Module 1, Practical 1¶

Setting up the environment¶

We will need to install several pieces of software to get a working programming environment suitable for this practical. In this section we will install everything that we are going to need in the next few weeks.

Python3 is available for Windows, Linux and Mac, therefore you can run it on your preferred platform.

The console¶

To access the console on Linux just open a terminal and type:

python3

while in Windows you have to look for “Python” and run “Python 3.x”. The console should look like this:

Now we are all set to start interacting with the Python interpreter. In the console, type the following instructions (i.e. the first line and then press ENTER):

[2]:

5 + 3

[2]:

8

All as expected. The “In [1]” line is the input, while the “Out [1]” reports the output of the interpreter. Let’s challenge python with some other operations:

[3]:

12 / 5

[3]:

2.4

[4]:

1/133

[4]:

0.007518796992481203

[5]:

2**1000

[5]:

10715086071862673209484250490600018105614048117055336074437503883703510511249361224931983788156958581275946729175531468251871452856923140435984577574698574803934567774824230985421074605062371141877954182153046474983581941267398767559165543946077062914571196477686542167660429831652624386837205668069376

And some assignments:

[6]:

a = 10
b = 7
s = a + b
d = a / b

print("sum is:",s, " division is:",d)

sum is: 17  division is: 1.4285714285714286

In the first four lines, values have been assigned to variables through the = operator. In the last line, the print function is used to display the output. For the time being, we will skip all the details and just notice that the print function somehow managed to get text and variables in input and coherently merged them in an output text. Although quite useful in some occasions, the console is quite limited therefore you can close it for now. To exit press Ctrl-D or type exit() and press ENTER.

VScode¶

Open VScode and create a file name test.py, copy and paste the following code and try to execute it!

[2]:

"""
This is the first example of Python script.
"""
a = 10 # variable a
b = 33 # variable b
c = a / b # variable c holds the ratio

# Let's print the result to screen.
print("a:", a, " b:", b, " a/b=", c)

a: 10  b: 33  a/b= 0.30303030303030304

A couple of things worth nothing: the first three lines opened and closed by “”” are some text describing the content of the script. Moreover, comments are proceeded by the hash key (#) and they are just ignored by the python interpreter.

Please notice that Visual Studio Code will help you writing your Python scripts. For example, when you start writing the print line it will complete the code for you (if the Pylint extension mentioned above is installed), suggesting the functions that match the letters typed in. This useful feature is called code completion and, alongside suggesting possible matches, it also visualizes a description of the function and parameters it needs. Here is an example:

Save the file (Ctrl+S as shortcut). It is convenient to ask the IDE to highlight potential syntactic problems found in the code. You can toggle this function on/off by clicking on View –> Problems. The Problems panel should look like this

Visual Studio Code is warning us that the variable names a,b,c at lines 4,5,6 do not follow Python naming conventions for constants (do you understand why? Check here to find the answer). This warning is because they have been defined at the top level (there is no structure to our script yet) and therefore are interpreted as constants. The naming convention for constants states that they should be in capital letters. To amend the code, you can just replace all the names with the corresponding capitalized name (i.e. A,B,C). If you do that, and you save the file again (Ctrl+S), you will see all these problems disappearing as well as the green underlining of the variable names. If your code does not have an empty line before the end, you might get another warning “Final new line missing”.

Had we by mistake mispelled the print function name (something that should not happen with the code completion tool that suggests functions names!) writing printt (note the double t), upon saving the file, the IDE would have underlined in red the function name and flagged it up as a problem.

This is because the builtin function printt does not exist and the python interpreter does not know what to do when it reads it. Note that printt is actually underlined in red, meaning that there is an error which will cause the interpreter to stop the execution with a failure. Please remember ALL ERRORS MUST BE FIXED before running any piece of code.

Now it is time to execute the code. By right-clicking in the code panel and selecting Run Python File in Terminal (see picture below) you can execute the code you have just written.

Upon clicking on Run Python File in Terminal a terminal panel should pop up in the lower section of the coding panel and the result shown above should be reported.

Saving script files like the example1.py above is also handy because they can be invoked several times (later on we will learn how to get inputs from the command line to make them more useful…). To do so, you just need to call the python intepreter passing the script file as parameter. From the folder containing the example1.py script:

python3 example1.py

will in fact return:

a: 10 b: 33 a/b= 0.30303030303030304

Before getting into that, consider the following lines of code (do not focus on the import line, this is only to load the mathematics module and use its method sqrt to compute the square root of its parameter):

[7]:

"""
Runtime error example, compute square root of numbers
"""
import math

A = 16
B = math.sqrt(A)
C = 5*B
print("A:", A, " B:", B, " C:", C)

D = math.sqrt(A-C) # whoops, A-C is now -4!!!
print(D)

A: 16  B: 4.0  C: 20.0

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Input In [7], in <cell line: 11>()
      8 C = 5*B
      9 print("A:", A, " B:", B, " C:", C)
---> 11 D = math.sqrt(A-C) # whoops, A-C is now -4!!!
     12 print(D)

ValueError: math domain error

If you add that code to a python file (e.g. sqrt_example.py), you save it and you try to execute it, you should get an error message as reported above. You can see that the interpreter has happily printed off the vaule of A,B and C but then stumbled into an error at line 9 (math domain error) when trying to compute \(\sqrt{A-C} = \sqrt{-4}\), because the sqrt method of the math module cannot be applied to negative values (i.e. it works in the domain of real numbers).

Please take some time to familiarize with Visual Studio Code (creating files, saving files etc.) as in the next practicals we will take this ability for granted.

The debugger¶

Another important feature of advanced Integrated Development Environments (IDEs) is their debugging capabilities. Visual Studio Code comes with a debugging tool that can help you trace the execution of your code and understand where possible errors hide.

Write the following code on a new file (let’s call it integer_sum.py) and execute it to get the result.

[8]:

""" integer_sum.py is a script to
 compute the sum of the first 1200 integers. """

S = 0
for i in range(0, 1201):
    S = S + i

print("The sum of the first 1200 integers is: ", S)

The sum of the first 1200 integers is:  720600

Without getting into too many details, the code you just wrote starts initializing a variable S to zero, and then loops from 0 to 1200 assigning each time the value to a variable i, accumulating the sum of S + i in the variable S.

A final thing to notice is indentation.

How does this code work? How does the value of S and i change as the code is executed? These are questions that can be answered by the debugger.

To start the debugger, click on Debug –> Start Debugging (shortcut F5). The following small panel should pop up:

We will use it shortly, but before that, let’s focus on what we want to track. On the left hand side of the main panel, a Watch panel appeared. This is where we need to add the things we want to monitor as the execution of the program goes. With respect to the code written above, we are interested in keeping an eye on the variables S, i and also of the expression S+i (that will give us the value of S of the next iteration). Add these three expressions in the watch panel (click on + to add new expressions). The watch panel should look like this:

do not worry about the message “name X is not defined”, this is normal as no execution has taken place yet and the interpreter still does not know the value of these expressions.

The final thing before starting to debug is to set some breakpoints, places where the execution will stop so that we can check the value of the watched expressions. This can be done by hovering with the mouse on the left of the line number. A small reddish dot should appear, place the mouse over the correct line (e.g. the line corresponding to S = S + 1 and click to add the breakpoint (a red dot should appear once you click).

Now we are ready to start debugging the code. Click on the green triangle on the small debug panel and you will see that the yellow arrow moved to the breakpoint and that the watch panel updated the value of all our expressions.

The value of all expressions is zero because the debugger stopped before executing the code specified at the breakpoint line (recall that S is initialized to 0 and that i will range from 0 to 1200). If you click again on the green arrow, execution will continue until the next breakpoint (we are in a for loop, so this will be again the same line - trust me for the time being).

Now i has been increased to 1, S is still 0 (remember that the execution stopped before executing the code at the breakpoint) and therefore S + i is now 1. Click one more time on the green arrow and values should update accordingly (i.e. S to 1, i to 2 and S + i to 3), another round of execution should update S to 3, i to 3 and S + i to 6. Got how this works? Variable i is increased by one each time, while S increases by i. You can go on for a few more iterations and see if this makes any sense to you, once you are done with debugging you can stop the execution by pressing the red square on the small debug panel.

Other editors are available, if you already have your favourite one you can stick to it. Some examples are:

• Spyder

• PyCharm Community Edition

• Jupyter Notebook. Note: we might use it later on in the course.

A quick Jupyter primer (just for your information, skip if not interested)¶

Jupyter allows to write notebooks organized in cells (these can be saved in files with .ipynb extension). Notebooks contain both the code, some text describing the code and the output of the code execution, they are quite useful to produce some quick reports on data analysis. where there is both code, output of running that code and text. The code by default is Python, but can also support other languages like R). The text is formatted using the Markdown language - see cheatsheet for its details. Jupyter is becoming the de-facto standard for writing technical documentation.

Installation¶

To install it (if you have not installed python with Anaconda otherwise you should have it already):

python3 -m pip install jupyter

you can find more information here

Upon successful installation, you can run it with:

jupyter-notebook

This should fire up a browser on a page where you can start creating your notebooks or modifying existing ones. To create a new notebook you simply click on New:

and then you can start adding cells (i.e. containers of code and text). The type of each cell is specified by selecting the cell and selecting the right type in the dropdown list:

Cells can be executed by clicking on the Run button. This will get the code to execute (and output to be written) and text to be processed to provide the final page layout. To go back to the edit mode, just double click on an executed cell.

Please take some more time to familiarize with Visual Studio Code (creating files, saving files, interacting with the debugger etc.) as in the next practicals we will take this ability for granted. Once you are done you can move on and do the following exercises.

Let’s try together¶

1. Compute the area of a triangle having base 120 units (B) and height 33 (H). Assign the result to a variable named area and print it.

[17]:

B = 120
H = 33
Area = B*H/2
print("Triangle area is:", Area)

Triangle area is: 1980.0

2. Aquire the side S (base of a square) from the user at runtime and compute the area of a square. Then, assign the result to a variable named area and print it. Hint: use the input function (details here) and remember to convert the acquired value into an int.

[19]:

S_str = input("Insert size: ")
print(type(S_str))
print(S_str)
S = int(S_str)
print(type(S))
print(S)
Area = S**2
print("Square area is:",Area)

<class 'str'>
5
<class 'int'>
5
Square area is: 25

Exercises¶

1. If you have not done so already, put the two previous scripts in two separate files (e.g. triangle_area.py and square_area.py and execute them from the terminal).

2. Write a small script (trapezoid.py) that computes the area of a trapezoid having major base (MB) equal to 30 units, minor base (mb) equal to 12 and height (H) equal to 17. Print the resulting area. Try executing the script from inside Visual Studio Code and from the terminal.

[5]:

"""trapezoid.py"""
MB = 30
mb = 12
H = 17
Area = (MB + mb)*H/2
print("Trapezoid area is: ", Area)

Trapezoid area is:  357.0

3. Write a program to compute the sum of the first 1200 integers by using the following equation: \(\sum\limits_{i=1}^n i = \frac{n (n+1)}{2}\).

[21]:

N = 1200

print("Sum of first 1200 integers: ", N*(N+1)/2)

Sum of first 1200 integers:  720600.0

4. Modify the program at point 3. to make it acquire the number of integers to sum N from the user at runtime.

[23]:

print("Input number N:")
N = int(input())
print("Sum of first ", N, " integers: ", N*(N+1)/2)

Input number N:
Sum of first  6  integers:  21.0

5. Write a small script to compute the length of the hypotenuse (c) of a right triangle having sides a=133 and b=72 units (see picture below). Hint: remember the Pythagorean theorem and use math.sqrt.

[24]:

import math

a = 133
b = 72

c = math.sqrt(a**2 + b**2)

print("Hypotenuse: ", c)

Hypotenuse:  151.23822268196622

6. Rewrite the trapezoid script making it compute the area of the trapezoid starting from the major base (MB), minor base (mb) and height (H) taken in input. (Hint: use the input function and remember to convert the acquired value into an int).

[25]:

"""trapezoidV2.py"""
MB = int(input("Input the major base (MB):"))
mb = int(input("Input the minor base (mb):"))
H = int(input("Input the height (H):"))
Area = (MB + mb)*H/2
print("Given MB:", str(MB) , " mb:", str(mb) , " and H:", H)
print("The trapezoid area is: ", Area)

Given MB: 12  mb: 55  and H: 66
The trapezoid area is:  2211.0

7. Write a script that reads the side of an hexagon in input and computes its perimeter and area printing them to the screen. Hint: \(Area = \frac{3*\sqrt{3}*side^{2}}{2}\)

[26]:

import math

side = int(input("Please insert the side of the hexagon: "))

P = 6*side
A = (3*math.sqrt(3)*side**2)/2
print("Perimeter: ", P, " Area: ", A)

Perimeter:  3330  Area:  800272.4250021052

8. You are given a very important deadline in: days = 4 hours = 13 minutes = 52 Write some code that prints the total minutes.

[27]:

days = 4
hours = 13
minutes = 52

# write here
print("In total there are", days*24*60 + hours*60 + minutes, "minutes left")

In total there are 6592 minutes left

Exercises (Difficult)¶

1. Difficult Write a function is_balanced(word) that: 1)takes a string of lowercase letters, 2)returns True iff for every letter the number of occurrences equals the number of occurrences of its opposite in the alphabet (a↔z, b↔y, …). The empty string counts as balanced.

is_balanced("azby")   # True
is_balanced("aazz")   # True
is_balanced("abc")    # False
is_balanced("")       # True

[1]:

import string
def is_balanced(input_string: str) -> bool:

    # Dictionary to store the frequency of each lowercase letter (a–z)
    letter_counts = {letter: 0 for letter in string.ascii_lowercase}

    # Input string to check
    input_string = "aazzb"

    # Count the occurrences of each letter in the input string
    for char in input_string:
        letter_counts[char] += 1

    # Convert the frequency dictionary to a list of counts (ordered a–z)
    frequency_list = list(letter_counts.values())

    # Check whether the frequency list is symmetrical (palindromic)
    for i in range(len(frequency_list) // 2):
        if frequency_list[i] != frequency_list[-1 - i]:
            return False  # no need to keep checking once asymmetry is found

    # Print the result: True if symmetric, False otherwise
    return True

2. Difficult Write a function named is_magic_square(matrix), that takes a square matrix of integers (list of lists) and returns True if the sum of every row and every column is the same number, otherwise False. (We do not require diagonal sums to match; we only check rows and columns.)

Examples

m1 = [
    [2, 2],
    [2, 2]
]
# all rows and columns sum to 4 → True
print(is_magic_square(m1))   # True

m2 = [
    [1, 2],
    [3, 4]
]
print(is_magic_square(m2))   # False

[6]:

def is_magic_square(matrix):
    n = len(matrix)
    target = sum(matrix[0])  # sum of first row as reference

    # check all rows
    for row in matrix:
        if sum(row) != target:
            return False

    # check all columns
    for c in range(n):
        if sum(matrix[r][c] for r in range(n)) != target:
            return False

    return True

Let’s try Linux¶

The following section explains how to install Linux on a windows machine. This is for your reference, you can read the following instructions before the next practical and try to instally Linux if you want to test it out.

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