1_for_loop.md

For Loop In Python

The for loop is used to iterate over a sequence of items, such as a list, str, tuple. The for loop will iterate over each item in the sequence, and it will execute the code in the loop block for each item.

The Python documentation:

The for statement in Python differs a bit from what you may be used to in other programming languages. Python’s for statement iterates over the items of any sequence (a list or a string), in the order that they appear in the sequence.

animals = ['cat', 'elephant', 'tiger']
for w in animals:
    print(w, len(w))

# Output:
cat 3
elephant 8
tiger 5

For loop: Strings

word = 'abracadabra'
for letter in word:
    print(letter)

For loop: Lists

# Define a list of numbers
numbers = [1, 2, 3, 4, 5, 6, 7, 8]

# Use the for loop to iterate over the list
for number in numbers:
    # This code will be executed for each item in the list
    print(number)

For loop: Dictionaries

We can use the for loop to iterate over the keys, values, or items in a dictionary. For example, using a for loop to iterate over the keys in a dictionary:

# Define a dictionary
my_dict = {"name": "John", "age": 25, "gender": "Male"}

# Use the for loop to iterate over the keys in the dictionary
for key in my_dict:
    # This code will be executed for each key in the dictionary
    print(key)


# Output: 
name
age
gender

Using the values() method to iterate over the values in a dictionary, like this:

# Define a dictionary
my_dict = {"name": "John", "age": 25, "gender": "Male"}

# Use the for loop to iterate over the values in the dictionary
for value in my_dict.values():
    # This code will be executed for each value in the dictionary
    print(value)

# Output:
John
25
Male

Finally, we can use the items() method to iterate over the items in a dictionary, which are represented as key-value pairs. For example:

# Define a dictionary
my_dict = {"name": "John", "age": 25, "gender": "Male"}

# Use the for loop to iterate over the items in the dictionary
for value in my_dict.items():
    # This code will be executed for each item in the dictionary
    print(value)

# Output:
('name', 'John')
('age', 25)
('gender', 'Male')

range()

We can also use the range() function to define a range of numbers to iterate over. For example:

# Use the range() function to define a range of numbers
for number in range(1, 11):
    # This code will be executed for each number in the range
    print(number)

Iterating over a reversed list:

for i in reversed(range(1, 10, 2)):
    print(i, end=' ')

# Output:
9 7 5 3 1

Iterating on a list after sorting the elements:

basket = ['orange', 'apple', 'pear', 'orange', 'banana']
for fruit in sorted(basket):
    print(fruit.capitalize())

# Output:
Apple
Banana
Orange
Orange
Pear

To loop over two or more sequences at the same time, the entries can be paired with the zip() function. For example:

questions = ['name', 'school', 'roll number']
answers = ['John', 'ABC School', 20]
for que, ans in zip(questions, answers):
    print(f'What is your {que}?')
    print(f'It is {ans}.')

# Output:
What is your name?
It is John.
What is your school?
It is ABC School.
What is your roll number?
It is 20.

Enumerate()

The enumerate() method is used to iterate over a sequence of items and retrieve the index and value of each item. This is useful when we require both the index and the value of each item in a list or other sequence.

Here is an example of using the enumerate() method in a for loop:

# Define a list of numbers
letters = ['a', 'b', 'c', 'd', 'e']

# Use the enumerate() method to iterate over the list.
# The enumerate object yields pairs containing a count (from start, which defaults to zero) 
# and a value yielded by the iterable argument.
for index, value in enumerate(letters):
    # This code will be executed for each item in the list
    print(f"Index: {index}, Value: {value}")

# Output:
Index: 0, Value: a
Index: 1, Value: b
Index: 2, Value: c
Index: 3, Value: d
Index: 4, Value: e

We can also specify a start index for the enumerate() method by passing it as an argument. Enumerate takes an optional argument start, to start index from. For example:

# Define a word
word = "Python"

# Use the enumerate() method to iterate over each letter of the word, and start indexing from 11.
for index, value in enumerate(word, 11):
    # This code will be executed for each item in the list
    print(f"Index: {index}, Value: {value}")

# Output:
Index: 11, Value: P
Index: 12, Value: y
Index: 13, Value: t
Index: 14, Value: h
Index: 15, Value: o
Index: 16, Value: n

for-else

In Python, the else keyword can be used in combination with a for loop to specify a block of code that should be executed when the loop completes successfully.

# Define a letter.
guess = 'k'

# Define a list of letters
letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']

# Use the for loop to iterate over the list
for letter in letters:
    # This code will be executed for each item in the list
    if letter == guess:
        print(f"Found guessed letter '{guess}' in the list.")
        # Use the break statement to exit the loop
        break
else:
    # This code will only be executed if the loop completes successfully, without break.
    print(f"The guessed letter '{guess}' is not found!")

# Output(guess = 'k'): The guessed letter 'k' is not found!
# Output(guess = 'e'): Found guessed letter 'e' in the list.