Strings 3 - Methods

Every data type has associated particular methods for that type, let’s see the simple ones associated to type string (str):

Method	Result	Meaning
str.upper()	str	Return the string with all characters uppercase
str.lower()	str	Return the string with all characters lowercase
str.capitalize()	str	Return the string with the first uppercase character
str.startswith(str)	bool	Check if the string begins with another one
str.endswith(str)	bool	Check whether the string ends with another one
str.isalpha(str)	bool	Check if all characters are alhpabetic
str.isdigit(str)	bool	Check if all characters are digits
str.isupper()	bool	Check if all characters are uppercase
str.islower()	bool	Check if all characters are lowercase

The others are described at the page Search methods

WARNING: ALL string methods ALWAYS generate a NEW string

The original string object is NEVER changed (strings are immutable).

Example - upper

A method is a function of an object that takes as input the object to which is it is applied and performs some calculation.

The string type str has predefined methods like str.upper() which can be applied to other string objects (i.e.: 'hello' is a string object)

The method str.upper() takes the string to which it is applied, and creates a NEW string in which all the characters are in uppercase. To apply a method like str.upper() to the particular string object 'hello', we must write:

'hello'.upper()

Frst we write the object on which apply the method ('hello'), then a dot ., and afterwards the method name followed by round parenthesis. The brackets can also contain further parameters according to the method.

Examples:

'hello'.upper()

'HELLO'

"I'm important".upper()

"I'M IMPORTANT"

WARNING: like ALL string methods, the original string object on which the method is called does NOT get modified.

Example:

x = "hello"
y = x.upper()    # generates a NEW string and associates it to the variables y

x                # x variable is still associated to the old string

'hello'

y                #  y variable is associated to the new string

'HELLO'

Exercise - walking

Write some code which given a string x (i.e.: x='walking') prints twice the row:

walking WALKING walking WALKING
walking WALKING walking WALKING

• DO NOT create new variables

• your code must work with any string

x = 'walking'

# write here

Show solution

x = 'walking'

print(x, x.upper(), x, x.upper())
print(x, x.upper(), x, x.upper())

Help: If you are not sure about a method (for example, strip), you can ask Python for help this way:

WARNING: when using help, DON’T put parenthesis after the method name !!

help("hello".strip)

Help on built-in function strip:

strip(chars=None, /) method of builtins.str instance
    Return a copy of the string with leading and trailing whitespace removed.

    If chars is given and not None, remove characters in chars instead.

lower method

Return the string with all lowercase characters

my_string = "HEllo WorLd"

another_string = my_string.lower()

print(another_string)

hello world

print(my_string)  # didn't change

HEllo WorLd

Exercise - lowermid

Write some code that given any string x of odd length, prints a new string like x having the mid-character as lowercase.

• your code must work with any string !

• HINT: to calculate the position of the mid-character, use integer division with the operator //

Example 1 - given:

x = 'ADORATION'

it should print:

ADORaTION

Example 2 - given:

x = 'LEADINg'

it should print:

LEAdINg

    #012345678
x = 'ADORATION'
#x = 'LEADINg'

# write here

Show solution

    #012345678
x = 'ADORATION'
#x = 'LEADINg'

k = len(x) // 2
print(x[:k] + x[k].lower() + x[k+1:])

capitalize method

capitalize() creates a NEW string having only the FIRST character as uppercase:

"artisan".capitalize()

'Artisan'

"premium".capitalize()

'Premium'

x = 'goat'
y = 'goat'.capitalize()

x      #  x remains associate to the old value

'goat'

y      #  y is associated to the new string

'Goat'

Exercise - Your Excellence

Write some code which given two strings x and y returns the two strings concatenated, separating them with a space and both as lowercase except the first two characters which must be uppercase

Example 1 - given:

x = 'yoUR'
y = 'exCelLenCE'

it must print:

Your Excellence

Example 2 - given:

x = 'hEr'
y = 'maJEsty'

it must print:

Her Majesty

x,y = 'yoUR','exCelLenCE'
#x,y = 'hEr','maJEsty'

# write here

Show solution

x,y = 'yoUR','exCelLenCE'
#x,y = 'hEr','maJEsty'

print(x.capitalize() + " " + y.capitalize())

startswith method

str.startswith takes as parameter a string and returns True if the string before the dot begins with the string passed as parameter. Example:

"the dog is barking in the road".startswith('the dog')

True

"the dog is barking in the road".startswith('is barking')

False

"the dog is barking in the road".startswith('THE DOG')  # uppercase is different from lowercase

False

"THE DOG BARKS IN THE ROAD".startswith('THE DOG')       # uppercase is different from lowercase

True

Exercise - by Jove

Write some code which given any three strings x, y and z, prints True if both x and y start with string z, otherwise prints False

Example 1 - given:

x = 'by Jove'
y = 'by Zeus'
z = 'by'

it should print:

True

Example 2 - given:

x = 'by Jove'
y = 'by Zeus'
z = 'from'

it should print:

False

Example 3 - given:

x = 'from Jove'
y = 'by Zeus'
z = 'by'

it should print:

False

x,y,z = 'by Jove','by Zeus','by'     # True
#x,y,z = 'by Jove','by Zeus','from'  # False
#x,y,z = 'from Jove','by Zeus','by'  # False

# write here

Show solution

x,y,z = 'by Jove','by Zeus','by'     # True
#x,y,z = 'by Jove','by Zeus','from'  # False
#x,y,z = 'from Jove','by Zeus','by'  # False

print(x.startswith(z) and y.startswith(z))

endswith method

str.endswith takes as parameter a string and returns True if the string before the dot ends with the string passed as parameter. Example:

"My best wishes".endswith('st wishes')

True

"My best wishes".endswith('best')

False

"My best wishes".endswith('WISHES')    # uppercase is different from lowercase

False

"MY BEST WISHES".endswith('WISHES')    # uppercase is different from lowercase

True

Exercise - Snobbonis

Given couple names husband and wife, write some code which prints True if they share the surname, False otherwise.

• assume the surname is always at position 9

• your code must work for any couple husband and wife

                 #0123456789               #0123456789
husband, wife  = 'Antonio  Snobbonis',     'Carolina Snobbonis'   # True
#husband, wife = 'Camillo  De Spaparanzi', 'Matilda  Degli Agi'   # False

# write here

Show solution

                 #0123456789               #0123456789
husband, wife  = 'Antonio  Snobbonis',     'Carolina Snobbonis'   # True
#husband, wife = 'Camillo  De Spaparanzi', 'Matilda  Degli Agi'   # False

print(wife.endswith(husband[9:]))

isalpha method

The method isalpha returns True if all characters in the string are alphabetic:

'CoralReel'.isalpha()

True

Numbers are not considered alphabetic:

'Route 666'.isalpha()

False

Also, blanks are not alphabetic:

'Coral Reel'.isalpha()

False

… nor punctuation:

'!'.isalpha()

False

… nor weird Unicode stuff:

'♥'.isalpha()

False

''.isalpha()

False

Exercise - Fighting the hackers

In the lower floors of Interpol, it is well known international hackers communicate using a slang called Leet. This fashion is also spreading in schools, where you are considered K001 (cool) if you know this inconvenient language. The idea is trying to substitute characters with numbers in written text ( Complete guide ).

1 -> i
2 -> z
3 -> e
4 -> h, a, y
etc

Write some code which checks name and surname given by students to detect Leet-like language.

• print True if at least one of the words contains numbers instead of alphabet characters, otherwise print False

• code must be generic, so must work with any word

• DO NOT use if command

name, surname =  'K001',   'H4ck3r'    # True
#name, surname = 'Cool',   'H4ck3r'    # True
#name, surname = 'Romina', 'Rossi'     # False
#name, surname = 'Peppo',  'Sbirilli'  # False
#name, surname = 'K001',   'Sbirilli'  # True

# write here

Show solution

name, surname =  'K001',   'H4ck3r'    # True
#name, surname = 'Cool',   'H4ck3r'    # True
#name, surname = 'Romina', 'Rossi'     # False
#name, surname = 'Peppo',  'Sbirilli'  # False
#name, surname = 'K001',   'Sbirilli'  # True

print(not (name.isalpha() and surname.isalpha()))

isdigit method

isdigit method returns True if a string is only composed of digits:

'391'.isdigit()

True

'400m'.isdigit()

False

Floating point and scientific notations are not recognized:

'3.14'.isdigit()

False

'4e29'.isdigit()

False

Exercise - Selling numbers

The multinational ToxiCorp managed to acquire a wealth of private data of unaware users, and asks you to analyze it. They will then sell private information to the highest bidder on the black market. The offer looks questionable, but they pay well, so you accept.

We need to understand the data and how to organize it. You found several strings which look like phone numbers.

Every number should be composed like so:

+[national prefix 39][10 numbers]

For example, this is a valid number: +392574856985

Write some code which prints True if the string is a phone number, False otherwise

• Try the various combinations by uncommenting phone =

• Your code must be generic, should be valid for all numbers

phone = '+392574856985'   # True
#phone = '395851256954'   # False (missing '+')
#phone = '++7485125874'   # False (missing prefix)
#phone = '+3933342Blah'   # False (obvious :D )
#phone = "+3912"          # False (too short)
#phone = '+393481489942'  # True

# write here

Show solution

phone = '+392574856985'   # True
#phone = '395851256954'   # False (missing '+')
#phone = '++7485125874'   # False (missing prefix)
#phone = '+3933342Blah'   # False (obvious :D )
#phone = "+3912"          # False (too short)
#phone = '+393481489942'  # True

number = phone[3:]  # I save the string excluding +39

# Let's make 4 bool to keep track of all conditions
has_plus = phone.startswith('+')                  # Does it start with + ?
has_national_prefix = phone[1:].startswith('39')  # Is there 39 after? We could have done it also in the row above
is_10_long = len(number) == 10       # Excluding the prefix, are the others 10 characters?
has_all_digits = number.isdigit()    # Are they all numbers?

# This is just a debug print
print("Variables check:", has_plus, has_national_prefix, is_10_long, has_all_digits)
# Final solution, combines everything
print("Is it a phone number?", has_plus and has_national_prefix and is_10_long and has_all_digits)

isupper and islower methods

We can check whether a character is uppercase or lowercase with isupper and islower methods:

'q'.isupper()

False

'Q'.isupper()

True

'b'.islower()

True

'B'.islower()

False

They also work on longer strings, checking if all characters meet the criteria:

'GREAT'.isupper()

True

'NotSoGREAT'.isupper()

False

Note blanks and punctuation are not taken into account:

'REALLY\nGREAT !'.isupper()

True

We could check whether a character is upper/lower case by examining its ASCII code but the best way to cover all alphabets is by using isupper and islower methods. For example, they also work with accented letters:

'à'.isupper()

False

'Á'.isupper()

True

Exercise - dwarves and GIANTS

In an unknown and exciting fantasy world live two populations, dwarves and GIANTS:

• dwarves love giving their offspring names containing only lowercase characters

• GIANTS don’t even need to think about it, because it’s written on the tablets of GROCK that GIANT names can only have uppercase characters

One day, a threat came from a far away kingdom, and so a team of fearless adventurers was gathered. The prophecy said only a mixed team of GIANTS and dwarves for a total of 4 people could defeat the evil.

1. Write some code which checks whether or not four adventurers can gather into a valid team:

• print True if the four names are both of dwarves and GIANTS, otherwise if they are of only one of the populations print False

• your code must be generic, valid for all strings

2. Find some GIANT names and dwarves names and try to put them, making sure to translate them with the all uppercase / all lowercase capitalization, es “Jisog” is not a valid giant name, it must be translated into the gigantic writing “JISOG”

adv1, adv2, adv3, adv4 = 'gimli', 'savorlim', 'glazouc', 'hondouni'       # False
#adv1, adv2, adv3, adv4 = 'OXLOR', 'HIVAR', 'ELOR', 'SUXGROG'             # False
#adv1, adv2, adv3, adv4 = 'krakrerlig', 'GUCAM', 'SUXGROG', 'kodearen'    # True
#adv1, adv2, adv3, adv4 = 'yarnithra', 'krakrerlig', 'jandreda', 'TOVIR'  # True

# write here

Show solution

adv1, adv2, adv3, adv4 = 'gimli', 'savorlim', 'glazouc', 'hondouni'       # False
#adv1, adv2, adv3, adv4 = 'OXLOR', 'HIVAR', 'ELOR', 'SUXGROG'             # False
#adv1, adv2, adv3, adv4 = 'krakrerlig', 'GUCAM', 'SUXGROG', 'kodearen'    # True
#adv1, adv2, adv3, adv4 = 'yarnithra', 'krakrerlig', 'jandreda', 'TOVIR'  # True

a1 = adv1.isupper()
a2 = adv2.isupper()
a3 = adv3.isupper()
a4 = adv4.isupper()

print(not(a1 == a2 == a3 == a4))

Continue

Go on with Strings 4 - Search Methods