Strings in Python: Indexing and Slicing for Beginners

Strings in Python are a cornerstone of programming, and Python makes working with them incredibly flexible. In this guide, we’ll explore some fundamental concepts of working with strings, including indexing, slicing, and how to handle common challenges. Whether you’re a complete beginner or brushing up on your skills, this guide will equip you with the tools to manipulate and extract data from strings effortlessly.

Getting Started with Strings in Python

In Python, a string is simply a sequence of characters enclosed in either single (') or double (") quotes. Both styles work interchangeably, but you’ll need to choose the appropriate one when handling specific cases like apostrophes.

Example: Handling Apostrophes in Strings

# Using single quotes
sentence = 'This is a sentence.'
print(sentence)  # Output: This is a sentence.

# Handling an apostrophe
sentence_with_apostrophe = "I'm coming home"
print(sentence_with_apostrophe)  # Output: I'm coming home

Using double quotes (") avoids errors when your string includes an apostrophe (').

Escape Sequences

Escape sequences let you control how strings are printed. For instance, n represents a new line. Let’s look at an example:

# Using n to print text on separate lines
sentence = "I'm comingnhome"
print(sentence)

Output:

I'm coming
home

Notice the n creates a new line. Avoid spaces after n unless you intentionally want them.

Indexing in Strings

Every character in a Python string has a unique index. Indexing starts at 0 for the first character and continues sequentially. Additionally, you can use negative indexing to count backward, starting from -1.

Example: Accessing Characters by Index

sentence = "This is a sentence"

# Positive indexing
print(sentence[0])  # Output: T
print(sentence[3])  # Output: s

# Negative indexing
print(sentence[-1])  # Output: e
print(sentence[-3])  # Output: c

Using negative indexes is a handy way to access characters without counting the total length.

Slicing Strings

Slicing allows you to extract substrings (a portion of the string) by specifying a range of indexes. The syntax for slicing is:

string[start:end]

• start: The index to begin the slice (inclusive).
• end: The index to stop at (exclusive).

Example: Basic Slicing

sentence = "This is a sentence"

# Extracting "This"
print(sentence[0:4])  # Output: This

# Extracting "is"
print(sentence[5:7])  # Output: is

If the start index is omitted, slicing begins at the start of the string. Similarly, if the end index is omitted, it slices until the end of the string.

# Slicing from the start
print(sentence[:4])  # Output: This

# Slicing to the end
print(sentence[10:])  # Output: sentence

Skipping Characters in Slices

You can add a third value, called a step, to specify how many characters to skip.

# Syntax: string[start:end:step]
sentence = "abcdefg"

# Extract every second character
print(sentence[0:7:2])  # Output: aceg

If you only want to slice from a given index to the end while skipping characters, simply omit the end value.

Reversing a String

You can reverse a string by using slicing with a step of -1.

sentence = "abcdefg"

# Reverse the string
print(sentence[::-1])  # Output: gfedcba

Practice Challenge

Try extracting the word is from the string below:

Hint: Identify the indexes of the first letter (i) and the position right after the last letter (s). Don’t forget that the second index in slicing is exclusive!

Resources

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• Watch the following free lecture on the Strings in Python:

Wrapping Up

String manipulation in Python is powerful and straightforward once you master the basics of indexing and slicing. Whether you’re extracting specific characters, creating substrings, or skipping characters, these techniques provide a strong foundation for handling text data.