Strings

Strings

String Literals

A can normally be specified by enclosing it within a pair of '' or a pair of "" e.g., 'How is life?'. However, this will not work if the string has a in it e.g., 'How's life?' is not acceptable to Python because it contains a ' which has the special meaning 'end of string', confusing Python as to which ' of the string literal indicates the end of the string. This is similarly confusing: "Say "wow"".

An escape sequence is a sequence of characters in a string literal that is taken together and interpreted in a special way. You can use an escape sequence to include a special character in a string literal without interpreting it as a special character. Given below are some examples:

Escape Sequence	Meaning	Example	Output
`\'`	single quote	`print('How\'s Life')`	`How's Life?`
`\"`	double quote	`print("Say \"wow\"")`	`Say "wow"`
`\\`	back slash	`print('files\\text')`	`files\text`

Another use of escape sequences is to give a special meaning to a character that normally does not have a special meaning. Here are some examples:

Escape Sequence	Meaning	Example	Output
`\t`	horizontal tab	`print('aaa\tbbb')`	`aaa` `bbb`
`\n`	line break	`print('hi\nthere!')`	`hi` `there!`

Exercise: Escape Sequences

Exercise : Escape Sequences

Modify the get_string() function so that the code prints the given output.
Note that these words are separated by tabs, not normal spaces:"oops"{tab here}"ok"{tab here}"oh?"

def get_string():
  return '' # REPLACE WITH YOUR CODE

print(get_string())

Which word didn't he/she say? ["oops" "ok" "oh?"]

💡 Tips

Partial solution

You can use a pair of triple quotes to indicate a multi-line string literal.

Here is an example multi-line string that uses triple quotes.

print('''Hi,
How's life?
bye!
-me''')

→

Hi,
How's life?
bye!
-me

def get_email_body():
  body = '''This is the first line of the email.
This is the second line.
This is the third line.
          - bye!'''
  return body

print(get_email_body())

This is the first line of the email.
This is the second line.
This is the third line.
          - bye!

It is optional to escape ' and " inside a mult-line string within triple quotes e.g., How's life? in the example above.

Exercise: Multi-Line String

Exercise : Multi-Line String

Modify the get_multiline_string() function so that the code prints the given output exactly as given.

def get_multiline_string():
  return '' #REPLACE WITH YOUR CODE
    
print(get_multiline_string())

Which word didn't he/she say?
* "oops"
  * "ok"
    * "oh?"

💡 Tips

Partial solution

Triple double-quotes (""") are commonly used to show documentation of code. Such comments are called docstrings.

The remove_head(items) function below has a docstring that explains its behavior.

def remove_head(items):
  """Remove the first item of the items.

  The list should have at least one item.
  Arguments:
  items -- (type: list) the list of items to be modified
  """
  print('removing head of list ', items)
  del items[0]

📎 Vist this page to learn more about docstrings

Working with Strings

As you have seen before, you can use + and * operators to concatenate and replicate strings
e.g., 'abc' + '!'*5 evaluates to 'abc!!!!!'.

You can use indexes and slices to access characters of a string, just like if a string is a simply a list of characters.

i.e., 'Hi there' is same as a list:

H	i		t	h	e	r	e	!
0	1	2	3	4	5	6	7	8

The code below shows how to use index and slice notations to get parts of a string.

s = 'Hi there!'
print(s[0])
print(s[-1])
print(s[3:6])

→

H
!
the

Strings are immutable. The following code will not work: s[0] = 'h'

Exercise: Shorten String

Exercise : Shorten String

Complete the function given below, to behave as described by its docstring.

def shorten(text):
  """ Return a 10-character version of the string if it is longer.

  If text is longer than 10 characters, return the first four characters
  followed by '..' followed by the last four characters.
  If text is not longer than 10 characters, return text.

  Example:
  shorten('1234567890abcd') returns '1234..abcd'
  """
  pass # REPLACE WITH YOUR CODE

Example usage:

print(shorten('1234'))
print(shorten('1234567890'))
print(shorten('1234567890abcd'))

→

1234
1234567890
1234..abcd

Partial solution

You can use the in and not in operator to see if one string is a sub-string of another.

Examples of checking for the existence of a sub-string:

s = 'Hi there!'
print('Hi' in s)
print('hi' in s) # matching is case-sensitive
print('Hello' not in s)

→

True
False
True

Exercise: Has All Characters

Exercise : Has All Characters

Complete the function given below so that it returns True if text has all the characters in the list characters.

def has_all_characters(text, characters):
  #pass # REPLACE WITH YOUR CODE

Example usage:

print(has_all_characters('abccde', ['a', 'c']))
print(has_all_characters('ab cde', ['a', 'a', ' ']))
print(has_all_characters('apple', ['a', 'f']))

→

True
True
False

Partial solution

String Methods

String objects have many methods (the full list is here).

Here are some string methods related to the nature of the string.

upper(): returns a string with all characters in upper case
lower(): returns a string with all characters in lower case
isupper(): returns True if all characters are in upper case
islower(): returns True if all characters are in lower case
isalpha(): returns True if the string consists only of letters and is not blank.
isalnum(): returns True if the string consists only of letters and numbers and is not blank.
isdecimal(): returns True if the string consists only of numeric characters and is not blank.
isspace(): returns True if the string consists only of spaces, tabs, and new-lines and is not blank.
startswith(s): returns True if the substring s appears at the start of the string
endswith(s): returns True if the substring s appears at the end of the string

Examples of string methods mentioned above:

print('Hi there!'.upper())
print('Hi there!'.lower())

→

HI THERE!
hi there!

print('ABC'.isupper(), 'Abc'.isupper())
print('abc'.islower(), 'Abc'.islower())
print('abc'.isalpha(), 'A12'.isalpha())
print('A23'.isalnum(), 'A+1'.isalnum())
print('123'.isdecimal(), 'A12'.isdecimal())
print(' \t\n'.isspace(), 'a b'.isspace())

→

True False
True False
True False
True False
True False
True False

s = 'Hi there!'
print(s.startswith('Hi'), s.startswith('hi'))
print(s.endswith('!'), s.endswith('?'))

→

True False
True False

Exercise: Rectify Case

Exercise : Rectify Case

Complete the rectify_case(text) function given below, to behave as follows:

If text is all upper case, return text in lower case
If text is all lower case, return text in upper case
Return text otherwise

def rectify_case(text):
  #pass # REPLACE WITH YOUR CODE

Example usage:

print(rectify_case('Mrs. Fox'))
print(rectify_case('MR. FOX'))
print(rectify_case('baby fox'))

→

Mrs. Fox
mr. fox
BABY FOX

Partial solution

Exercise: Is Doctor

Exercise : Is Doctor

Complete the is_doctor(name) function to return True for the following cases:

If name has a Dr. at the start
If name has a (Dr) at the end

Note that the matching should be case-insensitive.

def is_doctor(name):
  #pass # REPLACE WITH YOUR CODE

Example usage:

print(is_doctor('Dr. Jekyll'))
print(is_doctor('DR. Jekyll'))
print(is_doctor('dR. Jekyll'))
print(is_doctor('dr. Jekyll'))
print(is_doctor('Jekyll (Dr)'))
print(is_doctor('Jekyll (DR)'))
print(is_doctor('Jekyll (dr)'))
print(is_doctor('Mr. Hyde'))
print(is_doctor('Miss Dr. B.more'))

→

True
True
True
True
True
True
True
False
False

💡 Tips

Partial solution

The find(s) method gives index of s in the string, if it is found. It returns -1 if s is not found.

Examples of the find() method:

s = 'Monty Python'
print(s.find('Monty'))
print(s.find('Python'))
print(s.find('Spam'))

→

0
6
-1

Exercise: Remove From Word

Exercise : Remove From Word

Complete the remove_from_word(text, word) function given below, to behave as follows:

If word is found in text, return text minus the word (its first appearance) and any characters that appears after the word
Return text otherwise

def remove_from_word(text, word):
  #pass # REPLACE WITH YOUR CODE

Example usage:

print('>' + remove_from_word('red-hot-lava', 'red'))
print('>' + remove_from_word('red-hot-lava', 'hot'))
print('>' + remove_from_word('red-hot-lava', 'lava'))
print('>' + remove_from_word('red-hot-lava', 'bat'))
print('>' + remove_from_word('red-hot-lava', '-'))

→

>
>red-
>red-hot-
>red-hot-lava
>red

Partial solution

The join() method joins a list of string items while using the as a .

Examples of the join() method:

print(', '.join(['tom', 'dick', 'harry']))
print('-'.join(['one', 'to', 'one']))

→

tom, dick, harry
one-to-one

The split() method is the opposite of join(). It splits a string into a list of strings based on a given delimiter string. If no delimiter is given, any in the string are used as delimiters.

Some examples of using the split() method:

print('hi, how are you?'.split())
print('A1\t\tA2\nA3'.split())
print('''Todo:
1. eat
2. sleep'''.split('\n')) # split into lines
print('1-to-1-talk'.split('-'))

→

['hi,', 'how', 'are', 'you?']
['A1', 'A2', 'A3']
['Todo:', '1. eat', '2. sleep']
['1', 'to', '1', 'talk']

There are some string methods to help you to strip trailing/leading spaces.

Examples of stripping leading/trailing spaces from a string:

s = '  hello  there!  '
print('['+ s.strip() + ']')
print('['+ s.lstrip() + ']') #left side strip
print('['+ s.rstrip() + ']') #right side strip

→

[hello  there!]
[hello  there!  ]
[  hello  there!]

Exercise: Get Part

Exercise : Get Part

Complete the get_part(text, index) function given below, to behave as follows:

Split text using | as the delimiter
Return the part indicated by the index

The return value should not have leading/trailing spaces.

def get_part(text, index):
  #pass # REPLACE WITH YOUR CODE

Example usage:

print(get_part(' John Doe | Male |    X', 0))
print(get_part('John Doe | Male |    X', 2))
print(get_part('Dog|Cat', 0))

→

John Doe
X
Dog

Partial solution

The replace() method can replace a character (or a phrase) with another character/phrase.

Some examples of using replace() method:

print('face to face'.replace(' ', '-'))  # replace space with a dash
print('1,2,3,4'.replace(',', '\t'))  # replace comma with a tab
print('Yup, Yup, I agree'.replace('Yup', 'Yes'))

face-to-face
1	2	3	4
Yes, Yes, I agree

There are some string methods to help you to align text.

Examples of aligning text using string methods:

print('Here:'.rjust(20, '>')) # right-justify
print('Price'.ljust(20, '=')) # left-justify
print('Title'.center(16, ':')) # center

→

>>>>>>>>>>>>>>>Here:
Price===============
:::::Title::::::

Exercise: Print Formatted Item

Exercise : Print Formatted Item

Complete the print_formatted_item(name, count, width) function given below, to behave as follows:

Print name and count in the format name.................: count
Width used for printing should be width (counted in number of characters)
Use 3 spaces for count i.e., assume count is 0..999
Left-justify the name and right-justify the count
Replace any square brackets [] in the name with normal brackets ()

def print_formatted_item(name, count, width):
  print('') # REPLACE WITH YOUR CODE

Example usage:

w = 25
print_formatted_item('pens', 2, w)
print_formatted_item('books[old]', 50, w)
print_formatted_item('pins[new]', 110, w)

→

pens.................:  2
books(old)...........: 50
pins(new)............:110

w = 20
print_formatted_item('pens', 2, w)
print_formatted_item('books[old]', 50, w)
print_formatted_item('pins[new]', 110, w)

→

pens............:  2
books(old)......: 50
pins(new).......:110

Partial solution