Dalziel2016_pseudo.md

Hints and pseudo code for Exercise 3.8.1 (data from Dalziel et al., 2016)

1. Write a program that extracts the names of all the cities in the database (one city per entry)

   Hints each city is reported several times: we need a way to remove duplicates. For example, we could use a set. We will need to read a csv file: this task is easier if you import the csv module.

   Pseudocode:

   import csv
   cities = an empty set
   open data for reading
   create dictionary reader
   for each row in the file
       add the city to the set

2. Write a program that creates a dictionary where the keys are the cities, and the values are the number of records (rows) for that city in the data.

   Hints now we need to keep track of the number of occurrences for each city. A good data structure for this task is a dictionary in which cities are the keys and the values store the number of occurrences. Note that each time you see a new city, you need to add it to the dictionary. To do this, you can use the method get for dictionaries, which allows you to initialize a default value for a key that is not already present in the dictionary. For example, the code:

   dd = {} # empty dictionary
   my_list = ['a', 'b', 'a', 'c', 'd', 'b', 'a']
   for element in my_list:
       dd[element] = dd.get(element, 0) + 1

   returns

   {'a': 3, 'b': 2, 'c': 1, 'd': 1}

   Pseudocode:

   import csv library
   citycount = an empty dictionary
   open file for reading
   set up dictionary reader
       for each line in data
   	my_city = extract the city
   	citycount[my_city] = use get to update value

3. Write a program that calculates the mean population for each city, obtained by averaging the value of pop.

   Hints to compute the mean, keep summing the population for the city, and at the end divide by the number of records. A good data structure for this problem would be a dicitionary in which cities are keys, and the total population and the number of occurrences are stored in a list.

   Pseudocode:

   import csv
   citypop = an empty dictionary
   open data file reading
   set up dictionary reader
   for each line in data
          my_city = extract the city
          my_pop = extract population
          if this is the first time you see this city, initialize:
   	 citypop[my_city] = [0.0, 0]
          citypop[my_city][0] = what it was before + my_pop
          citypop[my_city][1] = what it was before + 1
   
   for each city
       divide the first element by the second to obtain the mean

4. Write a program that calculates the mean population for each city and year.

   Hints while the task is similar to the exercise above, we need to build a more complex data structure. For each city and each year, we should find multiple records and we need to average the population for each year. A good data structure would be a dictionary where each key is a city, and the value is another dictionary where keys are years, and the values are a list storing the population for that year, and the number of occurrences.