Strings, Control flow and Loops¶

Karën Fort (CC BY-NC-SA) -- 2024

Fanny Ducel slightly modified the material.

This notebook corresponds to your second lecture and covers :

  • strings
  • control flow: if, else, etc
  • for loops

It is inspired from different sources:

  • Loic Grobol's material
  • Gaël Guibon's slides and lab material from Master TAL (Univ. de Lorraine)
  • W3School tutorial

Before we begin¶

You will have to download the .ipynb files for the labs (TDs) from my website instead of Karën's, as I slightly modified the content.

The URL will be provided during the lab session.

Strings¶

Reminders¶

🥳 What do you already know about strings?

Strings are declared using double quotes or single quotes:

In [1]:
my_string = "saperlipopette !"
my_other_string = 'pouettt'

print(my_string)
print(my_other_string)

saperlipopette !
pouettt

If you need to include quotes in your string, use the other type of quote

In [5]:
chaine = "This is a 'new' string"

print(chaine)

This is a 'new' string

🆕 you can write a multiple line string with three quotes:

In [23]:
bella = '''Una mattina mi sono alzato
O bella ciao, bella ciao, bella ciao ciao ciao
Una mattina mi sono alzato
E ho trovato l'invasor'''

print(bella)

Una mattina mi sono alzato
O bella ciao, bella ciao, bella ciao ciao ciao
Una mattina mi sono alzato
E ho trovato l'invasor

Strings are in Unicode, which means you can represent any language

In [7]:
my_chinese_string = "猫" #the word "cat" in simplified Chinese (according to Google translate)

print(my_chinese_string)

猫

Strings in Python are arrays of bytes.

This means you can access a character of a string like an element in an array.

We'll see soon that we can iterate on a string

In [7]:
poppins_string = "superfragilistiexpialidocious"

print(poppins_string[-2])

u

and yes, do not forget that Python counts from 0! (you will forget, but keep that in mind)

drawing

Some operations on strings¶

You already know one of them:

  • len: returns the size of the string
In [9]:
print(len(poppins_string))

29

There are also a lot of methods (see here), here are some of them:

In [8]:
print(poppins_string.lower()) #note the difference of syntax: len(x) vs x.upper(), we'll come back to it later 

superfragilistiexpialidocious
In [15]:
print(poppins_string.capitalize())

Superfragilistiexpialidocious

Note that some methods take parameters:

In [17]:
trouve = "We will rock you!"

found = trouve.find("rock")#finds the first occurrence of the specified value

print(found) 

not_found = trouve.find("show")#returns -1 if not found

print(not_found)

8
-1
In [14]:
song = "only our rivers run free"

final = song.replace("rivers", "people")

print(final) 

only our people run free

🥳 Replace "alzato" by "svegliato" in bella:

In [12]:
bella = '''Una mattina mi sono alzato
O bella ciao, bella ciao, bella ciao ciao ciao
Una mattina mi sono alzato
E ho trovato l'invasor'''

# Code Me!
bella.replace("alzato", "svegliato")
Out[12]:
"Una mattina mi sono svegliato\nO bella ciao, bella ciao, bella ciao ciao ciao\nUna mattina mi sono svegliato\nE ho trovato l'invasor"
In [ ]:

⚠️ Strings are immutable

This means they cannot be modified, once created.

In [19]:
first_string = "Hello, world!"

first_string[0] = "V" #trying to modify the array

print(first_string)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-19-e396701c91c2> in <module>
      1 first_string = "Hello, world!"
----> 2 first_string[0] = "V" #trying to modify the array
      3 
      4 print(first_string)

TypeError: 'str' object does not support item assignment

However, you can modify the reference to the string:

In [20]:
song = "only our rivers run free"

song = song.replace("rivers", "people")

print(song) 

only our people run free

More about print()¶

By default, the print() function adds a newline character \n at the end of the output.

In [27]:
print("this is a")
print("test")

this is a
test

print() can take several parameters and concatenates them with spaces:

In [3]:
import math #you need this to use pi

pi = math.pi

print("The value of pi is", pi,"!")#here, print is used with 3 parameters of different types

The value of pi is 3.141592653589793 !

Control flow¶

if instruction¶

Theif operator introduces a block of code which will be executed if and only if the expressed condition is met:

In [18]:
answer = input("Do you feel ok?")

if "no" in answer:                    # note the ":"!
    print("Poor dear! What's wrong?") # note the indentation!

Do you feel ok?yes
In [19]:
if "yes" in answer:
    print("Cool!")

Cool!

if can be included in ifs:

In [22]:
answer = input("how old are you?")

if int(answer) > 50:                        # note int() to change the type of the answer
    if int(answer) > 70:
        print("Ok, boomer!")                # this print() depends on the "so bad" condition
        
    print("So how was it to live the 80s?") # this print() depends only on the first condition
    
print("Thank you!")                         # this print() will execute in all cases

how old are you?10
Thank you!

and and or¶

and and or are logical operators allowing you to specify (typically) two conditions at the same time:

In [23]:
upper_age = 65
lower_age = 27

my_age = input ("What's your age? ")

if int(my_age) < upper_age and int(my_age) > lower_age:          # AND: the two conditions have to be met at the same time
    print("There is no SNCF reduction available for you, sorry!")
else:
    print("In order for you to get your reduction we need a blood sample and a liver biopsy.")
    

What's your age? 52
There is no SNCF reduction available for you, sorry!
In [10]:
upper_age = 25
upper_revenue = 35000

my_age = input("What's your age?")
my_revenue = input("What's your annual revenue?")


if int(my_age) > upper_age or int(my_revenue) > upper_revenue:          # OR: one of the conditions has to be met
    print("There is no benefit available for you, sorry!")
else:
    print("In order for you to get your benefit we need a blood sample and a liver biopsy.")

What's your age?26
What's your annual revenue?34000
There is no benefit available for you, sorry!

not¶

You can negate a condition:

In [26]:
upper_age = 25

my_age = input("What's your age?")

#if not int(my_age) > upper_age:   
if int(my_age) < upper_age:  
    print("You're too young!")
else:
    print("You're too old!")

What's your age?23
You're too young!
In [ ]:

else instruction¶

The else operator introduces a block of code which will be executed if the if condition is not met. else is linked to the preceeding if and does not allow to add a condition:

In [44]:
answer = input("how old are you?")

if int(answer) > 50:                        # note int() to change the type of the answer

    if int(answer) > 70:
        print("Ok, boomer!")                # this print() depends on the "so bad" condition
        
    print("So how was it to live the 80s?") # this print() depends only on the first condition

else:
    print("You're so young!")               # this print() will execute only if the > 50 condition is not met
    
print("Thank you!")                         # this print() will execute in all cases

how old are you?49
You're so young!
Thank you!

elif instruction¶

elif is an else with a condition: it checks both that the condition is true and that all the previous conditions are false.

In [45]:
answer = input("how old are you?")

if int(answer) > 50:                        # note int() to change the type of the answer

    if int(answer) > 70:
        print("Ok, boomer!")                # this print() depends on the "so bad" condition
        
    print("So how was it to live the 80s?") # this print() depends only on the first condition

elif int(answer) > 40:
    print("You're older than I thought")

else:
    print("You're so young!")               # this print() will execute only if none of the previous conditions are met
    
print("Thank you!")                         # this print() will execute in all cases

how old are you?45
You're older than I thought
Thank you!

for loops¶

for loops allow to iterate over elements of containers such as strings, and to access items of those containers individually, in order:

In [27]:
for character in "chat":    # this string means "cat" in standard Arabic (according to Google translate)
    print(character)

c
h
a
t

Of course, loops and control flow can be mixed:

In [30]:
for character in "cat":    # Note the ":"
    if character == "c":
        print("This is the first character:",character, end=" ")
    else:
        print(character, end=" ")

This is the first character: c a t

🥳 Print the number of occurrences of "a" and "e" in the following string (please use for...)

In [33]:
Perec = "Anton Voyl n'arrivait pas à dormir. Il alluma. Son Jaz marquait minuit vingt. Il poussa un profond soupir, s'assit dans son lit, s'appuyant sur son polochon. Il prit un roman, il l'ouvrit, il lut; mais il n'y saisissait qu'un imbroglio confus, il butait à tout instant sur un mot dont il ignorait la signification. "

nb_a = 0
nb_e = 0

for char in Perec:
    if char == "a" or char=="A":
        nb_a += 1 # nb_a = nb_a + 1
    elif char=="e" or char=="E":
        nb_e +=1
        
print("The number of a is", nb_a)
print("The number of e is", nb_e)

The number of a is 23
The number of e is 0
In [34]:
nb_a_or_e = 0

for char in Perec:
    if char == "a" or char=="A" or char=="e" or char=="E":
        nb_a_or_e += 1 # nb_a = nb_a + 1

print(nb_a_or_e)

23

Practical reminders:¶

  1. You must work in pairs for the labs and the pairs must change every week.
  2. NLP people are in my group.
  3. You must use the files that are on my website (not Karën's)
  4. You must send your file by the end of the session, to your teacher: NLP people -> to me, SC/Mlex -> Nasser

Before we go back on while loops and move on, let's recap what we saw last time...¶

Quizz time! - on the lecture¶

  • What to use to create a string?
  • How to access the second character of a word?
  • How to print a string and an int together?
  • How to create three conditions? (what keywords to use?)
  • What is the difference between and, or, not?
  • What happens in a for loop?
In [1]:
# Waiting for your answers!

Before we go back on while loops and move on, let's recap what we saw last time...¶

Quizz time! - on the lecture > answers¶

  • strings : simple, double, triple quotes
  • word[i] to access element (= character) at position i
  • print(my_string, my_int) with a comma, or print(my_string + str(my_int))
  • if/elif/else
  • and = both statements must be True, or = only one must be True, not = the statement must be False
  • At each iteration, the loop will "see" one element (= char) of the string. It will continue until it reaches the end of the string.

Before we go back on while loops and move on, let's recap what we saw last time...¶

Quizz time! - on the lab¶

What is the problem with this code (for exercise 4)?

In [1]:
round = 3
stitch = 1
print("round 1 : 7 regular stitches")
print("round 2 : 7 increases")
while round < 11: 
    print("round", round,": (", stitch,"regular stitches", "1 increase)*7")
    round += 1

round 1 : 7 regular stitches
round 2 : 7 increases
round 3 : ( 1 regular stitches 1 increase)*7
round 4 : ( 1 regular stitches 1 increase)*7
round 5 : ( 1 regular stitches 1 increase)*7
round 6 : ( 1 regular stitches 1 increase)*7
round 7 : ( 1 regular stitches 1 increase)*7
round 8 : ( 1 regular stitches 1 increase)*7
round 9 : ( 1 regular stitches 1 increase)*7
round 10 : ( 1 regular stitches 1 increase)*7
In [2]:
round(1.543)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[2], line 1
----> 1 round(1.543)

TypeError: 'int' object is not callable

while loops¶

while loops indicate a stopping condition for a loop.

In [35]:
i = 1

while i < 10:           # Note the ":"
    print(i)
    i += 1               # Note this operator, which is equivalent to i = i +1

1
2
3
4
5
6
7
8
9

🥳 Print the characters of a string using while instead of for (tip: use len)

In [39]:
# CODE ME!

string = "hello"
print(string[0])

counter = 0
while counter < len(Perec):
    print(Perec[counter])
    counter += 1

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What will happen if i write this (you can interrupt the execution by entering i twice):

In [1]:
while True: 
    print("spam")

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---------------------------------------------------------------------------
KeyboardInterrupt                         Traceback (most recent call last)
<ipython-input-1-e61e1f19f6ea> in <module>
      1 while True:
----> 2     print("spam")

~/Dev/notebook/jupyterenv/lib/python3.6/site-packages/ipykernel/iostream.py in write(self, string)
    400             is_child = (not self._is_master_process())
    401             # only touch the buffer in the IO thread to avoid races
--> 402             self.pub_thread.schedule(lambda : self._buffer.write(string))
    403             if is_child:
    404                 # mp.Pool cannot be trusted to flush promptly (or ever),

~/Dev/notebook/jupyterenv/lib/python3.6/site-packages/ipykernel/iostream.py in schedule(self, f)
    201             self._events.append(f)
    202             # wake event thread (message content is ignored)
--> 203             self._event_pipe.send(b'')
    204         else:
    205             f()

~/Dev/notebook/jupyterenv/lib/python3.6/site-packages/zmq/sugar/socket.py in send(self, data, flags, copy, track, routing_id, group)
    618                 )
    619             data.group = group
--> 620         return super().send(data, flags=flags, copy=copy, track=track)
    621 
    622     def send_multipart(

zmq/backend/cython/socket.pyx in zmq.backend.cython.socket.Socket.send()

zmq/backend/cython/socket.pyx in zmq.backend.cython.socket.Socket.send()

zmq/backend/cython/socket.pyx in zmq.backend.cython.socket._send_copy()

~/Dev/notebook/jupyterenv/lib/python3.6/site-packages/zmq/backend/cython/checkrc.pxd in zmq.backend.cython.checkrc._check_rc()

KeyboardInterrupt:

When using while, one has to be sure that the condition can become false (can be falsified), else, it will generate an infinite loop.

You can use break to stop a loop before the condition is false (it's always better not to use it, but you might see it):

In [2]:
n = 0
while n < 9:
    print(n)
    if n == 7:
        break
    n = n + 1

0
1
2
3
4
5
6
7

Of course, for loops are easier to write in Python, so they are used more often than while. However, there are times when while loops will be easier to use:

In [4]:
available_ingredients = ["pepper", "leek", "mushroom", "carot"]
ingredient = input("Please enter an ingredient: ")

while ingredient not in available_ingredients:
    print("I don't have that!")
    ingredient = input("Please enter an ingredient: ")
    
print("Ah, yes,", ingredient, "I have some")

Please enter an ingredient: tomato
I don't have that!
Please enter an ingredient: brussel
I don't have that!
Please enter an ingredient: mushroom
Ah, yes, mushroom I have some

🥳 Write a program that will:

  • randomly select a number between 0 and 20
  • ask the user to guess this number and if it is not found, to try again
In [53]:
import random

to_find = random.randint(0, 20)

chosen_number = int(input("Choose a number: "))
while chosen_number != to_find:
    print("Oops, try again!")
    chosen_number = input("Try again: ")
In [2]:
import random

to_find = random.randint(0, 20)
guess = int(input("Guess a number between 0 and 20: "))
while guess != to_find:
    guess = int(input("Guess a number between 0 and 20: "))
if guess == to_find:
    print("Congrats! The number was", to_find)

Guess a number between 0 and 20: 20
Guess a number between 0 and 20: 15
Guess a number between 0 and 20: 10
Guess a number between 0 and 20: 5
Guess a number between 0 and 20: 1
Guess a number between 0 and 20: 0
Guess a number between 0 and 20: 2
Guess a number between 0 and 20: 3
Guess a number between 0 and 20: 4
Guess a number between 0 and 20: 6
Guess a number between 0 and 20: 8
Guess a number between 0 and 20: 11
Congrats! The number was 11

Good Practices¶

Testing your code¶

  • check the values stored in your variables regularly (use print() for example)
  • check with extreme values to see if your code behaves as expected:
    • what if the string is "" (empty string)?
    • what if the number if negative? 0? a floating number?

Reminders¶

  • write clear, explicit code (avoid "geeky" versions of the code)
  • add comments

drawing

Going further¶

  • Other String methods: Check the W3School doc