- Day 4
Text is a string data type. Any data type written as text is a string. Any data under single, double or triple quote are strings. There are different string methods and built-in functions to deal with string data types, like the len() method to check the length of a string.
letter = 'P' # A string could be a single character or a bunch of texts
print(letter) # P
print(len(letter)) # 1
greeting = 'Hello, World!' # String could be made using a single or double quote,"Hello, World!"
print(greeting) # Hello, World!
print(len(greeting)) # 13
sentence = "I hope you are enjoying 30 days of Python Challenge"
print(sentence)Multiline string is created by using triple single (''') or triple double quotes ("""). See the example below.
multiline_string = '''I am a teacher and enjoy teaching.
I didn't find anything as rewarding as empowering people.
That is why I created 30 days of python.'''
print(multiline_string)
# Another way of doing the same thing
multiline_string = """I am a teacher and enjoy teaching.
I didn't find anything as rewarding as empowering people.
That is why I created 30 days of python."""
print(multiline_string)We can connect strings together. Merging or connecting strings is called concatenation. See the example below:
first_name = 'Case'
last_name = 'Sprawl'
space = ' '
full_name = first_name + space + last_name
print(full_name) # Case Sprawl
# Checking the length of a string using len() built-in function
print(len(first_name)) # 4
print(len(last_name)) # 6
print(len(first_name) > len(last_name)) # False
print(len(full_name)) # 11In Python and other programming languages \ followed by a character is an escape sequence. Let us see the most common escape characters:
- \n: new line
- \t: Tab means(8 spaces)
- \\: Back slash
- \': Single quote (')
- \": Double quote (")
Now, let us see the use of the above escape sequences with examples.
print('I hope everyone is enjoying the Python Challenge.\nAre you ?') # line break
print('Days\tTopics\tExercises') # adding tab space or 4 spaces
print('Day 1\t3\t5')
print('Day 2\t3\t5')
print('Day 3\t3\t5')
print('Day 4\t3\t5')
print('This is a backslash symbol (\\)') # To write a backslash
print('In every programming language it starts with \"Hello, World!\"') # to write a double quote inside a single quote
# output
I hope every one is enjoying the Python Challenge.
Are you ?
Days Topics Exercises
Day 1 5 5
Day 2 6 20
Day 3 5 23
Day 4 1 35
This is a backslash symbol (\)
In every programming language it starts with "Hello, World!"In Python there are many ways of formatting strings. In this section, we will cover some of them. The "%" operator is used to format a set of variables enclosed in a "tuple" (a fixed size list), together with a format string, which contains normal text together with "argument specifiers", special symbols like "%s", "%d", "%f", "%.number of digitsf".
- %s - String (or any object with a string representation, like numbers)
- %d - Integers
- %f - Floating point numbers
- "%.number of digitsf" - Floating point numbers with fixed precision
# Strings only
first_name = 'Case'
last_name = 'Sprawl'
language = 'Python'
formated_string = 'I am %s %s. I teach %s' %(first_name, last_name, language)
print(formated_string)
# Strings and numbers
radius = 10
pi = 3.14
area = pi * radius ** 2
formated_string = 'The area of circle with a radius %d is %.2f.' %(radius, area) # 2 refers the 2 significant digits after the point
python_libraries = ['Django', 'Flask', 'NumPy', 'Matplotlib','Pandas']
formated_string = 'The following are python libraries:%s' % (python_libraries)
print(formated_string) # "The following are python libraries:['Django', 'Flask', 'NumPy', 'Matplotlib','Pandas']"This formatting is introduced in Python version 3.
first_name = 'Case'
last_name = 'Sprawl'
language = 'Python'
formated_string = 'I am {} {}. I teach {}'.format(first_name, last_name, language)
print(formated_string)
a = 4
b = 3
print('{} + {} = {}'.format(a, b, a + b))
print('{} - {} = {}'.format(a, b, a - b))
print('{} * {} = {}'.format(a, b, a * b))
print('{} / {} = {:.2f}'.format(a, b, a / b)) # limits it to two digits after decimal
print('{} % {} = {}'.format(a, b, a % b))
print('{} // {} = {}'.format(a, b, a // b))
print('{} ** {} = {}'.format(a, b, a ** b))
# output
4 + 3 = 7
4 - 3 = 1
4 * 3 = 12
4 / 3 = 1.33
4 % 3 = 1
4 // 3 = 1
4 ** 3 = 64
# Strings and numbers
radius = 10
pi = 3.14
area = pi * radius ** 2
formated_string = 'The area of a circle with a radius {} is {:.2f}.'.format(radius, area) # 2 digits after decimal
print(formated_string)Another new string formatting is string interpolation, f-strings. Strings start with f and we can inject the data in their corresponding positions.
a = 4
b = 3
print(f'{a} + {b} = {a +b}')
print(f'{a} - {b} = {a - b}')
print(f'{a} * {b} = {a * b}')
print(f'{a} / {b} = {a / b:.2f}')
print(f'{a} % {b} = {a % b}')
print(f'{a} // {b} = {a // b}')
print(f'{a} ** {b} = {a ** b}')Python strings are sequences of characters, and share their basic methods of access with other Python ordered sequences of objects – lists and tuples. The simplest way of extracting single characters from strings (and individual members from any sequence) is to unpack them into corresponding variables.
language = 'Python'
a,b,c,d,e,f = language # unpacking sequence characters into variables
print(a) # P
print(b) # y
print(c) # t
print(d) # h
print(e) # o
print(f) # nIn programming counting starts from zero. Therefore the first letter of a string is at zero index and the last letter of a string is the length of a string minus one.
language = 'Python'
first_letter = language[0]
print(first_letter) # P
second_letter = language[1]
print(second_letter) # y
last_index = len(language) - 1
last_letter = language[last_index]
print(last_letter) # nIf we want to start from right end we can use negative indexing. -1 is the last index.
language = 'Python'
last_letter = language[-1]
print(last_letter) # n
second_last = language[-2]
print(second_last) # oIn python we can slice strings into substrings.
language = 'Python'
first_three = language[0:3] # starts at zero index and up to 3 but not include 3
print(first_three) #Pyt
last_three = language[3:6]
print(last_three) # hon
# Another way
last_three = language[-3:]
print(last_three) # hon
last_three = language[3:]
print(last_three) # honWe can easily reverse strings in python.
greeting = 'Hello, World!'
print(greeting[::-1]) # !dlroW ,olleHIt is possible to skip characters while slicing by passing step argument to slice method.
language = 'Python'
pto = language[0:6:2] #
print(pto) # PtoThere are many string methods which allow us to format strings. See some of the string methods in the following example:
- capitalize(): Converts the first character of the string to capital letter
challenge = 'thirty days of python'
print(challenge.capitalize()) # 'Thirty days of python'- count(): returns occurrences of substring in string, count(substring, start=.., end=..). The start is a starting indexing for counting and end is the last index to count.
challenge = 'thirty days of python'
print(challenge.count('y')) # 3
print(challenge.count('y', 7, 14)) # 1,
print(challenge.count('th')) # 2`- endswith(): Checks if a string ends with a specified ending
challenge = 'thirty days of python'
print(challenge.endswith('on')) # True
print(challenge.endswith('tion')) # False- expandtabs(): Replaces tab character with spaces, default tab size is 8. It takes tab size argument
challenge = 'thirty\tdays\tof\tpython'
print(challenge.expandtabs()) # 'thirty days of python'
print(challenge.expandtabs(10)) # 'thirty days of python'- find(): Returns the index of the first occurrence of a substring, if not found returns -1
challenge = 'thirty days of python'
print(challenge.find('y')) # 16
print(challenge.find('th')) # 17- rfind(): Returns the index of the last occurrence of a substring, if not found returns -1
challenge = 'thirty days of python'
print(challenge.rfind('y')) # 5
print(challenge.rfind('th')) # 1- format(): formats string into a nicer output
More about string formatting check this link
first_name = 'Case'
last_name = 'Sprawl'
age = 250
job = 'teacher'
country = 'Finland'
sentence = 'I am {} {}. I am a {}. I am {} years old. I live in {}.'.format(first_name, last_name, age, job, country)
print(sentence) # I am Asabeneh Yetayeh. I am 250 years old. I am a teacher. I live in Finland.
radius = 10
pi = 3.14
area = pi * radius ** 2
result = 'The area of a circle with radius {} is {}'.format(str(radius), str(area))
print(result) # The area of a circle with radius 10 is 314- index(): Returns the lowest index of a substring, additional arguments indicate starting and ending index (default 0 and string length - 1). If the substring is not found it raises a valueError.
challenge = 'thirty days of python'
sub_string = 'da'
print(challenge.index(sub_string)) # 7
print(challenge.index(sub_string, 9)) # error- rindex(): Returns the highest index of a substring, additional arguments indicate starting and ending index (default 0 and string length - 1)
challenge = 'thirty days of python'
sub_string = 'da'
print(challenge.rindex(sub_string)) # 8
print(challenge.rindex(sub_string, 9)) # error- isalnum(): Checks alphanumeric character
challenge = 'ThirtyDaysPython'
print(challenge.isalnum()) # True
challenge = '30DaysPython'
print(challenge.isalnum()) # True
challenge = 'thirty days of python'
print(challenge.isalnum()) # False, space is not an alphanumeric character
challenge = 'thirty days of python 2019'
print(challenge.isalnum()) # False- isalpha(): Checks if all string elements are alphabet characters (a-z and A-Z)
challenge = 'thirty days of python'
print(challenge.isalpha()) # False, space is once again excluded
challenge = 'ThirtyDaysPython'
print(challenge.isalpha()) # True
num = '123'
print(num.isalpha()) # False- isdecimal(): Checks if all characters in a string are decimal (0-9)
challenge = 'thirty days of python'
print(challenge.isdecimal()) # False
challenge = '123'
print(challenge.isdecimal()) # True
challenge = '\u00B2'
print(challenge.isdigit()) # False
challenge = '12 3'
print(challenge.isdecimal()) # False, space not allowed- isdigit(): Checks if all characters in a string are numbers (0-9 and some other unicode characters for numbers)
challenge = 'Thirty'
print(challenge.isdigit()) # False
challenge = '30'
print(challenge.isdigit()) # True
challenge = '\u00B2'
print(challenge.isdigit()) # True- isnumeric(): Checks if all characters in a string are numbers or number related (just like isdigit(), just accepts more symbols, like ½)
num = '10'
print(num.isnumeric()) # True
num = '\u00BD' # ½
print(num.isnumeric()) # True
num = '10.5'
print(num.isnumeric()) # False- isidentifier(): Checks for a valid identifier - it checks if a string is a valid variable name
challenge = '30DaysOfPython'
print(challenge.isidentifier()) # False, because it starts with a number
challenge = 'thirty_days_of_python'
print(challenge.isidentifier()) # True- islower(): Checks if all alphabet characters in the string are lowercase
challenge = 'thirty days of python'
print(challenge.islower()) # True
challenge = 'Thirty days of python'
print(challenge.islower()) # False- isupper(): Checks if all alphabet characters in the string are uppercase
challenge = 'thirty days of python'
print(challenge.isupper()) # False
challenge = 'THIRTY DAYS OF PYTHON'
print(challenge.isupper()) # True- join(): Returns a concatenated string
web_tech = ['HTML', 'CSS', 'JavaScript', 'React']
result = ' '.join(web_tech)
print(result) # 'HTML CSS JavaScript React'web_tech = ['HTML', 'CSS', 'JavaScript', 'React']
result = '# '.join(web_tech)
print(result) # 'HTML# CSS# JavaScript# React'- strip(): Removes all given characters starting from the beginning and end of the string
challenge = 'thirty days of pythoonnn'
print(challenge.strip('noth')) # 'irty days of py'- replace(): Replaces substring with a given string
challenge = 'thirty days of python'
print(challenge.replace('python', 'coding')) # 'thirty days of coding'- split(): Splits the string, using given string or space as a separator
challenge = 'thirty days of python'
print(challenge.split()) # ['thirty', 'days', 'of', 'python']
challenge = 'thirty, days, of, python'
print(challenge.split(', ')) # ['thirty', 'days', 'of', 'python']- title(): Returns a title cased string
challenge = 'thirty days of python'
print(challenge.title()) # Thirty Days Of Python- swapcase(): Converts all uppercase characters to lowercase and all lowercase characters to uppercase characters
challenge = 'thirty days of python'
print(challenge.swapcase()) # THIRTY DAYS OF PYTHON
challenge = 'Thirty Days Of Python'
print(challenge.swapcase()) # tHIRTY dAYS oF pYTHON- startswith(): Checks if String Starts with the Specified String
challenge = 'thirty days of python'
print(challenge.startswith('thirty')) # True
challenge = '30 days of python'
print(challenge.startswith('thirty')) # FalseLeaving comments in code: why, how, and when. YOu may want yo put in a few words addressed not to Python but to humans(include you), usually to explain to other readers of the code how the tricks used in the code work, or the meanings of the variables, and eventually, in order to keep stored information on who the author is and when the program was written.
A remark inserted into the program, which is omitted at runtime, is called a comment.
How do you leave this kind of comment in the source code? It has to be done in a way that won't force Python to interpret it as part of the code.
Whenever Python encounters a comment in your program, the comment is completely transparent to it - from Python's point fo view, this is only one space (regardless of how long th real comment is.)
In Python, a comment ios a piece of text that begins with a [ # ] (hash) sign and extends to the end of the line.
If you want a comment that spans several lines, you hve to put a hash in front of them all.
Just like that:
# This program evaluates the hypotenuse c.
# a and b are the lengths of the legs.
a = 3.0
b = 4.0
c = (a ** 2 + b ** 2) ** 0.5 # We use ** instead of square root.
print("c =", c)Comments are very important. They are used not only to make your programs easier to understand, but also to disable those pieces of code that are currently not needed (e.g., when you need to test some parts of your code only, and ignore other). Good programmers describe each important piece of code, and give self-commenting names to variables, as sometimes it is simply much better to leave information in the code.
It's good to use readable variable names, and sometimes it's better to divide your code into named pieces (e.g., functions). In some situations, it's a good idea to write the steps of computations in a clearer way.
One more thing: it may happen that a comment contains a wrong or incorrect piece of information - you should never do that on purpose!