Learning Perl: 3.1. Accessing Elements of an Array

 

3.1. Accessing Elements of an Array

If you've used arrays in another language, you won't be surprised to find Perl provides a way to subscript an array to refer to an element by a numeric index.

The array elements are numbered using sequential integers, beginning at zero and increasing by one for each element, like this:

    $fred[0] = "yabba";    $fred[1] = "dabba";    $fred[2] = "doo";

The array name (in this case, "fred") is from a completely separate namespace than scalars use. You could have a scalar variable named $fred in the same program. Perl treats them as different things and doesn't get confused.^[*] (Your maintenance programmer might be confused though, so don't capriciously make all of your variable names the same.)

^[*] The syntax is always unambiguous; tricky perhaps, but unambiguous.

You can use an array element like $fred[2] in every place^[] where you could use any other scalar variable like $fred. For example, you can get the value from an array element or change that value by the same sorts of expressions we used in the previous chapter:

^[] Well, almost. The most notable exception is that the control variable of a foreach loop, which you'll see later in this chapter, must be a simple scalar. And there are others, like the "indirect object slot" and "indirect filehandle slot" for print and printf.

    print $fred[0];    $fred[2]  = "diddley";    $fred[1] .= "whatsis";

Of course, the subscript may be any expression that gives a numeric value. If it's not an integer, it'll automatically be truncated to the next lower integer:

    $number = 2.71828;    print $fred[$number - 1]; # Same as printing $fred[1]

If the subscript indicates an element that would be beyond the end of the array, the corresponding value will be undef. This is the same as ordinary scalars; if you've never stored a value into the variable, it's undef.

    $blank = $fred[ 142_857 ]; # unused array element gives undef    $blanc = $mel;             # unused scalar $mel also gives undef