Lesson 3c - Strings

Another one of Python’s built-in data types are strings. Strings represent a sequence of characters.

Lesson Objectives

• Learn about strings
• Explore some of the operations and functions Python has to offer for strings.
• Convert variables into strings.
• Format strings for more informative print() output.

Creating a String

In Python, strings are surrounded by single quotes (‘ ‘) or double quotes (“ “).

Don’t mix and match! You can’t have a string start with single quotes and end with double quotes.

myString = "Hello, World!"

print(myString)

Hello, World!

Using Quotations Inside Strings

If you need to use single quotes ‘ ‘, use double quotes “ “ to define the string.

myString = "I'm happy!"

If you need to use double quotes “ “, use single quotes ‘ ‘ to define the string.

myString = 'He said "hello".'

If you need to use both single quotes ‘ ‘ and double quotes “ “, you need to put a backslash \ before every quotation you defined the string with.

myString = "He said that \"He's happy\""
myString = 'He said that "He\'s happy"'

String Concatenation and Repetition

You can combine string together by using the + operator.

myString = "abc"
myString2 = "ABC"

print(myString + myString2)

abcABC

Similarly, you can repeat a string by using the * operator.

myString = "abc"

# Repeats myString 5 times.
print(myString * 5)

abcabcabcabcabc

String Operations and Functions

Length of a String

Before we get to indexing and slicing strings, let’s briefly mention the len() function. It returns the number of characters in a string.

myString = "abc"

print(len(myString))

3

Indexing

If we wanted to get the character at a specific index of a string, we have to use a special notation. It’s best to show an example.

myString = "Hello."

print(myString[0])
print(myString[1])
print(myString[2])
print(myString[3])
print(myString[4])
print(myString[5])

H
e
l
l
o
.

As you can see, to index a string, you have to mention the variable name followed by a number surrounded by square brackets [ ].

In most programming languages, including Python, the first index starts at 0. If we want to get the 1st character of a string, we need to get the 0th index. If we want to get the 4th character of a string, we need to get the 3rd index.

Python will give you an error if you try to access a string index that does not exist. Using myString = "Hello." as an example, accessing the 8th index by doing myString[8] will give you an error because the 8th index does not exist.

Backwards Indexing

You can also index starting from the end of the string by using negative numbers for your index.

myString = "Hello."

print(myString[-1])
print(myString[-2])
print(myString[-3])
print(myString[-4])
print(myString[-5])
print(myString[-6])

.
o
l
l
e
H

You can see here that the last character can be found by using -1 as your index.

Slicing

Using indexes, we can also slice the string into a substring.

myString = "Hello."

print(myString[1:3])  # Gets index 1 (inclusive) to index 3 (exclusive).
print(myString[1:])   # Gets index 1 (inclusive) to the end of the string.
print(myString[:3])   # Gets the entire string up until index 3 (exclusive).

el
ello.
Hel

Skipping Letters and Reversing a String

Using the same notation as slicing, we can also reverse a string.

myString = "Hello."

print(myString[::2])   # From the start to the end of the string, get every 2nd letter
print(myString[::-1])  # From the start to the end of the string, go backwards

Hlo
.olleH

Membership Checking

Using membership checking, we can check if a string is part of another string.

There are two operators for Membership Checking.

in

Checks if a string is part of another string.

not in

Checks if a string is not part of another string.

myString = "Hello! Welcome to the SCDS."
animal = "Penguin."

print("Welcome" in myString)   # Checks if the string "Welcome" is in myString.
print(animal not in myString)  # Checks if the string "Penguin." is not in myString.

True
True

Converting Variables to Strings

The str() function converts a variable into a string.

testScore = 18/24*100

# str(testScore) turns the test score into a string, and appends it into a sentence. 
print("Your test score was " + str(testScore) + "%.")

Your test score was 75.0%.

String Formatting

Another way to insert variables into strings is by using the format() function.

numerator = 18
denominator = 24

print("Your test score was {}/{}.".format(numerator, denominator))

Your test score was 18/24.

The format() function has a lot of other cool functionalities, which can be found here.

You can also accomplish the same thing by adding the letter f at the start of the string, before the surrounding quotes.

testScore = 18/24*100

print(f"Your test score was {testScore}%.")

Your test score was 75.0%.

Key Points / Summary

• Strings are sequences of characters.
• The first character in a string is at index 0.
• You can convert variables into strings.