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May 17, 2018
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Combined files and strings section into a tutorial on survey analysis… #24

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7 changes: 2 additions & 5 deletions _config.yml
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Original file line number Diff line number Diff line change
Expand Up @@ -18,11 +18,8 @@ map:
- title: Introducing IPython Notebook
path: /core/notebook.html
caption: A whole new way to work with Python!
- title: Working With Text Files
path: /core/text-files.html
caption: What is a text file? How do we get them in and out of Python?
- title: Working With Strings
path: /core/strings.html
- title: A typical problem -- Analyzing a survey
path: /core/survey.html
caption: Once we have our text in Python, what can we do with it?
- title: Creating Charts
path: /core/charts.html
Expand Down
167 changes: 88 additions & 79 deletions core/strings.md → core/survey.md
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@@ -1,13 +1,11 @@
---

layout: ots
title: Working with Strings
title: A typical problem -- Analyzing a survey

---

# A problem

Now we know how to work with text files, we'll use that knowledge to solve a problem:
# Our very, very important problem

Suppose you're a greengrocer, and you run a survey to see what radish varieties your customers prefer the most. You have your assistant type up the survey results into a text file on your computer, so you have 300 lines of survey data in the file [radishsurvey.txt](../files/radishsurvey.txt). Each line consists of a name, a hyphen, then a radish variety:

Expand All @@ -26,6 +24,8 @@ Suppose you're a greengrocer, and you run a survey to see what radish varieties

<a href="http://www.flickr.com/photos/brixton/2045816352/" title="Radishes radishes radishes by brixton, on Flickr"><img src="http://farm3.staticflickr.com/2298/2045816352_25cba9e434_m.jpg" width="240" height="180" alt="Radishes radishes radishes"></a>

(You may have noticed that this is a very simple file: Unlike on a document or web page, there is no formatting whatsoever. It doesn't look pretty, but it has one big advantage: This is the simplest type of text format to work with on a computer, so it is also the most easy to process and analyze.)

You want to know:

* What's the most popular radish variety?
Expand All @@ -41,24 +41,26 @@ You want to know:

Save the file [radishsurvey.txt](../files/radishsurvey.txt) to your computer. How do we write a program to find out which person voted for each radish preference?

From the previous chapter, we know that we can easily go through the file line by line, and each line will have a value like `"Jin Li - White Icicle\n"`. We also know that we can strip off the trailing newline with the `strip()` method:
We can easily open the file with Python and go through the file line by line. Each line will have a value like `"Jin Li - White Icicle\n"`. Then we can strip off the trailing newline with the `strip()` method. (If you are curious, you can look at the documentation for [open](https://docs.python.org/3/tutorial/inputoutput.html#reading-and-writing-files) and [split](https://docs.python.org/3/library/stdtypes.html?highlight=strip#str.strip) )

for line in open("radishsurvey.txt"):
line = line.strip()
# Do something with each line
whith open("radishsurvey.txt") as file:
for line in file:
line = line.strip()
# Do something with each line

We need a way to split each line into the name and the vote. Thankfully, Python comes with dozens of string methods including one called `split()`. [Have a look at the documentation for split()](http://docs.python.org/3.3/library/stdtypes.html#str.split) and see if you can figure out how to split each line into the name and the vote.

(Don't worry if you can't write a program that does this just yet, but at least have a think about it before you skip to the solution.)

### Solution

for line in open("radishsurvey.txt"):
line = line.strip()
parts = line.split(" - ")
name = parts[0]
vote = parts[1]
print(name + " voted for " + vote)
whith open("radishsurvey.txt") as file:
for line in file:
line = line.strip()
parts = line.split(" - ")
name = parts[0]
vote = parts[1]
print(name + " voted for " + vote)

There's a few things going on here, so let's go through it line by line. *Walking through a program in your head and thinking about what each line does by itself is a good way to start to understand it*

Expand Down Expand Up @@ -105,12 +107,13 @@ Use the previous example as a base. You'll need to compare the vote with the str

### Solution

for line in open("radishsurvey.txt"):
line = line.strip()
parts = line.split(" - ")
name, vote = parts
if vote == "White Icicle":
print(name + " likes White Icicle!")
with open("radishsurvey.txt") as file:
for line in file:
line = line.strip()
parts = line.split(" - ")
name, vote = parts
if vote == "White Icicle":
print(name + " likes White Icicle!")

You might notice that the code splitting the line has become even shorter here. Instead of assigning each element of parts separately, we can assign them together using a technique called "multiple assignment". The line `name, vote = parts` means to assign each variable to the corresponding item in the list.

Expand Down Expand Up @@ -159,11 +162,12 @@ Use your previous solution as a base. You'll need a variable to hold the number

print("Counting votes for White Icicle...")
count = 0
for line in open("radishsurvey.txt"):
line = line.strip()
name, vote = line.split(" - ")
if vote == "White Icicle":
count = count + 1
whith open("radishsurvey.txt") as file:
for line in file:
line = line.strip()
name, vote = line.split(" - ")
if vote == "White Icicle":
count = count + 1
print(count)


Expand All @@ -178,11 +182,12 @@ Using your function, can you write a program which counts votes for White Icicle
def count_votes(radish):
print("Counting votes for " + radish + "...")
count = 0
for line in open("radishsurvey.txt"):
line = line.strip()
name, vote = line.split(" - ")
if vote == radish:
count = count + 1
whith open("radishsurvey.txt") as file:
for line in file:
line = line.strip()
name, vote = line.split(" - ")
if vote == radish:
count = count + 1
return count

print(count_votes("White Icicle"))
Expand Down Expand Up @@ -241,15 +246,16 @@ Remember that for dictionaries `counts[vote]` means "the value in `counts` which
# with vote counts
counts = {}

for line in open("radishsurvey.txt"):
line = line.strip()
name, vote = line.split(" - ")
if vote not in counts:
# First vote for this variety
counts[vote] = 1
else:
# Increment the vote count
counts[vote] = counts[vote] + 1
whith open("radishsurvey.txt") as file:
for line in file:
line = line.strip()
name, vote = line.split(" - ")
if vote not in counts:
# First vote for this variety
counts[vote] = 1
else:
# Increment the vote count
counts[vote] = counts[vote] + 1
print(counts)

### Pretty printing
Expand Down Expand Up @@ -319,17 +325,18 @@ There are lots of functions which could remove the case distinction. `str.lower(
# with vote counts
counts = {}

for line in open("radishsurvey.txt"):
line = line.strip()
name, vote = line.split(" - ")
# munge the vote string to clean it up
vote = vote.strip().capitalize()
if not vote in counts:
# First vote for this variety
counts[vote] = 1
else:
# Increment the vote count
counts[vote] = counts[vote] + 1
whith open("radishsurvey.txt") as file:
for line in file:
line = line.strip()
name, vote = line.split(" - ")
# munge the vote string to clean it up
vote = vote.strip().capitalize()
if not vote in counts:
# First vote for this variety
counts[vote] = 1
else:
# Increment the vote count
counts[vote] = counts[vote] + 1
print(counts)

If you're having trouble spotting the difference here, it's
Expand Down Expand Up @@ -386,24 +393,25 @@ This is just one of many ways to do this:
# Create an empty list with the names of everyone who voted
voted = []

for line in open("radishsurvey.txt"):
line = line.strip()
name, vote = line.split(" - ")
# clean up the person's name
name = name.strip().capitalize().replace(" "," ")
# check if this person already voted
if name in voted:
print(name + " has already voted! Fraud!")
continue
voted.append(name)
# munge the vote string to clean it up
vote = vote.strip().capitalize().replace(" "," ")
if not vote in counts:
# First vote for this variety
counts[vote] = 1
else:
# Increment the vote count
counts[vote] += 1
whith open("radishsurvey.txt") as file:
for line in file:
line = line.strip()
name, vote = line.split(" - ")
# clean up the person's name
name = name.strip().capitalize().replace(" "," ")
# check if this person already voted
if name in voted:
print(name + " has already voted! Fraud!")
continue
voted.append(name)
# munge the vote string to clean it up
vote = vote.strip().capitalize().replace(" "," ")
if not vote in counts:
# First vote for this variety
counts[vote] = 1
else:
# Increment the vote count
counts[vote] += 1

print("Results:")
print()
Expand Down Expand Up @@ -473,15 +481,16 @@ This is just one possible way to break it down:
counts[radish] = counts[radish] + 1


for line in open("radishsurvey.txt"):
line = line.strip()
name, vote = line.split(" - ")
name = clean_string(name)
vote = clean_string(vote)

if not has_already_voted(name):
count_vote(vote)
voted.append(name)
whith open("radishsurvey.txt") as file:
for line in file:
line = line.strip()
name, vote = line.split(" - ")
name = clean_string(name)
vote = clean_string(vote)

if not has_already_voted(name):
count_vote(vote)
voted.append(name)

print("Results:")
print()
Expand Down Expand Up @@ -524,7 +533,7 @@ The loop shown above keeps track of one name, `winner_name`, and the number of v

## Challenge

Can you refactor the part of the program that finds the winner into a function?
Can you extract the part of the program that finds the winner into a function?

## Bigger Challenge

Expand All @@ -534,4 +543,4 @@ Can you write a winner function that could deal with a tie?

## Next Chapter

When you're done counting radish votes, the next chapter is [Creating Charts](charts.html)
That became complicated pretty quickly, didn't it? In the next chapter, we will try an easier way to [analyze the survey using pandas](pandas.html), a Python library designed for data analysis.
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