Python 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
Add values to a list
Use the raw_input() function

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/python heights = [] amt = [] title = raw_input("Enter Tally Title: ") ht = 0 n = 0 #populate our lists while True: ht = raw_input("Enter height in inches (hit q to quit): ") if(ht == 'q'): break else: heights.append(ht) n = raw_input("Enter number of subjects for that height: ") amt.append(n) #print the lists print "--------------\n\n\n" print title print "\n" print "HEIGHT NUMBER" for i in range(0,len(heights)): print str(heights[i]) + ": " + str(amt[i])

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: loop in this script will never end unless a certain condition is met. 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 break statement. The break statement will end this otherwise infinite loop.
The raw_input Function - The raw_input function is the counter-part to the input function, which you should feel free to experiment with, but long story made short: the raw_input function will work for this assignment better than the input function.
Adding Values to a List - The append() function allows us to add values to a list. The most recently appended value occupies the last position in the list (unless we sort it or do something to alter the order of items in the list).

When it comes to converting a value in feet-inches format to straight inches, the following trick will come in handy. (Run this script to see the output it produces. Also change the input to 5'11" to see how it deals with a two-digit inches value.)

#!/usr/bin/python input = "5'3\"" print input feet = input[0] single = input.index("'") print single double = input.index('"') print double inches = input[single+1:double] print inches

ASSIGNMENT:

Extend the sample script so that it calculates the mean for your data points. Take input for height in feet and inches and convert it to straight inches. Report the mean after displaying your tally. (NOTE: You will need to use the int() and the float() functions in order to convert the input to numbers so you can perform the arithmetic.)