For Loops in Python

For Loop

In Python, the for loop is used to iterate over a sequence (e.g., List, String) or other iterable objects like range.

Key Concepts

  • Iteration: Repeating a process for each item in a sequence.
  • Automation: Automating repetitive tasks efficiently.

Syntax

for var in sequence:
    statement
  • sequence: A collection of objects (e.g., a list).
  • var: Takes items from the sequence one by one.
  • statement: Loop body; executed once for each item in the sequence.
  • Indentation: Loop body must be indented.

Working with For Loop

Example:

Display "Hello"
Display "Hello"
Display "Hello"
Display "Hello"

Can be automated using a for loop:

for i in range(5):
    print("Hello")
  • range(5) creates a list [0, 1, 2, 3, 4].
  • The loop repeats five times, displaying "Hello" each time.

The range() Function

Used to iterate over a sequence of numbers.

Syntax:
range(start, stop, step)
  • start (Optional): Integer specifying the start position (default is 0).
  • stop (Required): Integer specifying the stop position (not included).
  • step (Optional): Integer specifying the incrementation (default is 1).
Examples:
for i in range(5):
    print(i)

Output:

0
1
2
3
4

range(5, 10)

Output:

[5, 6, 7, 8, 9]

range(100, 50, -10)

Output:

[100, 90, 80, 70, 60]

Program to Print Individual Letters in a String

name = input("Enter a string: ")
x = len(name)
for i in range(x):
    print("The Character at {} Index Position = {}" .format(i, name[i]))
  • Get a string as user input.
  • Use a for loop with range() to iterate over the string's elements.
  • len(name) specifies the end value of the range() function.
  • Print characters along with their index using the format() method.

Program to Find Unique Vowels in a String

name = input("Enter your name: ")
lower_name = str.lower(name)
vowels = ['a', 'e', 'i', 'o', 'u']
foundvowel = []  # list of vowels found in the name

for i in lower_name:
    if i in vowels:
        if i not in foundvowel:
            foundvowel.append(i)

print("The unique vowels found in the name {} are {}" .format(name, foundvowel))

Explanation:

  1. Get user input for a name.
  2. Convert the name to lowercase.
  3. Create a list of vowels.
  4. Initialize an empty list foundvowel.
  5. Iterate through the characters of the name.
  6. Check if each character is in the vowels list.
  7. If True, check if the vowel is already in the foundvowel list.
  8. If not, append the vowel to the foundvowel list.
  9. Print the unique vowels found.

The in Operator

  • Checks if a specified value is an element of a sequence (string or list).
  • Returns a Boolean result (True or False).
  • In loops, it fetches the next value in the sequence.
  • The not in operator checks if a value is not an element in a sequence.

Prime Number

  • A natural number greater than 1 that is divisible only by 1 and itself.
  • Examples: 2, 3, 5, 7, 11, 13, 17, 19, 23, …
Program to Check if a Number is Prime
num = int(input("Enter the number: "))
prime = True

if num <= 1:
    prime = False
else:
    for i in range(2, num):
        if (num % i) == 0:
            prime = False
            break

if prime:
    print(num, "is a prime number")
else:
    print(num, "is not a prime number")

Nested Loops

A loop inside another loop. The inner loop executes for each iteration of the outer loop.

Program to Print a Pyramid Pattern of Stars
rows = int(input('Enter the number of rows: '))

for i in range(0, rows):
    for j in range(0, i + 1):
        print("* ", end=" ")
    print()

Explanation:

  1. Get the number of rows from the user.
  2. Use an outer for loop to iterate through each row.
  3. Use a nested for loop to print stars in each row.
  4. The number of stars increases with each row.
  5. The end parameter in the print() function adds a space after each asterisk.

The end Parameter of print() Function

  • By default, the print() function ends with a newline character.
  • The end parameter can be used to change this behavior.

Challenges

Challenge 1

Write a program using a for loop to display the multiplication table of a given number up to 10 times.

Solution:

num = int(input("Enter the number: "))
for i in range(1, 11):
    print(num, "X", i, "=", num * i)
Challenge 2

Write a program to find the next 100 leap years. A leap year is exactly divisible by four except for the century years (years ending with 00). The century year is a leap year only if it is perfectly divisible by 400.

Solution:

year = int(input("Enter the Year to be checked: "))
leap = year % 4 == 0 and year % 100 != 0 or year % 100 == 0 and year % 400 == 0
if leap:
    print("%d is Leap Year" % year)
    print('Next 100 Leap years are')
    for i in range(100):
        year = year + 4
        print(year)
else:
    print("%d is Not the Leap Year" % year)
Challenge 3

Write a program to print all the prime numbers between a certain range using a ‘for’ loop. Ask the user to input the upper and lower limit for the range. The program should display the prime numbers within the specified range.

Solution:

print('Enter numbers to find the prime numbers within that range')
lower = int(input("Enter the Lower limit: "))
upper = int(input("Enter the Upper limit: "))
print("Prime numbers between", lower, "and", upper, "are:")

for num in range(lower, upper + 1):
    if num > 1:
        for i in range(2, num):
            if (num % i) == 0:
                break
        else:
            print(num)