Understanding matplotlib xticks syntax

Question

I am reading a book and I came across this code:

import matplotlib.pyplot as plt
plt.scatter(x,y)
plt.title(\"Web traffic over the last month\")
plt.xlabel(\"Tim         


        

Answer 1

range is a function in python2 which makes a list for the argument given to it:

range(5) -> [0,1,2,3,4]
range(1,5) -> [1, 2, 3, 4]

in general range(lower_index, upper_index+1) will generate a list equivalent to [ lower_index, upper_index] in python2,

you can use xrange for better performance ( as it's uses lazy evaluation, calculating when it is needed) or range in python3 will do the work as xrange in python2.

now for the line:

plt.xticks([w*24*7 for w in range(10)],['week %i'%w for w in range(10)])

actually xticks is the interval for your x axis ticks or measurement, so as your level of measurement is in hours so it is better to tick for each hour in a week (i.e. 7 days * 24 hours) for the week's in the data set, and the second list comprehension put's the label's for that one week interval( week 0, week 1 .....),

one point to notice is that actually the data set you have used from the book have 748 rows so approximately (748/(24*7)) = 4.45 weeks ,,

so you really can plot the graph using range(5), the reason the output plot is scaled to week0 - week4 is because of the line plt.autoscale(tight=True), well without plt.autoscale the plot would have shown something like this.

hope it helps.

Answer 2

In order to understand range, open python and write in sequence the following commands:

 range(7) 
 range(4,8) 
 range(3,11,2)

For the list comprehensions within the plt.xticks, they are basically a compact way of writing loops. They are very common, useful and neat. In order to understand them:

 [w*2 for w in range(10)] 
 [w*2 for w in range(10) if w < 4] 

Finally, for the command plt.xticks itself you can check http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.xticks for a very brief explanation with simple examples.