Making a histogram in Python

Question

I have been trying to make a histogram using the data given in survey below.

#represents the "information" (the summarized data)
ranking = [0,0,0,0,0,0,0,0,0,0,0]
survey = [1,5,3,4,1,1,1,2,1,2,1,3,4,5,1,7]

for i in range(len(survey)):
    ranking[survey[i]]+=1

#create histogram
print("\nCreating a histogram from values: ")
print("%3s %5s %7s"%("Element", "Value", "Histogram"))
for i in range(len(ranking)):
    print("%7d %5d %-s"%(i+1, ranking[i+1], "*" * ranking[i+1]))

Here is exactly what the shell displays when I run my code:

Creating a histogram from values: 
Element Value Histogram
      1     7 *******
      2     2 **
      3     2 **
      4     2 **
      5     2 **
      6     0 
      7     1 *
      8     0 
      9     0 
     10     0 
Traceback (most recent call last):
  File "C:\folder\file23.py", line 17, in <module>
    print("%7d %5d %-s"%(i+1, ranking[i+1], "*" * ranking[i+1]))
IndexError: list index out of range

My expected output is the above thing just without the traceback.

The shell is displaying the right thing, I'm just unsure of the error message. How can I fix this?

Answer 1

When i reaches its highest value, len(ranking) - 1, your use of ranking[i+1] is clearly "out of range"! Use range(len(ranking) - 1 in the for loop to avoid the error.

The counting can be simplified, too:

import collections
ranking = collections.Counter(survey)

for i in range(min(ranking), max(ranking)+1):
    print("%7d %5d %-s"%(i, ranking[i], "*" * ranking[i]))

Here you need min and max because Counter is mapping-like, not sequence-like. But it will still work fine (and you can use range(0, max(ranking)+1) if you prefer!-)

Answer 2

Your index i is incremented up to len(ranking)-1, which is the last valid index in ranking, but you're trying to access ranking[i+1], hence the IndexError.

Fix:

for i in range(len(ranking)-1):