Author: Jesús Fernández Bes
Jerónimo Arenas García (jeronimo.arenas@uc3m.es)
Jesús Cid Sueiro (jcid@tsc.uc3m.es)
Vanessa Gómez Verdejo (vanessa@tsc.uc3m.es)
Óscar García Hinde (oghinnde@tsc.uc3m.es)
Simón Roca Sotelo (sroca@tsc.uc3m.es)
Notebook version: 1.1 (Sep 20, 2017)
Changes: v.1.0 - First version.
v.1.1 - Compatibility with python 2 and python 3
v.2.0 - (Sep 3, 2018) - Adapted to TMDE
v.2.1 - (Sep 2, 2019) - Dictionaries added
v.2.2 - (Sep 6, 2019) - ExercisesFrom Wikipedia: "Python is a widely used general-purpose, high-level programming language. Its design philosophy emphasizes code readability, and its syntax allows programmers to express concepts in fewer lines of code than would be possible in languages such as C++ or Java. The language provides constructs intended to enable clear programs on both a small and large scale."
To easily work with Python from any computer you can use , which provides a free envoriment to run Python Jupyter notebooks.
Throughout this tutorial, students will learn some basic characteristics of the Python programming language, as well as the main characteristics of the Python notebooks.
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a = 'house'
print(a)
To run this code and check its output, you firstly have to be conected to a Python runtime server (check the top right pannel) and then you can:
Besides, you can use the options of the Runtime (on the top menu) to run several cells at the same time or stop a execution.
Among the different native python types, we will focus on strings, since they will be the core type that we will recur to represent text. Essentially, a string is just a concatenation of characters.
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str1 = '"Hola" is how we say "hello" in Spanish.'
str2 = "Strings can also be defined with quotes; try to be sistematic and consistent."
It is easy to check the type of a variable with the type() command:
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print(str1)
print(type(str1))
print(type(3))
print(type(3.))
The following commands implement some common operations with strings in Python. Take a look at them, and try to deduce what the result of each operation will be. Then, execute the commands and check what the actual results are.
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print(str1[0:5])
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print(str1 + str2)
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print(str1.lower())
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print(str1.upper())
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print(len(str1))
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print(str1.replace('h','H'))
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str3 = 'This is a question'
str3 = str3.replace('i', 'o')
str3 = str3.lower()
print(str3[0:3])
It is interesting to notice the difference in the use of commands 'lower' and 'len'. Python is an object-oriented language, and str1 is an instance of the Python class 'string'. Then, str1.lower() invokes the method lower() of the class string to which object str1 belongs, while len(str1) or type(str1) imply the use of external methods, not belonging to the class string. In any case, we will not pay (much) attention to these issues during the session.
Finally, we remark that there exist special characters that require special consideration. Apart from language-oriented characters or special symbols (e.g., \euro), the following characters are commonly used to denote carriage return and the start of new lines
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print('This is just a carriage return symbol.\r Second line will start on top of the first line.')
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print('If you wish to start a new line,\r\nthe line feed character should also be used.')
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print('But note that most applications are tolerant\nto the use of \'line feed\' only.')
Exercise 1: Define variables str1 and str2 containing strings "The greatest glory in living lies not in never falling, " and "but in rising every time we fall", respectively. Concatenate both strings to compose a sentence, transform it to capital letters and print the result.
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# Write your solution here
# <SOL>
str1 = "The greatest glory in living lies not in never falling, "
str2 = "but in rising every time we fall"
str3 = (str1 + str2).upper()
print(str3)
# </SOL>
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# A list of integers
mylist0 = [1, 2, 3, 4, 5]
print(mylist0)
# A list of trings
mylist1 = ['a', 'b', 'c', 'd']
print(mylist1)
# A list of strings, integers and float
mylist2 = ['student', 'teacher', 1997, 2000, 8.9]
print(mylist2)
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print(mylist0[0])
print(mylist1[2:4])
print(mylist2[-1])
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mylist = ['student', 'teacher', 1997, 2000]
mylist.append(3)
print(mylist)
mylist.remove('teacher')
print(mylist)
You can also concatenate lists by simple "addition"
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fruits = ['apple', 'orange']
drinks = ['water', 'juice']
fruits_and_drinks = fruits + drinks
print(fruits_and_drinks)
Other useful functions are:
len(list): Gives the number of elements in a list.
max(list): Returns item from the list with max value.
min(list): Returns item from the list with min value.
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mylist = [1, 2, 3, 4, 5]
print(len(mylist))
print(max(mylist))
print(min(mylist))
Exercise 2: Define variable odds containing all odd numbers betewen 1 and 9. Also, define variable prime containing all prime numbers between 1 and 9. Now, define variable odd_or_prime as the concatenation of odd and prime. Some numbers are repeated. User method remove to supress elements appearing twice. Print the result.
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# Write your solution here
# <SOL>
odd = [1, 3, 5, 7, 9]
prime = [2, 3, 5, 7]
odd_or_prime = odd + prime
odd_or_prime.remove(3)
odd_or_prime.remove(5)
odd_or_prime.remove(7)
print(odd_or_prime)
# </SOL>
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my_marks = {"calculus": 9.5, "programming": 7.5, "statistics": 4.5}
print(my_marks)
In order to access a particular value in the dictionary we need to use the corresponding key:
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print(my_marks["programming"])
If we want to display all the keys in a dictionary we can use the .keys() method. Likewise we can view all the stored values in the dictionary by using the .values() method:
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print(my_marks.keys())
print(my_marks.values())
Exercise 3: After the exam review, you teacher has updated your mark in Statistics to 7.0. Also, the scores of "communication theory" have been just published, and you've got 10.0. How would you modify your dictionary with the new marks?
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# Write your solution here
# <SOL>
my_marks['statistics'] = 7.0
my_marks['communication theory'] = 10.0
# </SOL>
print(my_marks)
As in other programming languages, python offers mechanisms to loop through a piece of code several times, or for conditionally executing a code fragment when certain conditions are satisfied.
For conditional execution, you can we use the 'if', 'elif' and 'else' statements.
Try to play with the following example:
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x = int(input('Please enter an integer: '))
if x < 0:
x = 0
print('Negative changed to zero')
elif x == 0:
print('Zero')
elif x == 1:
print('Single')
else:
print('More')
The above fragment, allows us also to discuss some important characteristics of the Python language syntaxis:
The statement 'for' lets you iterate over the items of any iterable (a list, a string, a dictionary... ), in the order that they appear.
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words = ['cat', 'window', 'open-course']
for w in words:
print (w, len(w))
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my_dict = {"key1": 10, "key2": 25, "key3": 30}
for key in my_dict.keys():
print (key, my_dict[key])
In combination with enumerate(), you can iterate over the elements of the sequence and have a counter over them
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words = ['cat', 'window', 'open-course']
for (i, w) in enumerate(words):
print ('element ' + str(i) + ' is ' + w)
Python is very expressive and flexible when it comes to list creation and manipulation. Check out this interesting block of code and try to figure out what it's doing:
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# Python one-liners are great:
x = [1, 2, 3, 4, 5]
y = [i**2 for i in x]
print('x = ', x, '\ny = ', y)
Exercise 4: Create a list powers2 of all powers of 2 between $2^0$ (2**0) and $2^{16}$ (2**16). Loop over the list to create a new list y with all powers of 2 that are greater than 100 and lower than 1000
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# Write your solution here
# <SOL>
powers2 = [2**x for x in range(16)]
y = []
for p in powers2:
if p > 100 and p < 10000:
y.append(p)
# </SOL>
print(y)
First of all, you need to open a file with the open() function (if the file does not exist, the open() function will create it).
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f = open('workfile', 'w')
The first argument is a string containing the filename. The second argument defines the mode in which the file will be used:
'r' : only to be read,
'w' : for only writing (an existing file with the same name would be erased),
'a' : the file is opened for appending; any data written to the file is automatically appended to the end.
'r+': opens the file for both reading and writing.
If the mode argument is not included, 'r' will be assumed.
Use f.write(string) to write the contents of a string to the file. When you are done, do not forget to close the file:
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f.write('This is a test\n with 2 lines')
f.close()
To read the content of a file, use the function f.read():
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f2 = open('workfile', 'r')
text = f2.read()
f2.close()
print(text)
You can also read line by line from the file identifier
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f2 = open('workfile', 'r')
for line in f2:
print(line)
f2.close()
Python lets you define modules which are files consisting of Python code. A module can define functions, classes and variables.
Most Python distributions already include the most popular modules with predefined libraries which make our programmer lives easier. Some well-known libraries are: time, sys, os, numpy, etc.
There are several ways to import a library:
1) Import all the contents of the library: import lib_name
Note: You have to call these methods as part of the library
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import time
print(time.time()) # returns the current processor time in seconds
time.sleep(2) # suspends execution for the given number of seconds
print(time.time()) # returns the current processor time in seconds again!!!
2) Define a short name to use the library: import lib_name as lib
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import time as t
print(t.time())
3) Import only some elements of the library
Note: now you have to use the methods directly
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from time import time, sleep
print(time())
sleep(2)
print(time())
A function is a block of organized, reusable code that is used to perform a single, related action. Functions provide better modularity and a high degree of code reusing.
As we have seen, Python gives us many built-in functions like print(), etc. but you can also create your own functions.
Function blocks begin with the keyword def followed by the function name and parentheses ( ( ) ).
Any input parameters or arguments should be placed within these parentheses. You can also define parameters inside these parentheses.
The first statement of a function can be an optional statement - the documentation string of the function or docstring. Docstring blocks start and ed with triple quotes (""").
The code block within every function starts with a colon (:) and is indented.
The statement return [expression] exits a function, optionally passing back an expression to the caller. A return statement with no arguments is the same as return None.
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# Example of a function:
def add_five(a):
"""This function adds the number five to its input argument."""
return a + 5
# Let's try it out!
x = int(input('Please enter an integer: '))
print('Our add_five function is super effective at adding fives:')
print(x, '+ 5 =', add_five(x))
Exercise 5: Now try it yourself! Write a function called my_factorial that computes the fatorial of a given number. If you're feeling brave today, you can try writing it recursively since Python obviously supports recursion. Remember that the factorial of 0 is 1 and the factorial of a negative number is undefined.
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# Write your solution here
# <SOL>
def my_factorial(n):
if n > 1:
return n * my_factorial(n - 1)
else:
return 1
# </SOL>
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# Let's try it out!
from math import factorial
x = int(input('Please enter an integer: '))
if my_factorial(x) == factorial(x):
print(x, '! = ', my_factorial(x))
print('It works!')
else:
print("Something's not working..." )
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