3. Python FOR cycle - Guide and examples for teaching Python | Pythonlib в питоне, теория питона, питон, python

A self-study guide for Python 3 compiled from the materials on this site. Primarily intended for those who want to learn the Python programming language from scratch.

What is the for loop in Python

The for loop in Python is a built-in construct for iterating over elements of collections (lists, strings, tuples, sets, and dictionaries) or for performing operations on sequences of numbers. The for loop is one of the most important tools in Python and allows efficient data processing.

The basic structure of the for loop

for element in collection:
    # A block of code that is executed for each element

Examples of using the for loop

Iterating through the list

One of the most common ways to use the for loop is to bypass the list elements.:

numbers = [1, 2, 3, 4, 5]

for num in numbers:
    print(num)

Iterating over the line

The for loop allows you to iterate through each character in a string:

message = "Hello, World!"

for char in message:
    print(char)

Dictionary iteration

When working with dictionaries, you can iterate through keys, values, or key-value pairs.:

person = {'name': 'Ivan', 'age': 30, 'city': 'Moscow'}

# Key iteration
for key in person.keys():
    print(key)

# Iterate over the values
for value in person.values():
    print(value)

# Key-value pair iteration
for key, value in person.items():
    print(f"{key}: {value}")

Iterating over a tuple

colors = ('red', 'green', 'blue')

for color in colors:
    print(color)

The range() function in the for loop

The range() function is used to generate sequences of numbers, which makes it indispensable for iteration in the for loop.

The main use cases for range()

# Iteration over a sequence of numbers from 0 to 4
for i in range(5):
print(i) # outputs: 0, 1, 2, 3, 4

# Iteration with initial value and final value
for i in range(2, 8):
print(i) # outputs: 2, 3, 4, 5, 6, 7

# Iterate in steps
for i in range(1, 10, 2):
print(i) # outputs: 1, 3, 5, 7, 9

Increment and decrement in Python

What is an increment and a decrement

An increment is an increase in the value of a variable by a certain number, usually by 1. A decrement is a decrease in the value of a variable by a certain number, usually by 1.

Increment in Python

In Python, there is no special operator for increment (like ++ in C++ or Java), but the += operator is used:

# Initialize the variable
number = 5
# Increasing the value of a variable by 1
number += 1
print(number)  # will output 6

Explanation:

number = 5 sets the initial value of the number variable to 5
the number += 1 means "increase the value of the number by 1". This is equivalent to writing number = number + 1
print(number) outputs the new value of the number variable, which is now 6

Decrement in Python

Python uses the -= operator for decrement:

# Initialize the variable
number = 5
# Reducing the value of a variable by 1
number -= 1
print(number)  # will output 4

Explanation:

number = 5 sets the initial value of the number variable to 5
the number -= 1 means "reduce the value of the number by 1". This is equivalent to writing number = number - 1
print(number) outputs the new value of the number variable, which is now 4

Assignment operators with other mathematical operations

Increment and decrement work with all mathematical operations. Instead of a number, you can use another variable:

a = 5
a += 5 # a = 10 (addition)
a -= 3 # a = 7 (subtraction)
a *= 4 # a = 28 (multiplication)
a /= 2 # a = 14.0 (division)
a //= 3 # a = 4.0 (integer division)
a %= 5 # a = 4.0 (remainder of division)
a **= 2 # a = 16.0 (exponentiation)

Cycle control: break, continue and else

The break operator

The break operator is used to exit the loop early.:

for i in range(10):
    if i == 5:
        break
    print(i)
# Outputs: 0, 1, 2, 3, 4

The continue operator

The continue operator is used to skip to the next iteration of the loop, skipping the remaining code in the current iteration.:

for i in range(10):
    if i % 2 == 0:
        continue
    print(i)
# Outputs: 1, 3, 5, 7, 9

The else block in the for loop

The for loop may have a else block, which is executed if the loop ended without using the break operator:

for i in range(5):
print(i)
else:
print("The loop is completed without interruptions")

Nested for loops

You can use one for loop inside another to work with multidimensional data structures:

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

for row in matrix:
    for element in row:
        print(element, end=' ')
print() # line break

The enumerate() function

The enumerate() function allows you to get the index of an element during iteration:

fruits = ['apple', 'banana', 'orange']

for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

Practical tips for using the for loop

Use clear variable names for iteration
Use list comprehensions for simple operations on lists
Use enumerate() when you need an element index
Keep performance in mind when working with large amounts of data

The for loop is a powerful Python tool that allows you to efficiently process data and perform repetitive operations. Mastering all its features will help you write more readable and efficient code.

Using Forbon Cycles in Python: Iteration on sequences and collections to perform repeated actions.