Exercises

Here are some exercises for you to try on your own. Solutions are available at http://pragprog.com/titles/gwpy3/practical-programming.

1. Write a function called find_dups that takes a list of integers as its input argument and returns a set of those integers occurring two or more times in the list.

2. Write the bodies of the new versions of functions read_molecule and read_all_molecules from ​Creating New Type Annotations​.

3. Python’s set objects have a method called pop that removes and returns an arbitrary element from the set. If the set gerbils contains five cuddly little animals, for example, calling gerbils.pop() five times will return those animals one by one, leaving the set empty at the end. Use this to write a function called mating_pairs that takes two equal-sized sets called males and females as input and returns a set of pairs; each pair must be a tuple containing one male and one female. (The elements of males and females may be strings containing gerbil names or gerbil ID numbers—your function must work with both.)

4. The PDB file format is often used to store information about molecules. A PDB file may contain zero or more lines that begin with the word AUTHOR (which may be in uppercase, lowercase, or mixed case), followed by spaces or tabs, followed by the name of the person who created the file. Write a function that takes a list of filenames as an input argument and returns the set of all author names found in those files.

5. The keys in a dictionary are guaranteed to be unique, but the values are not. Write a function called count_values that takes a single dictionary as an argument and returns the number of distinct values it contains. Given the input {’red’: 1, ’green’: 1, ’blue’: 2}, for example, it should return 2.

6. After doing a series of experiments, you have compiled a dictionary showing the probability of detecting certain kinds of subatomic particles. The particles’ names are the dictionary’s keys, and the probabilities are the values: {’neutron’: 0.55, ’proton’: 0.21, ’meson’: 0.03, ’muon’: 0.07, ’neutrino’: 0.14}. Write a function that takes a single dictionary of this kind as input and returns the particle that is least likely to be observed. Given the dictionary shown earlier, for example, the function would return ’meson’.

7. Write a function called count_duplicates that takes a dictionary as an argument and returns the number of values that appear two or more times.

8. A balanced color is one whose red, green, and blue values add up to 1.0. Write a function called is_balanced that takes a dictionary whose keys are ’R’, ’G’, and ’B’ and whose values are between 0 and 1 as input and that returns True if they represent a balanced color.

9. Write a function called dict_intersect that takes two dictionaries as arguments and returns a dictionary that contains only the key/value pairs found in both of the original dictionaries.

10. Programmers sometimes use a dictionary of dictionaries as a simple database. For example, to keep track of information about famous scientists, you might have a dictionary where the keys are strings and the values are dictionaries, like this:

    ​	{
    ​	​'jgoodall'​ : {​'surname'​ : ​'Goodall'​,
    ​	​'forename'​ : ​'Jane'​,
    ​	​'born'​ : 1934,
    ​	​'died'​ : None,
    ​	​'notes'​ : ​'primate researcher'​,
    ​	​'author'​ : [​'In the Shadow of Man'​,
    ​	​'The Chimpanzees of Gombe'​]},
    ​	​'rfranklin'​ : {​'surname'​ : ​'Franklin'​,
    ​	​'forename'​ : ​'Rosalind'​,
    ​	​'born'​ : 1920,
    ​	​'died'​ : 1957,
    ​	​'notes'​ : ​'contributed to discovery of DNA'​},
    ​
    ​
    ​	​'rcarson'​ : {​'surname'​ : ​'Carson'​,
    ​	​'forename'​ : ​'Rachel'​,
    ​	​'born'​ : 1907,
    ​	​'died'​ : 1964,
    ​	​'notes'​ : ​'raised awareness of effects of DDT'​,
    ​	​'author'​ : [​'Silent Spring'​]}
    ​	}

    Write a function called db_headings that returns the set of keys used in any of the inner dictionaries. In this example, the function should return set(’author’, ’forename’, ’surname’, ’notes’, ’born’, ’died’).

11. Write another function called db_consistent that takes a dictionary of dictionaries in the format described in the previous question and returns True if and only if every one of the inner dictionaries has exactly the same keys. (This function would return False for the previous example, since Rosalind Franklin’s entry doesn’t contain the ’author’ key.)

12. A sparse vector is a vector whose entries are almost all zero, like [1, 0, 0, 0, 0, 0, 3, 0, 0, 0]. Storing all those zeros in a list wastes memory, so programmers often use dictionaries instead to keep track of just the nonzero entries. For example, the vector shown earlier would be represented as {0:1, 6:3}, because the vector it is meant to represent has the value 1 at index 0 and the value 3 at index 6.

    1. The sum of two vectors is just the element-wise sum of their elements. For example, the sum of [1, 2, 3] and [4, 5, 6] is [5, 7, 9]. Write a function called sparse_add that takes two sparse vectors stored as dictionaries and returns a new dictionary representing their sum.

    2. The dot product of two vectors is the sum of the products of corresponding elements. For example, the dot product of [1, 2, 3] and [4, 5, 6] is 4+10+18, or 32. Write another function called sparse_dot that calculates the dot product of two sparse vectors.

    3. Your boss has asked you to write a function called sparse_len that will return the length of a sparse vector (just as Python’s len returns the length of a list). What do you need to ask her before you can start writing it?