Programmeerimine 2018/19 kevad

Before session 13

Dictionaries

Before the next class session, watch the videos about dictionaries, data structures that are useful for storing related sets of data:

Quiz

Go to Moodle and take the test on video lectures under Session 13.

Examples

This week we are going to get an insight into dictionaries.

Example 1. Meetings

The file meetings.txt contains records of meetings. Each number in the file indicates a day of the week when an appointment takes place (not the number of appointments): 1 means Monday, 2 means Tuesday etc. The following program reads the data from the file, creates a dictionary and prints out its keys and the values. The keys contain the days of a week, and the values are the numbers of planned appointments on that day.

def day_of_week(n):
    days = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']
    return days[n-1]

ffile = open("meetings.txt")

meetings = dict()

for line in ffile:
    try:
        number = int(line)
        day = day_of_week(number)
        meetings[day] = meetings.get(day,0) + 1
    except:
        print("Invalid value")

print(meetings)

Example 2. Inventory

This program helps compile an inventory of all fruits in a shop. Dictionary inventory contains the data about how many fruits there should be in the shop (fruit names and their amounts). The program has a loop which repeatedly asks the user for a fruit. If the fruit is in the dictionary, the program outputs the amount of that fruit in the shop, and asks the user if he wants to update the amount of the fruit. If the fruit is not in the dictionary, the programs asks the user about the amount of the fruit and adds a new entry into the dictionary. The program continues asking until the user enters “done”.

inventory = {'apple': 430, 'banana': 312, 'orange': 525, 'pear': 217}

while True:
       fruit = input('Enter a fruit (done to quit): ')
       if fruit == 'done':
           break

       if fruit in inventory:
           print('Stock information for', fruit, 'is', inventory[fruit])
           upd = input('Do you want to update it? (yes/no) ')
           if upd == 'yes':
               number = int(input('How many ' + fruit + ' do you have? '))
               inventory[fruit] = number
       else:
           print('There is no information about', fruit, 'in the shop')
           number = int(input('How many ' + fruit + ' do you have? '))
           inventory[fruit] = number

Exercises

1. Birthdays

A text file contains birthdays in the format yyyy-mm-dd (see example).

Write a function create_dictionary that takes a filename as a parameter. The function reads the data from the file and creates a dictionary with the following structure. The keys of the dictionary are names of weekdays (i.e. "Monday", "Tuesday" etc), and the value of each key is the number of people who have been born on that weekday. At the end the function returns the dictionary.

>>> create_dictionary('dates.txt')
{'Tuesday': 9, 'Sunday': 7, 'Monday': 8, 'Friday': 7, 'Saturday': 7, 'Thursday': 6, 'Wednesday': 7}

To find the weekday, use Python's standard module datetime:

import datetime
date = datetime.datetime(2019, 5, 3)
print(date.weekday())

2. Dictionary

A tool that is very convenient when learning a new language, is a dictionary, where we can look up words and add new words. Write a program that repeatedly prompts the user for a foreign word. If the word is in the dictionary, then the program prints out its translation. If there is no such word in the dictionary yet, the program should ask for its translations and add the word together with its translation to the dictionary.

If the user is asked for a new word and the user enters done, the program stops prompting and outputs the total number of words in the dictionary and all words together with their translation (one word and translation pair per line):

Here is an example of program output:

 Enter a word (done to quit): stjärtand
 There is no information for stjärtand
 What does stjärtand mean? pintail
 Enter a word (done to quit): silkesapa
 There is no information for silkesapa
 What does silkesapa mean? marmoset
 Enter a word (done to quit): brushane
 There is no information for brushane
 What does brushane mean? ruff
 Enter a word (done to quit): silkesapa
 silkesapa means marmoset
 Enter a word (done to quit): brushane
 brushane means ruff
 Enter a name (done to quit): done

 There are 3 entries in the dictionary:
 stjärtand - pintail
 silkesapa - marmoset
 brushane - ruff

Optional. Modify your program so that it reads the dictionary from the file at the beginning of the session and writes the new dictionary to the same file at the end of the session, so that words from previous sessions are already in the dictionary when the new session starts.

Submit your solutions

Go to Moodle and upload your solutions under homework for Session 13. The programs of exercises 1 and 2 should have names home1.py and home2.py, respectively. The optional part of exercise 2 should be a separate program that can have any name; there is no autograder for the optional part of exercise 2.