> For the complete documentation index, see [llms.txt](https://sai11101989.gitbook.io/iss_workbook/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://sai11101989.gitbook.io/iss_workbook/object-oriented-programming/python-conditions-and-loops.md). # Python - Conditions and Loops ## If and Else In Python, conditional statements are fundamental for controlling the flow of a program based on certain conditions. The primary types of conditional statements include: ### Types of Conditional Statements in Python 1. **If Statement** * The simplest form of a conditional statement that executes a block of code if a specified condition is true. * **Syntax:** ```python if condition: # code to execute if condition is true ``` * **Example:** ```python num = 8 if num > 0: print("The number is positive.") ``` * This code prints "The number is positive" if `num` is greater than zero. 2. **If-Else Statement** * This statement provides an alternative code block that executes when the condition in the `if` statement evaluates to false. * **Syntax:** ```python if condition: # code to execute if condition is true else: # code to execute if condition is false ``` * **Example:** ```python num = 11 if num % 2 == 0: print("The number is even.") else: print("The number is odd.") ``` * This code checks if `num` is even or odd and prints the corresponding message. 3. **Nested If-Else Statement** * This allows one `if` statement to be placed inside another, enabling complex decision-making. * **Syntax:** ```python if condition1: if condition2: # code for both conditions being true else: # code for condition1 true, condition2 false else: # code for condition1 false ``` * **Example:** ```python num = 3 if num > 0: if num % 2 == 0: print("The number is positive and even.") else: print("The number is positive but odd.") else: print("The number is not positive.") ``` * This checks multiple conditions about the variable `num`. 4. **If-Elif-Else Ladder** * This structure allows checking multiple expressions for truth and executing a block of code as soon as one of the conditions evaluates to true. * **Syntax:** ```python if condition1: # code block 1 elif condition2: # code block 2 elif condition3: # code block 3 else: # default code block ``` * **Example:** ```python x = 11 if x == 2: print("x is equal to 2") elif x == 3: print("x is equal to 3") elif x == 4: print("x is equal to 4") else: print("x is not equal to 2, 3, or 4") ``` * This allows for multiple conditions to be checked sequentially. 5. **Ternary Operator (Conditional Expressions)** * A shorthand way to write simple `if-else` statements in a single line. * **Syntax:** ```python value_if_true if condition else value_if_false ``` * **Example:** ```python result = 'pass' if grade >= 50 else 'fail' print(result) ``` * This assigns 'pass' or 'fail' based on the value of `grade`. These various forms of conditional statements allow Python programmers to implement complex logic and control the flow of their programs effectively. ## While loop Python has one primary type of `while` loop, but it can be used with several control statements to modify its behavior. **Types of `while` Loops** * **Basic `while` Loop**: This loop executes a block of code as long as a specified condition is true. The condition is checked at the beginning of each iteration. * **Syntax:** ```python while condition: # Code to execute ``` * **Example:** ```python i = 1 while i < 6: print(i) i += 1 ``` In this example, the loop prints the value of `i` as long as `i` is less than 6. **Control Statements in `while` Loops** * **`break` Statement**: This statement is used to exit the loop prematurely, even if the condition is still true. * **Example:** ```python i = 1 while i < 6: print(i) if i == 3: break i += 1 ``` In this case, the loop will stop when `i` equals 3. * **`continue` Statement**: This statement skips the rest of the current iteration and proceeds to the next iteration of the loop. * **Example:** ```python i = 0 while i < 6: i += 1 if i == 3: continue print(i) ``` Here, when `i` is 3, the `print(i)` statement is skipped, and the loop continues with `i = 4`. * **`else` Statement**: This block of code is executed when the loop condition becomes false. If the loop is terminated by a `break` statement, the `else` block is not executed. * **Example:** ```python pythoni = 1 while i < 6: print(i) i += 1 else: print("i is no longer less than 6") ``` The `else` block will be executed once `i` is no longer less than 6. **Other variations** * **Infinite Loops**: These occur when the loop condition never becomes false. They can be useful in certain situations, but it is important to ensure there is a way to exit the loop (e.g., using a `break` statement). * **Example:** ```python while True: # Code to execute if some_condition: break ``` * **Nested `while` Loops**: You can place one `while` loop inside another, allowing for more complex control flow. * **One-Line `while` Loops**: If the `while` loop contains simple statements, it can be written on a single line, with statements separated by semicolons. * **Example:** ```python n = 5 while n > 0: n -= 1; print(n) ``` ## For loop In Python, the `for` loop is a versatile control structure used primarily for iterating over sequences such as lists, tuples, sets, dictionaries, and strings. Here are the main types and features of `for` loops in Python: ### Types of `for` Loops in Python 1. **Basic `for` Loop** * This loop iterates over a sequence (like a list or string) and executes a block of code for each item. * **Syntax:** ```python for variable in sequence: # Code to execute ``` * **Example:** ```python fruits = ['apple', 'banana', 'cherry'] for fruit in fruits: print(fruit) ``` * This code will print each fruit in the list. 2. **`for` Loop with `range()`** * The `range()` function generates a sequence of numbers, which can be used to iterate a specific number of times. * **Syntax:** ```python for i in range(start, stop, step): # Code to execute ``` * **Example:** ```python for i in range(1, 5): print(i) ``` * This will print numbers from 1 to 4. 3. **Nested `for` Loops** * A `for` loop can be placed inside another `for` loop, allowing for iteration over multi-dimensional data structures. * **Syntax:** ```python for outer_variable in outer_sequence: for inner_variable in inner_sequence: # Code to execute ``` * **Example:** ```python matrix = [[1, 2, 3], [4, 5, 6]] for row in matrix: for item in row: print(item) ``` * This code will print each item in the two-dimensional list. 4. **List Comprehensions** * A concise way to create lists using a single line of code that includes a `for` loop. * **Syntax:** ```python new_list = [expression for item in iterable] ``` * **Example:** ```python squares = [x**2 for x in range(10)] print(squares) ``` * This creates a list of squares from 0 to 9. 5. **Iterating Over Dictionaries** * You can iterate through keys, values, or key-value pairs in a dictionary using a `for` loop. * **Example:** ```python my_dict = {'a': 1, 'b': 2, 'c': 3} for key, value in my_dict.items(): print(key, value) ``` * This will print each key-value pair in the dictionary. 6. **Using `break` and `continue` Statements** * The `break` statement can be used to exit the loop prematurely, while the `continue` statement skips the current iteration and moves to the next one. * **Example with `break`:** ```python for i in range(10): if i == 5: break print(i) ``` * This prints numbers from 0 to 4 and then exits the loop when `i` equals 5. * **Example with `continue`:** ```python for i in range(5): if i == 2: continue print(i) ``` * This prints numbers from 0 to 4 but skips printing the number 2.