Chapter 5.2 for loop.md

Chapter 5.2: For Loop

Table of Contents:

• Chapter 5.2: For Loop
• Introduction to For Loop
  • The Range Function
  • Using For loop with string, list, tuple, set, and dictionary
  • Nested For Loop
  • The enumerate() function
  • The zip() function

Introduction to For Loop

A For Loop iterates over a sequence of items over the iterable in the order that they appear in a sequence. While while loops generally use the conditions for breaking out of the loop, for loops have definite number of items to iterate over. Generally, We use generator function (eg: range()) to create a sequence of numbers and loop over them to achieve loop for n-times.

The Range Function

The range() function creates a range of numbers through which, we can iterate over using for loop. The range() function is a generator which yields an item at a time rather than returning the whole sequence of numbers which saves a lot of memory when generating a sequence of millions of items.

Example 1:

range(10)  # creates a range from 0 upto 9
range(1, 10)  # creates a sequence of numbers from 1 to 10
range(1, 10, 2)  # createa a sequence of numbers from 1 to 10 with step of 2

Example 2: Using range() function to iterate for 10 times

for x in range(10):
    print(f'The current value of x is: {x}')

Output

The current value of x is: 1
The current value of x is: 2
The current value of x is: 3
The current value of x is: 4
The current value of x is: 5
The current value of x is: 6
The current value of x is: 7
The current value of x is: 8
The current value of x is: 9

Example 3: different usage of range() function.

# generating odd numbers from 1 upto 19
odd = list(range(1, 20, 2))
print(odd)
# [1, 3, 5, 7, 9, 11, 13, 15, 17, 19]


# Generating multiples of 3
multiples = list(range(3, 31, 3))
print(multiples)
# [3, 6, 9, 12, 15, 18, 21, 24, 27, 30]

Using For loop with string, list, tuple, set, and dictionary

Example 1: For Loop with String

name = 'Hello World!'
for char in name:
    print(f'Ascii value of [ {char} ]: ', ord(char))

Output:

Ascii value of [ H ]:  72
Ascii value of [ e ]:  101
Ascii value of [ l ]:  108
Ascii value of [ l ]:  108
Ascii value of [ o ]:  111
Ascii value of [   ]:  32
Ascii value of [ W ]:  87
Ascii value of [ o ]:  111
Ascii value of [ r ]:  114
Ascii value of [ l ]:  108
Ascii value of [ d ]:  100
Ascii value of [ ! ]:  33

Example 2: For Loop with List / tuple / set

odd = [1, 3, 5, 7, 9]
double_odd = []
for num in odd:
    double_odd.append(2 * num)
print(double_odd)

# Output: [2, 6, 10, 14, 18]

even = (2, 4, 6, 8, 10)
for num in even:
    print(num, end=' ')

# Output: 2 4 6 8 10


prime = {2, 3, 5, 7, 11}
for num in prime:
    print(num, end=', ')

# Output: 2, 3, 5, 7, 11

Example 3: For loop with dictionary

person = {
    'name': 'Spock',
    'age': 90,
    'address': 'Vulcan'
}

for key, value in person.items():
    print(f'The {key} is {value}')

Output:

The name is Spock
The age is 90
The address is Vulcan

Nested For Loop

Example 1:

nested = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
for x in nested:
    for y in x:
        print(y, end=' ')

# Output: 1 2 3 4 5 6 7 8 9

Example 2:

lang = {
    'low level': ['Machine Level', 'Assembly'],
    'high level': ['C++', 'Java', 'Python']
}

for lang_type in lang:
    for lng in lang[lang_type]:
        print(f'The {lang_type} programming language is {lng}')

Output:

The low level programming language is Machine Level
The low level programming language is Assembly
The high level programming language is C++
The high level programming language is Java
The high level programming language is Python

The enumerate() function

The enumerate() function enumerates the value of an iterable with the index of that item. example if we have a list ['a', 'b', 'c', 'd', 'e'], then the enumerate() function gives the tuple of the index of the list and the item on typecasting.

Example:

items = [chr(val) for val in range(65, 70)]  # convert ASCII value to character
print(items)
# ['A', 'B', 'C', 'D', 'E']

print(list(enumerate(items)))
# [(0, 'A'), (1, 'B'), (2, 'C'), (3, 'D'), (4, 'E')]

We can also use for loop within enumerate() function

Example:

casts = ['Robert Downey Jr.', 'Scarlett Joahnsson', 'Chris Evans', 'Mark Ruffalo', 'Chris Hemsworth']

for index, cast in enumerate(casts):
    print(f'{index})\t{cast}')

Output:

0) Robert Downey Jr.
1) Scarlett Joahnsson
2) Chris Evans
3) Mark Ruffalo
4) Chris Hemsworth

The zip() function

The zip() function also does something similar to the enumerate() function but requires 2 or more set of items with equal length. It zips all the values of same index to a tuple of items.

Example:

casts = ['Robert Downey Jr.', 'Scarlett Joahnsson', 'Chris Evans', 'Mark Ruffalo', 'Chris Hemsworth']
characters = ['Iron Man', 'Black Widow', 'Captain America', 'Hulk', 'Thor']

print(list(zip(casts, characters)))
'''
[
    ('Robert Downey Jr.', 'Iron Man'),
    ('Scarlett Joahnsson', 'Black Widow'),
    ('Chris Evans', 'Captain America'),
    ('Mark Ruffalo', 'Hulk'),
    ('Chris Hemsworth', 'Thor')
]
'''

for cast, character in zip(casts, characters):
    print(f'- {cast} played as "{character}" in Avengers')

Output:

- Robert Downey Jr. played as "Iron Man" in Avengers
- Scarlett Joahnsson played as "Black Widow" in Avengers
- Chris Evans played as "Captain America" in Avengers
- Mark Ruffalo played as "Hulk" in Avengers
- Chris Hemsworth played as "Thor" in Avengers

We can also zip multiple lists

Example:

roll = [1, 2, 3, 4]
name = ['John Lennon', 'John Legend', 'John Mayer', 'John Denver']
songs = ['Imagine', 'All of Me', 'Gravity', 'Take Me home']

for items in zip(roll, name, songs):
    print(items)

Output:*

(1, 'John Lennon', 'Imagine')
(2, 'John Legend', 'All of Me')
(3, 'John Mayer', 'Gravity')
(4, 'John Denver', 'Take Me home')