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Refresher: Iteration 1

Question

What is the output of the following? 

a = 3
a *= 4
print(a)

Choices*

  • 3
  • 4
  • 7
  • 12

Question

What is the output of the following?

a = 5
if(a < 6):
    print('Option 1')
if(a < 3):
    print('Option 2')
else:
    print('Option 3')

Choices

  • Option 1
  • Option 2
  • Option 3
  • Option 1
    Option 3

Question

What is the output of the following?

a = 1
b = 0
c = 1
if ( a and b):
    print("Option 1")
elif (a and c):
    print("Option 2")
else:
    print("Option 3")

Choices

  • Option 1
  • Option 2
  • Option 3
  • Option 4

Question

What is the output of the following?

count = 0
while(count < 10):
    print(10, end = ' ')

Choices

  • 0 1 2 3 4 5 6 7 8 9
  • Infinite Loop
  • 10 10 10 10 10 10 10 10 10 10

Introduction

  • Imagine we wish to print the numbers from 1 to 1000.
  • So we can write the code as follows:
print(1)
print(2)
..
..
..
print(1000)
  • This is not an intelligent task to do. If the task is simple and we want to repeat it 5 times, we might write the same piece of code 5 times But this is not a practical approach if the repetition number is like 100 or 1000.
  • There is a principle in programming called DRY

Do you know the full form of it?

  • Don't Repeat Yourself
  • Even if you are writing the same code three times, you are not doing a good job at it. We can either use loops or functions.
  • Before seeing loops, let us see why Python does things in a certain way.
  • I have a friend Ramesh and I have to take 10 toffies from him. I will pick a chocolate and start counting from 1 until I reach 10 and then stop.
  • Let us see in code what we are doing. Each time in a loop we will check if condition until we reach the else part.

image

count = 0
if(count < 10):
    take toffee
    count += 1
else
    stop
  • There are four things we always do in a loop:
    1. Initialization
    2. Condition
    3. Action
    4. Updation
  • We will discuss the while loop in this class and the for loop in the next as it is different in different programming languages.
  • The while loop will only run till the condition is true. As soon as the condition becomes false we exit the loop and execute the rest of the code.

Syntax

variable initialization
while (condition)
{
    action
    update variable
}

image

This can be read as while this condition is true perform this action and update the variable.

Let us quickly move to an example to understand better.

Example 1 - Print Numbers from 1 to n

    n = input()
    count = 1
    while(count <= n): # can also be written as (count < n + 1)
        print(count)
        count += 1
    print("end")
  • Parenthesis are optional in while condition.
  • Dry run for n = 5

If I do not write the incrementing count statement what will happen?

It will fall in the infinite loop and 1 will be printed forever.

  • Always ensure you have all 4 things while writing a loop.
  • In any programming language all the loops will always have these four things.
  • An alternative way of writing this code is as follows:
n = input()
count = 0
while count < n:
    count += 1
    print(count)

This is because counting in programming languages starts from 0. Array indexing in most of the languages also starts from 0. This is doing the same thing except the statements are shuffled.

Example 2 - Print Number From n to 1

  • Go grab your notebook, iPad, or laptop try writing code for the above question, and put the solution in the chat section.
  • Writing in a book is best practice.
n = int(input())
while n >= 1:
    print(n)
    n -= 1
  • Dry run for n = 5

image

Example 3 - Print Even Numbers From 1 to 100

  • Once done paste it into the chat section Note: Don't forget the updation of a variable. Also, some of the solutions have conditions where the loop doesn't run at all.
n = int(input())
count = 1
while count <= n:
    if count % 2 == 0:
        print(count)
    count += 1
  • Write the updation statement in the beginning only.

Alternative Solution:

In each interaction, we can update the count by 2. Initially, instead of starting from 1, we can start from 2, and in each iteration, we can do count += 2 this will always result in an even number.

n = int(input()):
count = 2
while count <= n:
    print(count)
    count += 2

image

This is an optimization of the above code as we are not checking the condition as well as the loop will only run half the times of n.

Example 4 - Print the Sum of All Even Numbers From 1 to 20

We already know how to generate even numbers between 1 to 20.

Instead of printing it, we need to store it in the bucket.

So we can keep adding these to a number and return the sum at the end.

sum = 0
count = 2
while(count <= n):
    sum += count
    count += 2
print(sum)

Example (VVIMP) - Print the Digits of Number 459 (order Doesn't Matter)

  • How to get digits of a number in iteration
  • Scraper -> extracts the last digit
  • Scissors -> Cuts the last digit

image

We will scrape the last digit, print the result, and cut the last digit. This process is continued until all the digits are cut. This is a loop, right?

Is there any operation I told you in the past which takes 459 and returns 9?

  • taking % 10 (modulo 10) will return the last digit.
  • 459 % 10 -> 9
  • 45 % 10 -> 5
  • 4 % 10 -> 4

Is there any operation that takes 459 and returns 45, basically cutting the last digit?

  • taking / 10 (integer division by 10) will return the last digit.
  • int(459 / 10) -> 45
  • int(45 / 10) -> 4
  • int(4 / 10) -> 0

Only division will give the floor value such as 45.9. Also, 459 // 10 doesn't work with negative integers as -459 will give -45.9 -> -46. Thus, we can simply use int(val) to get the integer part of the remaining number.

n = int(input())
while n > 0:
    print(n % 10)
    n = int(n / 10)

image

Question

What is the output of the following?

print(149 % 10)

Choices

  • 14
  • 9
  • 1
  • 0

Break and Continue Statements

Break Statement

Note: Only for loops If you want to break out of the loop if a certain condition is met then you use break. It will break the entire loop and execute the next statement.

Definition Whenever a break is executed inside a loop, it terminates the loop.

What will be the output of the following code?

count = 1
while count <= 100:
    if count == 4:
        break
    print(count, end=" ")
    count += 1

Output 

1 2 3

Continue Statement

Note: Only for loops

Definition It skips the inside loop and continues the loop's execution.

count = 1
while count <= 10:
    count += 1
    if count == 4:
        continue
    print(count, end=" ")

Output

1 2 3 5 6 7 8 9 10 11
  • The continue statement simply skips the rest of the instructions of the current interaction and begins with the next iteration.

Look at the example below:

count = 1
while count <= 10:
    if count == 4:
        continue
    print(count, end=" ")
    count += 1
  • This will end up in an infinite loop. As the updation condition is never performed due to continue statment. So once it reaches 4, it keeps looping on 4.

While-else

  • Only and only in Python

  • Else block will run only after the loop successfully terminates without a break. 

    count = 1
while count <= 100:
    if count == 4:
        break
    print(count, end=" ")
    count += 1
else:
    print("Yayy!")

  • It will not print Yayy! as it is terminated via a break statement rather than terminating naturally.

Check whether a number is prime or not

Example - Check Whether a Number is Prime or Not

  • Prime number is only divisible by 1 or the number itself.
  • One is neither prime not consonant.
  • We can say that a is divisible by n if a % n == 0.
  • We can say that 24 is not prime as 24 divides 2, 4, 6, 8, etc.
  • We can say that 23 is a prime number cause it only divides 1 and 23.
  • Write a code to check if the number is prime or not. Start a loop from 2 to n-1 if it is dividing any number then return not prime else prime. We are not required to use break, continue, or while else.

Solution 1:

Let us say our number is 25.
We will use a variable flag to store whether a number is prime or not. It is simply a boolean variable initialized with false.

As soon as the number is divisible by some other number we can change its value to true and it is not prime. If the number is not divisible by any other number the flag remains false and we can conclude that it is a prime number.

n = int(input())
count = 2
flag = False
while count < n:
    if n % count == 0:
        flag = True
    count += 1
if flag:
    print("Not Prime")
else:
    print("Prime")

Solution 2:

  • Is not very different from the previous solution.
  • Now consider if the number is 9, it is divisible by 3. Do we need to check for other numbers? We don't! If the given number is divisible by any other number, we can break from the loop and declare that it is prime.
  • This is a great situation where we can use break.
n = int(input())
count = 2
flag = False
while count < n:
    if n % count == 0:
        flag = True
        break
    count += 1
if flag:
    print("Not Prime")
else:
    print("Prime")

Solution 2:

  • We can remove the flag. We can use the while else statement.
  • Instead of checking the flag, we can say that if we didn't break from the loop, it is a prime number.
  • So as soon as we encounter that the number is divisible by some other number we print it is not prime. In the else part we print it is prime.
n = int(input())
i = 2
while i < n:
    if n % i == 0:
        print("not prime")
        break
    i += 1
else:
    print("prime number")

Question

When can we say a number A is divisible by n?

Choices

  • A % n == 0
  • A / n == 0
  • A // n == 0
  • A % n != 0

Question

What is the output of the following?

count = 1
while(count <= 5):
    if(count == 2):
        break
    print(count, end = ' ')
    count += 1

Choices

  • 1 3 4 5
  • 1
  • 1 2
  • 0 1

Question

What is the output of the following?

count = 1
while(count <= 5):
    if(count == 3):
        continue
    print(count, end = ' ')
    count += 1

Choices

  • 1 2 3 4 5
  • 1 2 4
  • 0 1 2 4 5
  • Infinite Loop