two values in Perl, the result is either true or false. Using comparison operators you specify the criterion for the comparison.

For example, suppose that $USTotal contains the value 50 and $CDNTotal possesses 67.5. Using the arithmetic greater-than operator (>):

$USTotal > $CDNTotal	yields	false
$CDNTotal > $USTotal	yields	true

Typically, you would include one of these comparisons inside a larger expression, which makes use of the result. A simple example would be an assignment to a third variable:

$USgreater=($USTotal>$CDNTotal);

In a later line of code you might test whether $USgreater contained true or false, as a result of the above comparison. More commonly, though, these types of comparisons are used in conditional statements to direct the flow of the Perl program. We haven't yet looked at conditional statements, but they should be quite familiar to readers with basic programming experience. Non-programmers can simply read the example aloud to get an idea of the logic behind this code:

if ($USTotal>$CDNTotal)
 { ...execute some piece of code... }
else
 { ...execute some other piece of code... }

Now suppose that we wanted to test whether $USTotal and $CDNTotal were equal. Using the arithmetic equality operator, a double equal sign (==):

$USTotal == $CDNTotal

yields

false

Of course, if both $USTotal and $CDNTotal contained the same value, the result of this equality comparison would have been true. Programmers note: the arithmetic comparison operator is a double equal sign, not just one. Several other programming languages use a single equal sign to represent both the comparison operator and the assignment operator, and forgetting to use the double equal sign is a common cause of program errors in Perl scripts.

Recall our earlier discussion about data types, and how Perl infers the data type based on the operator being used. In the above example, we are comparing whether one value is greater than another -- this only makes sense with numbers, since we cannot say that "dog" is greater than "cat" (well, some might say that, but they would be asking for trouble). In fact, Perl can make this comparison, but it does so using different logic and different operators.

As far as Perl is concerned, "dog" is greater than "cat" because it comes after cat alphabetically. Conversely, we can say that "cat" is less than "dog". The string greater-than operator in Perl is "gt", and the string equality operator is "eq", thus if $petA contained "cat" and $petB contained "dog" we could compare:

$petA gt $petB	yields	false
$petA eq "cat"	yields	true

There are many possible reasons to compare variables -- this tutorial is not in-depth enough to explore these in detail. The table below summarizes the common comparison operators, which you can use when and if you encounter a situation where such a comparison would be useful.

Table 1. Basic Perl Operators
The Perl You Need to Know
Table 2. Perl Comparison Operators