មេរៀនសង្ខេប : Variables, Basic Operations & String Manipulation

1. Variables

A variable is a named storage location that holds data during program execution. A variable has a name, value, type, memory location, scope, and mutability. In Python, a variable is created by assigning a value with =. Example: x = 42. Python is dynamically typed, so its type is inferred from the assigned value.

Common types: int = whole numbers (x = 10); float = decimal numbers (y = 3.14); str = text in quotes (name = "Sok"); bool = True/False; list = ordered and mutable ([1, 2, 3]); tuple = ordered and immutable ((4, 5)); dict = key-value pairs ({'name':'Alice'}); set = unique unordered values ({1, 2, 3}); None = absence of a value.

Important: Python variable names are case-sensitive (myVar ≠ myvar), cannot start with a number, and should follow PEP 8 naming conventions.

2. Basic Operations

Arithmetic: + addition, − subtraction, * multiplication, / division, // floor division, % remainder (modulus), ** exponentiation. Examples: 10 + 5 = 15; 17 // 5 = 3; 17 % 5 = 2; 2 ** 3 = 8.

Assignment: = assigns a value. In-place operators update a variable: +=, -=, *=, /=. Example: y = 5; y += 3 → y = 8.

Comparison: == equal, != not equal, > greater than, < less than, >= greater/equal, <= less/equal. These comparisons return True or False.

Logical: and → True when both conditions are true; or → True when at least one is true; not → reverses the truth value.

Membership: in → checks whether a value exists in a sequence; not in → checks whether it does not exist. Example: 2 in [1,2,3] → True.

3. String Manipulation

Strings are sequences of characters. Common operations include:

• Concatenation: joins strings using +. Example: "Hello" + " " + "World" → "Hello World".

• Formatting: f-strings insert variables into text. Example: f"My name is {name} and I am {age} years old."

• Length: len(text) returns the number of characters.

• Indexing: text[0] gets the first character.

• Slicing: text[0:6] extracts characters from index 0 up to (not including) 6.

• Case conversion: text.upper() → uppercase; text.lower() → lowercase.

• Whitespace: text.strip() removes leading and trailing spaces.

• Replace: text.replace("Java", "Python") replaces a substring.

• Checking: "fun" in text checks whether a substring exists.

• Splitting: sentence.split() breaks a string into a list of words.

4. Key Exercise Skills

The exercise practices creating variables, using len(), converting text with upper(), replacing words with replace(), counting characters with count(), reversing a string with slicing [::-1], and printing results.