# 4.6: Dictionary Subtraction

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Finding the words from the book that are not in the word list from words.txt is a problem you might recognize as set subtraction; that is, we want to find all the words from one set (the words in the book) that are not in another set (the words in the list).

subtract takes dictionaries d1 and d2 and returns a new dictionary that contains all the keys from d1 that are not in d2. Since we don’t really care about the values, we set them all to None.

def subtract(d1, d2):
res = dict()
for key in d1:
if key not in d2:
res[key] = None
return res


To find the words in the book that are not in words.txt, we can use process_file to build a histogram for words.txt, and then subtract:

words = process_file('words.txt')
diff = subtract(hist, words)

print "The words in the book that aren't in the word list are:"
for word in diff.keys():
print word,


Here are some of the results from Emma:

The words in the book that aren't in the word list are:
rencontre jane's blanche woodhouses disingenuousness
friend's venice apartment ...


Some of these words are names and possessives. Others, like “rencontre,” are no longer in common use. But a few are common words that should really be in the list!

Exercise $$\PageIndex{1}$$

Python provides a data structure called set that provides many common set operations. Read the documentation at http://docs.python.org/2/library/stdtypes.html#types-set and write a program that uses set subtraction to find words in the book that are not in the word list.

Solution:

http://thinkpython.com/code/analyze_book2.py

This page titled 4.6: Dictionary Subtraction is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Allen B. Downey (Green Tea Press) .