8. Coding Exercises 

(Note: Solutions to most exercises are available in https://github.com/chrplr/PCBS/tree/master/coding-exercises )

8.1. Flow control 

You can read about loops in Python, and more generally about flow control.

8.1.1. Shining 

Write a python script that prints 1000 times the line All work and no play makes Jack a dull boy.

Check a solution at shining.py

8.1.2. Multiplication tables 

Write a script that displays the tables of multiplication from 1 to 10 as a table:

 2   3   4   5   6   7   8   9  10
 4   6   8  10  12  14  16  18  20
 6   9  12  15  18  21  24  27  30
 8  12  16  20  24  28  32  36  40
10  15  20  25  30  35  40  45  50
12  18  24  30  36  42  48  54  60
14  21  28  35  42  49  56  63  70
16  24  32  40  48  56  64  72  80
18  27  36  45  54  63  72  81  90
20  30  40  50  60  70  80  90 100

Check out multiplication_table.py for a solution.

8.1.3. Taxis 

Two taxi companies propose differents pricing schemes:

Company A charges 4.80€ plus 1.00€/km

Company B charges 3.20€ plus 1.20€/km

Write code to find which company is the cheapest as a function of the distance to travel.

Check out taxis.py

8.1.4. Estimation of PI by a Monte-Carlo method 

One way to estimate the value of the π is to generate a large number of random points in the unit square and see how many fall within the unit circle; their proportion is an estimate of the area of the circle. See https://academo.org/demos/estimating-pi-monte-carlo/

Implement the proposed algorithm to estimate the value of π.

Check out pi_monte_carlo.py

8.1.5. Computer-guess-a-number 

Read chapter 3 of Invent your own games with Python where the author presents a game where the computer chooses a random number that the user must guess. Study the code.

Now, your task is to write another program, where the roles are inverted: the computer tries to guess a number that the user has in mind. The computer proposes a number and the user answers with ‘+’ (the number he has is mind is larger), ‘-’ (if it is smaller), ‘y’ (if the guess is correct)

Check a solution at computer-guess-a-number.py

8.2. Lists 

These exercises require list manipulations. If you do not know Lists in Python, you can read:

Python Lists

List comprehensions

https://automatetheboringstuff.com/2e/chapter4/

Try to solve the following exercices:

Given a list of numbers, print their sum
Given a list of numbers, print their product
Given a list of numbers, print the sum of their squares
Given a list of numbers, print the largest one.
Given a list of numbers, print the second largest one.

After you have tried to solve these problems, you can check lists.py

8.2.1. Prime numbers 

Write a script that lists all prime numbers between 1 and 10000 (A prime number is a integer that has no divisors except 1 and itself). You can use the following function:

def is_factor(d, n):
    """ True if `d` is a divisor of `n` """
    return n % d == 0

Check a solution at prime-numbers.py

8.2.2. Pascal triangle 

Write a program that prints the first N rows of Pascal’s triangle (see https://www.youtube.com/watch?v=XMriWTvPXHI). For example:

1
 1
 2   1
 3   3   1
 4   6   4   1
 5  10  10   5   1
 6  15  20  15   6   1
 7  21  35  35  21   7   1
 8  28  56  70  56  28   8   1
 9  36  84 126 126  84  36   9   1

To solve this problem, one solution is to store the values of the current line in a Python list, and write a function that given a list as an argument, calculates and returns the following line in a new list.

Proposed solutions:

Pascal-triangle_v1.py

Pascal-triangle_v2.py

8.3. Functions 

Read about functions in Python: - https://www.w3schools.com/python/python_functions.asp - https://automatetheboringstuff.com/2e/chapter3/).

8.3.1. Convert temperatures from Fahrenheit to Celcius and vice-versa.

Read https://en.wikipedia.org/wiki/Fahrenheit and write a function that converts a temperature from Fahrenheit to Celsius, and another one that converts from Celsius to Fahrenheit
Add code that reads temperatures from the standard input and print the converted numbers.

A solution is available here: Fahrenheit_celsius.py.

8.3.2. Permutations 

Generate all the permutations of a set, e.g. (1..n).

Note: This is an advanced exercise, which requires mastery of recursive functions (functions that call themselves)

A solution is proposed at generate_all_permutations.py.

To run it:

python generate_all_permutations.py 4

8.3.3. Eight queens puzzle 

The eight queens puzzle is the problem of placing eight chess queens on an 8×8 chessboard so that no two queens threaten each other; thus, a solution requires that no two queens share the same row, column, or diagonal. See https://en.wikipedia.org/wiki/Eight_queens_puzzle

As there can be only one queen per column and per row, a winning solution can be represented by a set of 8 numbers, one per line, which represent the column in which there is a queen. Because the columns must be different, the solutions are a subset of the permutations of 8 numbers. We just have to check that no two queens are in the same diagonal.

In Python, you can get all the permutations of a set, with the permutations function from the itertools module

from itertools import permutations
list(permutations(range(3))

For a solution to the eight queens problem, check out https://code.activestate.com/recipes/576647-eight-queens-six-lines/

The code is available at eight_queens.py.

8.4. Strings 

8.4.1. Pseudowords 

Search the internet to find out how to generate a random integer number in a interval in Python
Read about strings in Python at https://realpython.com/python-strings/
Write functions that generate pseudowords from words. The first function will delete a character in a random position from a string passed as argument. The second will insert a random character at a random position. The third will swap two characters at random location.
If you know about file input/output (see https://automatetheboringstuff.com/2e/chapter9/), you can read a dictionary (e.g. http://www.pallier.org/extra/liste.de.mots.francais.frgut.txt) and use it to filter out any actual words.

8.5. Dictionaries 

8.5.1. unique 

Given a list of words, print how many different words are in that list (hint: use a dictionary or a set)

liste = ['bonjour', 'chat', 'chien', 'bonjour']

n = 0
d = dict()
for e in liste:
   if not e in d.keys():
      d[e] = 1
      n = n + 1
print(n)

print(len(set(liste)))  # shortest solution using a set

8.5.2. word count 

Given a list of words, count the number of times each word appears in the list. Eg. [Jim, Alan, Jim, Joe] -> Jim:2, Alan:1, Joe:1 (hint: use a dictionary)

liste = ['Jim', 'Alan', 'Jim', 'Joe']
counts = dict()
for word in liste:
    if word in counts.keys():
         counts[word] += 1
    else:
         counts[word] = 1
print(counts)

8.5.3. Anagrams 

Two words are anagrams if they contain the same letters in different orders, e.g., binary and brainy.

write a function that take two strings as arguments and returns True if they are anagrams.
Given a list of words, print all subsets that form anagrams.

Check my solution at anagrams.py. Running:

python anagrams.py < liste.de.mots.francais.frgut.txt

will list all anagrams in French! (liste.de.mots.francais.frgut.txt contains a list of French words)

8.6. File reading and writing 

Read the chapter about files reading and writing at https://automatetheboringstuff.com/2e/chapter9/

8.6.1. head 

Write a script that prints the first 10 lines of a file (or the whole file is it is less than 10 lines long).

with open('aga.txt', 'r', encoding='utf-8') as f:
   for l in f.readlines()[:10]:
       print(l, end='')

8.6.2. tail 

Write a script that prints the last 10 lines of a file (or the whole file is it is less than 10 lines long).

with open('aga.txt', 'r', encoding='utf-8') as f:
    all_lines = f.readlines()
    for l in all_lines[-10:]:
        print(l, end='')

8.6.3. string-detector 

Read Chap. 8 of Automate the boring stuff.

Write a script that opens and read a text file, and print all the lines that contain a given target word, say, cogmaster.

Check out search-file.py

8.6.4. Kaprekar numbers 

A Kaprekar number is a number whose decimal representation of the square can be cut into a left and a right part (no nil) such that the sum of these two parts gives the number initial. For example:

703 is a number of Kaprekar in base 10 because 703² = 494 209 and that
494 + 209 = 703.
4879 is a number of Kaprekar in base 10 because 4879² = 23 804 641 and
04641 + 238 = 4879

Write a program that returns all Kaprekar numbers between 1 and N.

Solution: Kaprekar-numbers.py

8.6.5. RPN Calculator 

Write a reverse Polish arithmetic expression evaluator (See https://en.wikipedia.org/wiki/Reverse_Polish_notation).

E.g. 3 4 * 5 - evaluate to 7.

Solution: rpn-calculator.py

8.6.6. Rodrego-simulator 

Write a Python script that simulates a RodRego machine with 10 registers. The program is stored in a string or in file that is read and then executed. Your program must contain a function which, given the 10 initial values of the registers, and the program, returns the new register values when the END command is reached.

Check two possible solutions: - rodrego_maxime_caute.py - rodrego_christophe_pallier.py

8.6.7. Cellular automata 

Implement a 1-dimension elementay cellular automata. (Further reading: https://en.wikipedia.org/wiki/A_New_Kind_of_Science)

Solution: 1d-ca.py

8.6.8. Analysis of a Signal Detection Experiment 

In a signal detection experiment, a faint stimulus (e.g. a faint sound or a faint visual target) is presented or not at each trial and the participant must indicate whether he has perceived it or not. There are four possible outcomes for each trial:

A hit occurs when the participant correctly detects the target.

A miss occurs when the target was there but the participant did not detect it.

A false alarm occurs when the participant reports the presence of the target when it was not actually there.

A correct rejection occurs when the participant correctly reports that the target was not present.

One defines;

The hit rate, equal to #hits / (#hits + #misses)

The false alarm rate, equal to #false alarms / (#false alarms + # correct rejections)

Let us first suppose that the data from a participant is represented as a string. This string represents a series of trials, each trial being represented by two characters indicating the trial type (1=target present, 0=target absent) and the participant’s response (Y=target perceived, N=No target perceived). For example:

data = "0Y,0N,1Y,1Y,0N,0N,0Y,1Y,1Y"

Exercise:

Write a function which, given such a string, returns the Hit rate and the False rate.

Now, the results from different participants are stored in different files subj*.dat (download the files from https://github.com/chrplr/PCBS/tree/master/coding-exercises/subjdat.zip`) Write a script that computes the hit rates and false alarms for each subject, and displays the group averages and standard deviations.

Solution sdt.py