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Perl. They allow you to build up complex databases while parsing text. It would be nearly impossible to use a standard array for this same task. We would first have to count the number of users in the input stream and then allocate an array of the appropriate size, assigning a position in the array to each user (through the use of a hash function or some other indexing scheme). An associative array, however, allows you to index data directly using strings and without regard for the size of the array in question. (Of course, performance issues always arise when attempting to use large arrays, but for most applications this isn't a problem.)

Let's move on. Line 14 uses the Perl foreach statement, which you may be used to if you write shell scripts. (The foreach loop actually breaks down into a for loop, much like that found in C.) Here, in each iteration of the loop, the variable $user is assigned the next value in the list given by the expression sort(keys %hours). %hours simply refers to the entire associative array hours that we have constructed. The function keys returns a list of all the keys used to index the array, which is in this case a list of usernames. Finally, the sort function sorts the list returned by keys. Therefore, we are looping over a sorted list of usernames, assigning each username in turn to the variable $user.

Lines 16 and 17 simply correct for situations where the number of minutes is greater than 60; it determines the total number of hours contained in the minutes entry for this user and increments hours accordingly. The int function returns the integral portion of its argument. (Yes, Perl handles floating-point numbers as well; that's why use of int is necessary.)

Finally, lines 19 to 22 print the total login time and number of logins for each user. The simple print function just prints its arguments, like the awk function of the same name. Note that variable evaluation can be done within a print statement, as on lines 19 and 22. However, if you want to do some fancy text formatting, you need to use the printf function (which is just like its C equivalent). In this case, we wish to set the minimum output length of the hours and minutes values for this user to two characters wide, and to left-pad the output with zeroes. To do this, we use the printf command on line 21.

If this script is saved in the file logintime, we can execute it as follows:

papaya$ last | logintime

User johnsonm, total login time 01:07, total logins 11.

User kibo, total login time 00:42, total logins 3.

User linus, total login time 98:50, total logins 208.

User mdw, total login time 153:03, total logins 290.

papaya$

Of course, this example doesn't serve well as a Perl tutorial, but it should give you some idea of what it can do. We encourage you to read one of the excellent Perl books out there to learn more.

More Features

The previous example introduced the most commonly used Perl features by demonstrating a living, breathing program. There is much more where that came from—in the way of both well-known and not-so-well-known features.

As we mentioned, Perl provides a report-generation mechanism beyond the standard print and printf functions. Using this feature, the programmer defines a report format that describes how each page of the report will look. For example, we could have included the following format definition in our example:

format STDOUT_TOP =

User Total login time Total logins

-------------- -------------------- -------------------

.

format STDOUT =

@<<<<<<<<<<<<< @<<<<<<<< @####

$user, $thetime, $logins{$user}

.

The STDOUT_TOP definition describes the header of the report, which will be printed at the top of each page of output. The STDOUT format describes the look of each line of output. Each field is described beginning with the @ character; @<<<< specifies a left-justified text field, and @#### specifies a numeric field. The line below the field definitions gives the names of the variables to use in printing the fields. Here, we have used the variable $thetime to store the formatted