String operations

Working with Strings in Python

In this chapter, you’ll learn how to do useful operations with strings: combining them, repeating them, checking their length, and picking out parts of them.

Assume you already know what a string is and how to store it in a variable (covered in the str chapter).

Throughout this chapter we’ll use examples like:

message = "Hello"
name = "Alice"

Concatenation: Joining Strings Together

Concatenation means joining strings end to end.

Use the + operator to concatenate strings:

first_name = "Alice"
last_name = "Smith"
full_name = first_name + last_name
print(full_name)        # AliceSmith
full_name_with_space = first_name + " " + last_name
print(full_name_with_space)  # Alice Smith

You can concatenate:

• variables that hold strings
• string literals like " ", "!", "Python"

Common mistake: trying to add a string and a number directly:

age = 30
# print("Age: " + age)   # This causes an error!

To fix this, convert the number to a string first (you’ll see more about this when combining text and variables):

age = 30
print("Age: " + str(age))   # Age: 30

Repeating Strings with `*`

You can repeat a string multiple times using the * operator with an integer:

laugh = "ha"
print(laugh * 3)   # hahaha
line = "-"
print(line * 10)   # ----------

This is useful for simple text effects:

title = "Python"
print("=" * 10)
print(title)
print("=" * 10)

Finding the Length of a String: `len()`

Use len() to count how many characters are in a string:

word = "Python"
print(len(word))       # 6
empty = ""
print(len(empty))      # 0
space = " "
print(len(space))      # 1

Remember:

• Spaces, punctuation, and symbols are all counted as characters.
• len() returns an int.

Accessing Individual Characters (Indexing)

You can access a single character in a string using square brackets [] with an index.

• Indexing starts at 0.
• For the string "Python":
• P is index 0
• y is index 1
• n is index 5

word = "Python"
print(word[0])   # P
print(word[1])   # y
print(word[5])   # n

Common mistakes:

• Using an index that’s too large causes an error:

  word = "Hi"
  # print(word[2])   # Error: index out of range

• Trying to change a character directly:

  text = "cat"
  # text[0] = "b"    # This is NOT allowed

Strings are immutable (you can’t change them in place). To “change” them, you build a new string (shown later in this chapter).

Negative Indexing (From the End)

You can also count from the end of the string using negative indices:

• -1 is the last character
• -2 is the second-to-last, and so on

For "Python":

• n is -1
• o is -2
• P is -6

word = "Python"
print(word[-1])   # n
print(word[-2])   # o
print(word[-6])   # P

Slicing: Getting Substrings

A slice is a part of a string. Use:

string[start:end]

• start = index where the slice begins (inclusive)
• end = index where the slice stops (exclusive, not included)

For "Python":

• string[0:2] gives "Py"
• string[1:4] gives "yth"

Examples:

text = "Python programming"
print(text[0:6])     # Python
print(text[7:18])    # programming
# from index 7 up to (but not including) index 11
print(text[7:11])    # prog

You can omit start or end:

• Omit start → slice from the beginning
• Omit end → slice to the end

text = "Hello, world!"
print(text[:5])    # Hello   (from start to index 5, not included)
print(text[7:])    # world!  (from index 7 to the end)
print(text[:])     # Hello, world! (whole string)

Negative Indices in Slices

You can combine slicing with negative indices:

text = "Hello, world!"
# last 6 characters
print(text[-6:])    # world!
# everything except last character
print(text[:-1])    # Hello, world
# slice using negative start and end
print(text[-6:-1])  # world

Basic String Methods

Strings have methods—functions that belong to the string object.
You call them with dot notation: string.method().

Here are some useful basic methods. They return new strings; they don’t change the original.

Changing Case: `lower()`, `upper()`, `title()`

text = "Hello, World!"
print(text.lower())   # hello, world!
print(text.upper())   # HELLO, WORLD!
print(text.title())   # Hello, World!

The original string stays the same:

print(text)           # Hello, World!

Removing Extra Spaces: `strip()`, `lstrip()`, `rstrip()`

These methods remove spaces (and some other whitespace) from the ends of a string.

raw = "   Python  "
print("[" + raw + "]")           # [   Python  ]
print("[" + raw.strip() + "]")   # [Python]
print("[" + raw.lstrip() + "]")  # [Python  ]
print("[" + raw.rstrip() + "]")  # [   Python]

Counting and Finding Text: `count()`, `find()`

count(sub) tells you how many times a substring appears:

text = "banana"
print(text.count("a"))   # 3
print(text.count("na"))  # 2

find(sub) gives the index where a substring first appears, or -1 if not found:

text = "Hello, world!"
print(text.find("world"))   # 7
print(text.find("Python"))  # -1

Replacing Text: `replace()`

replace(old, new) returns a new string with all old parts replaced by new:

text = "I like cats. Cats are cute."
new_text = text.replace("cats", "dogs")
print(new_text)   # I like dogs. Cats are cute.
new_text2 = text.replace("Cats", "Dogs")
print(new_text2)  # I like cats. Dogs are cute.

Note: replacement is case-sensitive ("cats" ≠ "Cats").

Escaping Special Characters

Some characters have a special meaning in strings, like the quote characters and backslash.
Use the backslash \ to escape them.

Quotes Inside Strings

If your string uses double quotes outside, you can use single quotes inside (and vice versa) without escaping:

msg1 = "She said 'hello'."
msg2 = 'He replied "hi".'

To use the same quote type inside and outside, escape the inner quotes:

msg = "She said \"hello\"."
print(msg)   # She said "hello".

New Lines and Tabs: `\n`, `\t`

Some escape sequences represent special characters:

• \n → new line
• \t → tab

text = "First line\nSecond line"
print(text)
# First line
# Second line
columns = "Name\tAge"
print(columns)
# Name    Age

Converting Between Strings and Numbers (Quick Overview)

When mixing text and numbers, you often need to convert between types:

• str(x) → turn x into a string
• int(s) → convert string s to an integer (if possible)
• float(s) → convert string s to a float (if possible)

You’ll use this heavily when combining text and variables, and for user input, but here’s a simple example focused on strings:

age = 25
# Convert number to string to join with +
message = "You are " + str(age) + " years old."
print(message)

Simple String Exercises

Try these small tasks to get comfortable with string operations:

1. Create a variable word = "Python" and:
• Print the first and last characters.
• Print the substring "tho" using slicing.
2. Ask the user (using input()) for their first and last name, then:
• Create a full name with a space between them.
• Print it in uppercase.
• Print how many characters are in the full name (including the space).
3. Given text = " I love Python! ", print:
• The text without leading and trailing spaces.
• The number of "o" characters in the cleaned-up text.
• The text with "Python" replaced by "coding".
4. Create a simple “banner” for a title:
• Ask for a title (e.g. "Welcome").
• Print a line of * characters above and below the title.
  The number of * should match the length of the title.

These exercises will give you practice with concatenation, repetition, slicing, string methods, and len().