python flow control - for loop

 

In Python, the for loop is a control flow statement used to iterate over a sequence (such as a list, tuple, dictionary, set, or string) or other iterable objects. The for loop allows you to execute a block of code for each element in the sequence.

Basic Structure

The basic structure of a for loop is as follows:

 

for variable in iterable:
    # Code block to execute for each element

 

• variable: This is a variable that takes the value of the item inside the iterable on each iteration.
• iterable: This can be any Python object that can return its elements one at a time, such as a list, tuple, string, etc.

Examples of Using for Loops

Iterating Over a List

 

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

for fruit in fruits:
    print(fruit)
# Output:
# apple
# banana
# cherry

 

Iterating Over a String

for char in "hello":
    print(char)
# Output:
# h
# e
# l
# l
# o

Using range() Function

The range() function generates a sequence of numbers. It's commonly used with for loops to iterate over a sequence of numbers.

# Iterating over a range of numbers from 0 to 4
for i in range(5):
    print(i)
# Output:
# 0
# 1
# 2
# 3
# 4

# Iterating with a starting and ending point
for i in range(2, 6):
    print(i)
# Output:
# 2
# 3
# 4
# 5

# Iterating with a step
for i in range(1, 10, 2):
    print(i)
# Output:
# 1
# 3
# 5
# 7
# 9

 

Iterating Over a Dictionary

When iterating over a dictionary, you can iterate over keys, values, or key-value pairs.

person = {'name': 'Alice', 'age': 30, 'city': 'New York'}

# Iterating over keys
for key in person:
    print(key)
# Output:
# name
# age
# city

# Iterating over values
for value in person.values():
    print(value)
# Output:
# Alice
# 30
# New York

# Iterating over key-value pairs
for key, value in person.items():
    print(f"{key}: {value}")
# Output:
# name: Alice
# age: 30
# city: New York

 

Using for Loop with else

An optional else block can follow a for loop. The else block executes only if the loop completes normally, without encountering a break statement.

for i in range(5):
    print(i)
else:
    print("Loop completed without break.")
# Output:
# 0
# 1
# 2
# 3
# 4
# Loop completed without break.

for i in range(5):
    if i == 3:
        break
    print(i)
else:
    print("Loop completed without break.")
# Output:
# 0
# 1
# 2

 

In this example, the else block is not executed because the loop was terminated with a break.

Nested for Loops

You can nest for loops to iterate over multiple sequences.

for i in range(3):
    for j in range(2):
        print(f"i = {i}, j = {j}")
# Output:
# i = 0, j = 0
# i = 0, j = 1
# i = 1, j = 0
# i = 1, j = 1
# i = 2, j = 0
# i = 2, j = 1

Using for Loop with enumerate()

The enumerate() function adds a counter to an iterable and returns it as an enumerate object.

 

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

for index, fruit in enumerate(fruits):
    print(f"Index {index}: {fruit}")
# Output:
# Index 0: apple
# Index 1: banana
# Index 2: cherry

for Loop with zip()

The zip() function takes multiple iterables and returns an iterator of tuples, where the i-th tuple contains the i-th element from each of the passed iterables.

 

names = ['Alice', 'Bob', 'Charlie']
ages = [25, 30, 35]

for name, age in zip(names, ages):
    print(f"{name} is {age} years old.")
# Output:
# Alice is 25 years old.
# Bob is 30 years old.
# Charlie is 35 years old.

 

 

The for loop is a fundamental construct in Python that allows you to iterate over elements of a collection or other iterable objects efficiently. It's commonly used for tasks such as processing data, executing repeated actions, and more.