More on Strings¶
In this section we will learn more about strings. Create a new Jupter Notebook and name it string_formatting_and_indexing so you can follow along with the code.
String Formatting¶
It is often the case that we want to substitute some variable or Python expression into a string. This is called string formatting. There is multiple ways to do this in Python. There is the “old way” of doing it and the newer way in Python 3.6. The easiest method is to use f-strings which will use a lot in this book. But here we will first show the old way of doing it to because you might see it while reading Python code.
String Formatting with the (% Operator)¶
This is the old way of doing it and we will not use
it much. But it’s still good to know about.
The %s is used to substitute the value
of the variable name into the string.
So if you ever see this in Python code
that is what is happening (string formatting).
name = 'chris'
print('my name is %s' %name)
my name is chris
String Formatting with .format¶
This is a newer way of formatting strings.
Again, we will not use this method because
we will opt to use f-strings. It’s
still good to know about. You can
see the syntax is nicer because
it does not require the use of %s.
print('my name is {}'.format(name))
my name is chris
f-strings¶
This is the preferred method to format strings and substitute variables and other Python expressions into strings. Learn and know this method because it is very useful!. They are called string literals or f-strings. Here is an example.
print(f'my name is {name}')
my name is chris
And that’s it! You simply put an f
at the beginning of the string in front of the quote.
You then use the braces {} and put the Python expression
between the braces. Here are some more examples.
print(f'one plus two is {1 + 2}')
one plus two is 3
name = "Chris"
age = 35
province = 'Nova Scotia'
country = 'Canada'
print(f"Hey there! My name is {name} and I am from {province}, {country}. I am {age} years old.")
Hey there! My name is Chris and I am from Nova Scotia, Canada. I am 35 years old.
Notice in the above example we were able to substitute an integer object, age directly
into the string with the use of f-strings. If we had to do the above with
string concatenation it would look more complex. We would also have to convert the
the age to a string with the str() function. But when using f-strings Python
will take care of these smaller details.
print("Hey there! My name is " + name + " and I am from " + province + ", " + country + ". I am " + str(age) + " years old.")
Hey there! My name is Chris and I am from Nova Scotia, Canada. I am 35 years old.
It is much easier to code it using the f-string method!
The f-strings work in multiline strings too.
my_multi_line_string = f"""
Name: {name}
Age: {age}
Province: {province}
Country: {country}
"""
print(my_multi_line_string)
Name: Chris
Age: 35
Province: Nova Scotia
Country: Canada
Indexing and Slicing Strings¶
Python strings are sequences made up of one or more individual characters.
You can access a specific character in a string at any position
by using its index. Index numbers allow us to access specific characters within a string.
You use the square brackets [] and the index number to get a character at the
position/index in the string.
Let’s look at some examples.
Note
We will start using Python comments in our code.
Comments are lines in your python code that are not
executed. They are any line that begins with a # symbol.
Comments can be used to explain Python code, make it more readable,
and can prevent execution when testing.
# These lines are an example of a Python comment. It is
# text in your Python code that is not executed.
# You can make comments through out your
# code to make it more readable and explain certain things.
# Here is an example of indexing a string to get specific characters at
# a position/index.
hello_str = 'Hello World!'
print(hello_str)
print(hello_str[0]) # counting and indexing in Python always starts at 0
print(hello_str[1])
print(hello_str[2])
print(hello_str[3])
print(hello_str[4])
print(hello_str[5])
print(hello_str[6])
print(hello_str[7])
print(hello_str[8])
print(hello_str[9])
print(hello_str[10])
print(hello_str[11])
Hello World!
H
e
l
l
o
W
o
r
l
d
!
If we try to index a string at a position that
is outside the length of the string we will get an index error.
The string in the above example, hello_str has a length of 12
because it has 12 characters. hello_str[0] is the first character,
hello_str[1] is the second character, and hello_str[11] is the last character.
Here we try and access hello_str[12] which will raise an error.
It’s an IndexError because we are trying to access index 12
which is outside the range of the string.
hello_str[12]
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
<ipython-input-9-cf1f2f026d16> in <module>
----> 1 hello_str[12]
IndexError: string index out of range
You can even use negative numbers for indexes
starting with -1 which will be the index
for the last character of a string.
-2 will be the index for the second last
character and so on.
print(hello_str[-1])
print(hello_str[-2])
print(hello_str[-3])
print(hello_str[-4])
print(hello_str[-5])
print(hello_str[-6])
print(hello_str[-7])
print(hello_str[-8])
print(hello_str[-9])
print(hello_str[-10])
print(hello_str[-11])
print(hello_str[-12])
!
d
l
r
o
W
o
l
l
e
H
print(f"""The first character of '{hello_str}' is {hello_str[0]},
the 5th character is {hello_str[4]} and the last character is
{hello_str[-1]}.""")
The first character of 'Hello World!' is H,
the 5th character is o and the last character is
!.
You can also access a slice of a string using the notation
[a:b] which will get every character from index a
up to index b but not including b.
print(hello_str[0:5]) # gets the characters at index 0,1,2,3,4
Hello
print(hello_str[6:11]) # gets the characters at index 6,7,8,9,10,
World
print(hello_str[6:]) # will index everything from index 6 and after
World!
print(hello_str[6:20]) # when slicing, you can access outside the range and it won't throw an error
World!
print(hello_str[-5:-2]) # you can even use negative indexes when slicing
orl
We can also set a stride when slicing strings. By default this stride is equal to one as we have not been specifying it in the above examples.
# This third number 2 is called the stride.
# Here we are using a stride of 2.
# It starts at index 0, up to and not including index 12,
# and takes every second character along the way.
# If the stride is 3 it takes every third character and so on.
print(hello_str[0:12:2])
print(hello_str[0:12:1]) # stride of 1 does no skipping
print(hello_str[0:12]) # stride of 1 by default
print(hello_str[0:12:3]) # stride of 3 example
HloWrd
Hello World!
Hello World!
HlWl
You can even set a negative value for the stride.
print(hello_str[-1:-12:-2])
print(hello_str[::-2]) # same thing as directly above.
print(hello_str[::-1]) # a stride of -1 is an easy way to reverse a string.
!lo le
!lo le
!dlroW olleH