Strings

1. len() function

The len() function returns the number of characters in a string.

text = "Hello, World!"
print(len(text))  # Output: 13

2. Accessing String Characters

1. String Indexing

Accessing individual characters in a string using their position (index).

text = "Python"
print(text[0])  # Output: P
print(text[-1])  # Output: n (negative indexing starts from the end)

2. String Looping

Iterating through each character in a string.

text = "Python"

# more commonly used is this shorthand
for char in text:
    print(char)

# using the range() and len() function
for char in range(len(text)):
	print(char)

3. String Concatenation

Combining two or more strings into a single string using the + operator.

str1 = "Hello"
str2 = "World"
result = str1 + " " + str2
print(result)  # Output: Hello World

4. String Slicing

Extracting a portion of a string using a start and end index. String slicing returns the sliced part as a new string, not making changes to actual string.

• The starting index is inclusive, and the ending index is exclusive.

# syntax
my_string[start:end]

text = "Python Programming"
print(text[0:6])  # Output: Python

With string slicing we are actually not required to specify either the start or the end index.

• If we don't specify the start, it's equivalent to starting from the beginning of the string.
• If we don't specify the end, it's equivalent to ending at the end of the string.
• If we don't specify either, we get the entire string.

text = "Python Programming"
print(text[:3])  # Output: Pyt
print(text[3:])  # Output: hon Programming
print(text[:])   # Output: Python Programming

Advanced slicing techniques, including step size.

# syntax
my_string[start:end:step]

text = "Python Programming"
print(text[::2])  # Output: Pto rgamn (every second character)

5. Reverse a String

We can also use slicing to reverse a string. By not specifying the starting index or the ending index, and setting the step to -1, the string will be reversed.

my_string = "Hello"

print(my_string[::-1])  # Output: olleH

6. String are Immutable

In Python, strings are immutable, which means they cannot be changed after they are created. It's important to know that whenever you slice a string, you are not modifying the underlying string. Instead, you are creating a new string with the sliced characters.

message = "I will never change."

message[0] = "X" # This will cause an error

message = "I will never change."

before_second = message[:1] # "I"
after_second = message[2:]  # "will never change."

new_message = before_second + after_second

7. String Formatting

1. format() method

We can format strings in Python using the .format() method.

Example:

• We have a string with two placeholders: {}.
• We then call the format method on the string and pass in the values we want to replace the placeholders with.
• The values are passed in the order they are to be inserted.
• The number of placeholders must match the number of arguments passed to the format method.

name = "Alice"
age = 25

msg = "Hello, {}. You are {} years old.".format(name, age)

print(msg)  # Output: Hello, Alice. You are 25 years old.

You can also use the index of the placeholders to specify the order of the arguments.

name = "Alice"
age = 25

msg = "Hello, {1}. You are {0} years old.".format(age, name)

print(msg)  # Output: Hello, Alice. You are 25 years old.

2. f-strings

An even more concise way to format strings is to use f-strings. These are prefixed with an f before the string and allow you to insert variables/expressions/function calls into the string.

name = "Alice"
age = 25

msg = f"Hello, {name}. You are {age} years old."

print(msg)  # Output: Hello, Alice. You are 25 years old.