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Strings

Below is a complete, detailed, and exam-oriented explanation of Strings in Python, including ALL major operations, functions, and methods.

STRINGS IN PYTHON

What is a String?

A string is a sequence of characters enclosed in:

  • Single quotes → 'hello'
  • Double quotes → "hello"
  • Triple quotes → """hello""" (for multi-line strings)

Characteristics of Strings

✔ Ordered (indexed)
✔ Immutable (cannot be modified)
✔ Iterable (can loop through characters)
✔ Supports slicing
✔ Can hold Unicode text (supports all languages)

1. Creating Strings

s1 = 'Hello'
s2 = "Python"
s3 = """Multi
line
string"""

2. Accessing String Characters (Indexing)

Positive indexing:

s = "Python"
print(s[0])  # P
print(s[3])  # h

Negative indexing:

print(s[-1])  # n
print(s[-3])  # h

3. Slicing Strings

Syntax:

string[start : end : step]

Examples:

s = "Programming"
print(s[0:5])   # Progr
print(s[:7])    # Program
print(s[3:])    # gramming
print(s[::-1])  # Reverse string

4. String Immutability

Strings cannot be modified:

s = "Hello"
s[0] = 'h'   # ❌ Error

To modify:

s = 'h' + s[1:]

5. String Operations

1. Concatenation (+)

"Hello " + "World"

2. Repetition (*)

"Hi" * 3   # HiHiHi

3. Membership (in, not in)

"Py" in "Python"    # True

4. Comparison

"abc" == "abc"
"apple" < "banana"

5. Iteration

for ch in "Python":
    print(ch)

6. Length

len("Python")   # 6

6. Built-in Functions for Strings

FunctionDescription
len(s)Length of string
max(s)Character with highest ASCII value
min(s)Character with lowest ASCII value
sorted(s)Returns characters in sorted order
str(obj)Converts object to string

Examples:

s = "python"
print(max(s))   # y
print(sorted(s))  # ['h','n','o','p','t','y']

7. STRING METHODS (VERY IMPORTANT)

Below is a complete list of essential string methods.

A. Case Conversion Methods

1. lower()

Converts to lowercase.

"HELLO".lower()

2. upper()

Converts to uppercase.

"hello".upper()

3. title()

Each word’s first letter becomes uppercase.

"hello world".title()

4. capitalize()

First letter capitalized.

"python programming".capitalize()

5. swapcase()

Uppercase ↔ lowercase.

"HeLLo".swapcase()

B. Search & Find Methods

1. find(sub)

Returns index of substring (or -1).

"hello".find("l")   # 2

2. rfind(sub)

Find from right side.

3. index(sub)

Like find() but gives error if substring not found.

4. startswith(prefix)

Checks starting.

"Python".startswith("Py")

5. endswith(suffix)

Checks ending.

C. Modification Methods

1. replace(old, new)

"hello world".replace("world", "Python")

2. strip() / lstrip() / rstrip()

Removes whitespace.

"  hello  ".strip()

3. join()

Joins list of strings.

"-".join(["a","b","c"])

4. split(sep)

Splits into list.

"apple,banana".split(",")

5. splitlines()

Splits lines in multi-line text.

D. Testing Methods (Return True/False)

1. isalnum() → only letters & numbers

2. isalpha() → only letters

3. isdigit() → only numbers

4. islower() / isupper()

5. isspace() → only whitespace

6. istitle()

Examples:

"123".isdigit()   # True
"Hello".isalpha() # True

E. Alignment and Formatting Methods

1. center(width)

"Hi".center(10)

2. ljust() / rjust()

Left/right alignment.

3. zfill(width)

Pads number with zeros.

"42".zfill(5)   # 00042

8. Escape Sequences

EscapeMeaning
\nNew line
\tTab
\\Backslash
\'Single quote
\"Double quote

Example:

print("Hello\nWorld")

9. Raw Strings

Used to ignore escape sequences:

s = r"C:\new\folder"

10. String Formatting

1. f-strings (best method)

name = "John"
print(f"My name is {name}")

2. format()

"{} scored {}".format("John", 90)

3. % formatting

print("Age = %d" % 25)

11. Important String Programs

1. Reverse String

s = "Python"
print(s[::-1])

2. Count vowels

vowels = 0
s = "Education".lower()
for ch in s:
    if ch in "aeiou":
        vowels += 1
print(vowels)

3. Check Palindrome

s = "madam"
print(s == s[::-1])

4. Remove all spaces

"Python Programming".replace(" ", "")

5. Frequency of characters

s = "apple"
freq = {}
for ch in s:
    freq[ch] = freq.get(ch, 0) + 1
print(freq)

SUMMARY TABLE

CategoryMethods
Case changelower, upper, title, capitalize, swapcase
Searchfind, rfind, index, startswith, endswith
Modifyreplace, strip, split, join
Testingisdigit, isalpha, isalnum, isspace, islower, isupper
Alignmentcenter, ljust, rjust, zfill