Strings in Python and Indexing

Strings in Python are sequences of characters enclosed in single quotes (' '), double quotes (" "), or triple quotes (''' ''' or """ """). They are immutable sequences of Unicode code points used to represent text.

String Characteristics

1. Immutable: Once created, strings cannot be modified
2. Ordered: Characters maintain their position
3. Indexable: Individual characters can be accessed via indices

Creating Strings

python

single_quoted = 'Hello'
double_quoted = "World"
triple_quoted = '''This is a 
multi-line string'''

String Indexing

Python uses zero-based indexing for strings, where:

• The first character has index 0
• The second character has index 1
• And so on…

Positive Indexing

python

text = "Python"
# P y t h o n
# 0 1 2 3 4 5

print(text[0])  # 'P'
print(text[3])  # 'h'

Negative Indexing

Python also supports negative indices that count from the end:

• The last character has index -1
• The second last has index -2
• And so on…

python

text = "Python"
# P y t h o n
# -6 -5 -4 -3 -2 -1

print(text[-1])  # 'n'
print(text[-3])  # 'h'

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String Length and Traversal in Python

1. Length of a String

You can find the length of a string using the built-in len() function:

python

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

2. Traversing a String Without Using range()

Method 1: Using a for loop directly

python

text = "Python"

# Traversing using for loop
for char in text:
    print(char)

# Output:
# P
# y
# t
# h
# o
# n

Method 2: Using while loop

python

text = "Python"
i = 0
while i < len(text):
    print(text[i])
    i += 1

# Output:
# P
# y
# t
# h
# o
# n

Method 1: Using range() with index

python

text = "Python"

for i in range(len(text)):
    print(f"Character at index {i}: {text[i]}")

Comparison

• Without range(): Simpler and more Pythonic when you just need the characters
• With range(): Useful when you need both the index and the character, or when you need to modify the traversal pattern (like stepping through every second character)

Example: Traversing Backwards

python

text = "Python"

# Without range
for char in reversed(text):
    print(char)

# With range
for i in range(len(text)-1, -1, -1):
    print(text[i])

# Both output:
# n
# o
# h
# t
# y
# P