Perl Basics and Biostatistics

Tallying Results

In this lesson the student will learn how to:

Implement a never ending loop
End a loop at any point within it's execution
Use the tab symbol
Use the chop function
Increment a scalar
Add values to a list

By the end of this lesson the student will be able to:


  Write a script which tallies measurments and calculates the 
  mean using this tally.

Let's say that the state decides for some reason that it needs to know the height of all the nine-year-old girls in the town of Trona. So, some teacher or administrator or public health official is given the task of measuring all the nine-year-old girls attending Trona Elementary School. In the process of recording these measurements this person might create a tally that looks something like this:


Height of Nine-Year-Old Girls in Trona, 2003:

 height    number
   3'11"     1
   4' 0"     2
   4' 1"     1
   4' 2"     0
   4' 3"     2
   4' 4"     2
   4' 5"     1
   4' 6"     0
   4' 7"     1
   4' 8"     0
   4' 9"     2
   4'10"     1
   4'11"     0
   5' 0"     0
   5' 1"     1

Using this data we can easily calculate an average height after converting it into inches.


47+48+48+49+51+51+52+52+53+55+57+57+58+61 = 739
  739 / 14 = 52.79

Here's a little script which may make tallying our information a little easier:

#!/usr/bin/perl @keys = (); @values = (); print "Enter Tally Title: "; $title = <STDIN>; chop($title); #populate our lists while(true){ #endless while loop print "Enter height in inches: "; $ht = <STDIN>; chomp($ht); if($ht eq 'q'){ last; } print "Enter number of subjects for that height: "; $n = <STDIN>; chomp($n); push(@keys, $ht); push(@values, $n); } #print results $cnt = 0; print "\n\n$title\n\n"; print "HEIGHT\tNUMBER\n"; while($cnt < scalar(@keys) ){ print "@keys[$cnt]\t@values[$cnt]\n"; $cnt++; }

Sample output:


Height of Dwarfs in Fairyland

HEIGHT  NUMBER
14      2
16      3
17      1
19      2
21      1

Inspect the script shown above (after executing it a few times) and consider the following points:

Infinite Loop - The while(true) { #do stuff } loop in this script will never end. It will continue on forever (or until the power goes out). The only thing that will stop it (besides pulling the plug out of the back of your computer or an interrupt signal from the operating system) is if the user enters a single 'q' when prompted to enter a height. This will trigger a call to the last function. The last function will end this otherwise infinite loop.
Using TABS - Sometimes you want to create rows when printing your output. The \t symbol is useful for this. You can use two \t symbols in a row to create a double tab if you want.
The chop Function - The chop function just chops off the last character in a string. In this script it will chop off the \n character in the $title varible.
Auto-incrementation - The $cnt++; statement is a shorthand way of adding one to the $cnt variable. The $cnt is used as a counter in the last while loop. This loop could be made into an infinite loop if we didn't have the $cnt++; statement inside it because without the $cnt++; statement the value of $cnt will never become greater than the length of @keys which is the condition which ends the loop!
The scalar Function - The scalar function returns the length of a list.
Adding Values to a List - First of all we create an empty list like this: "@vals = ( );". We populate this list using the push function. The push function takes two arguments: The first is the list into which we add the second argument.

ASSIGNMENT:

Extend the sample script so that it calculates the mean for your data points. Report the mean after displaying your tally.