Perl 101 (Part 3) - Looping The Loop - Grade School (
Page 7 of 9 )
Before we go on to our last - also known as final - control structure of
the day, we're going to take a quick detour and introduce you to a
different kind of variable.
As you already know, Perl comes with scalar variables, which can be used to
store a single value. But what you may not know is that it also comes with
a mechanism to store multiple values in a single variable. This variable is
known as an "array variable", and it is a useful method of storing and
representing related information.
All the standard rules which apply when naming scalar variables also apply
in the case of array variables, with one important exception - where a
scalar variable name is preceded by a dollar [$] sign, an array variable
name is preceded by an @ symbol.
Note also that Perl does not place any restrictions on array and scalar
variables sharing the same name in Perl - so the array @stuff is different
from the scalar $stuff.
Now, let's say that you wanted to create an array containing the names of
your friends:
@friends = ("Rachel", "Monica", "Phoebe", "Chandler", "Joey", "Ross");
Thus the array variable @friends contains six elements.
Each element of the array is a scalar variable, and can be accessed using
scalar notation, with a suffix denoting the element's position in the
array. So, in order to extract the first element of the array @friends,
you'd use
$friends[0]
while
$friends[5]
would give you the sixth element of the array, Ross.
Similarly, if you wanted to modify a particular element of the array, you
could use the scalar notation above to accomplish your task, like this:
$friends[3] = "Janice";
Your array would now contain
("Rachel", "Monica", "Phoebe", "Janice", "Joey", "Ross");
Note that the first element of an array is always referred to by the index
0 - this concept, known as "zero-based indexing" often confuses novice
programmers, and is just one more reason why geeks have so few friends.
An array can contain both string and numeric data - for example, this is
perfectly valid:
@mix = ("hello", 34747, 3, "bonjour");
How about a quick example to illustrate how data can be stored in a single
array variable:
#!/usr/bin/perl
# this example demonstrates how a single
# array variable can hold a student's gradea
# in six subjects
# set up some variables
@subjectlist = ("Math", "Lit.", "Physics", "Biology");
@gradelist = ();
$total = 0;
# get and store input
for($x=0; $x<4; $x++)
{
print("What was your grade in $subjectlist[$x]? ");
$gradelist[$x] = <STDIN>;
chomp($gradelist[$x]);
}
# display it in neatly formatted rows
print ("SUBJECT\t\tGRADE\n");
for($y=0; $y<4; $y++)
{
print ("$subjectlist[$y]\t\t$gradelist[$y]\n");
$total += $gradelist[$y];
}
# print a total
print ("TOTAL: $total\n");
And here's what it looks like:
What was your grade in Math? 10
What was your grade in Lit.? 20
What was your grade in Physics? 30
What was your grade in Biology? 40
SUBJECT GRADE
Math 10
Lit. 20
Physics 30
Biology 40
TOTAL: 100
Let's walk you through this: we've begun by initializing two array
variables and one scalar variable. The array @subjectlist contains a list
of the subjects to be graded, and the array @gradelist will be populated by
the user with the actual grades. The scalar variable $total will be used to
display a total figure at the end.
Next, we've used a "for" loop to display a question, and assign the user's
input to the @gradelist array in the appropriate slot via the $x counter.
Once all four subjects are taken care of [note the conditional expression
in the "for" loop, which automatically stops looping when the counter
reaches 4], we've simply used the print() function and a second "for" loop
to display a neatly-tabulated row of grades and subjects.
The second loop also adds each grade to the variable $total, and this
displays this total value at the end of the program.
A couple of other points to note:
* The \t character used in the print() statements above is used to generate
a single "tab" space.
* The notation
$total += $gradelist[$y];
is simply a Perl shortcut for
$total = $total + $gradelist[$y];
This article copyright Melonfire 2000. All rights reserved.
