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Refresher: Iteration 2

Introduction

Recap

  • introduction to loop
  • while, while else
  • break and continue
  • print 1 to n, n to 1
  • print even numbers, sum
  • prime numbers
  • scrapers and scissors

Question

What is the output of the following?

count = 0
while(count < 10):
    print(10, end = ' ')
    count += 1

Choices

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

Question

Which is true for an Odd number n?

Choices

  • n % 2 == 0
  • n % 2 = = 1

Question

What operation can be used to get the last digit of a number?

Choices

  • n - 10
  • n // 10
  • int(n / 10)
  • n % 10

Question

What will be 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

Range Function

  1. range(n) returns number from 0 to n-1.
    • start = 0
    • jump = 1
    • number line example

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* range(-5) -> nothing as decrementing -5 we will never reach anywhere.
* range(1) -> 0
  1. range(start, end) -> general numbers from [start, end-1]
    • jump -> +1
    • range(1,3) -> 1,2
    • range(-5, -1) -> -5, -4, -3, -2
    • range(-4, -10) -> nothing
    • range(5, 1) -> nothing

image

  1. range(start, end, jump)
    • start, end - 1
    • range(1,6,2) -> 1, 3, 5
    • range(0, -5, -1) -> 0, -1, -2, -3, -4

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Precautions

  • jump can not be zero
  • range always takes and returns an integer value

Iterables an Iterators

  • Assume that we have a bag of candies. I put my hand in the bag and take the candies out of the back one by one.

image

  • Examples - list, dict, set, string, range, enumerate, tuple
  • Iterables are groups of objects
  • Iterator can be related to the hand that we are using to take candies out of the iterables(bag of candies).

Range as an Iterable

  • Range is iterable, it is a collection of integers. If the range returns nothing we can say the bag is empty it doesn't return anything. print(range(3)) What will this return?

  • Print is not an iterator it will simply return range(3).

  • for loop is one of the best iterators.

For Loop

Syntax:

for variable in iterable:
    action
  • With for loop we can skip initialization, condition, and updation.
  • It is an alternative to the foreach loop.
  • The for loop can be used with iterables such as lists, dictionaries, etc. This will be covered when we will discuss lists and other iterables.

image

Question - Print 1 to 100?

    for i in range(1, 101):
        print(i)

Question

What will the sequence be generated by the following?

range(5)

Choices

  • 1 2 3 4 5
  • 0 1 2 3 4 5
  • 1 2 3 4
  • 0 1 2 3 4

Question

What will the sequence be generated by the following?

range(5,15, 2)
Choices

  • 5,6,7,8,9,10,11,12,13,14,15
  • 5,7,9,11,13,15
  • 5,7,9,11,13
  • 5,6,7,8,9,10,11,12,13,14

Question

What will the sequence be generated by the following?

range(-5,0,1)

Choices

  • -5,-4,-3,-2,-1,0
  • Nothing
  • 0,-1,-2,-3,-4,-5
  • -5,-4,-3,-2,-1

Question

What will the sequence be generated by the following?

range(-10,-5,-1)

Choices

  • -10,-9,-8,-7,-6,-5
  • -10,-9,-8,-7,-6
  • Nothing
  • -6,-7,-8,-9,-10

Question

What is the output of the following?

for i in range(0,1):
    print('Hello')

Choices

  • Hello
    Hello
  • Hello
  • Nothing
  • 0
    1

How many values will be returned by this:

range(n) -> n

If you want the loop to run n times just say range(n). The loop will run n times but start with zero till n-1 therefore n values.

range(5) -> 0, 1, 2, 3, 4 => 5 values

Break and Continue in For Loop

  • Break and Continue is same as we saw in while loop.
  • 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.
  • It skips the inside loop and continues the loop’s execution.

[ASK THE LEARNERS] is _(underscore) a valid variable name?

  • Variable should start with an alphabet or an _
  • can only have underscore, alphabets, and numbers.
  • It should not start with a number.
  • Many programmers use underscore when they don't need a name for a variable in the for loop.
for _ in range(10):
    print("hello")
  • If we use _ it won't give a warning in case we are not using it. With any other variable name, it will give a warning in Python.
  • What will be the output of the following:
for i in range(10):
    if(i == 4):
        break
    print(i)

Output: 

0
1
2
3

  • What will be the output of the following:
for i in range(6):
    if(i % 2 == 0):
        continue
    print(i)

Output: 

1
3
5

Pass Statement

  • It is not to be used in competitive programming or interviews. It is usually used in testing.
  • The pass does nothing. It signifies that the programmer will later add some code to it. Right now ignore this block.
for i in range(6):
    if(i % 2 == 0):
        pass
    print(i)
  • Pass will still print the i. In case of continuing it will directly begin with a new iteration.

For Else Loop

  • Else statement will execute if the loop terminates successfully i.e. without a break
  • Write a code for the prime number in the for loop.
n = int(input())
for i in range(2,n):
    if(n % i == 0):
        print("Not Prime")
        break
    else:
        print("Prime")

Question

What is the output of the following?

for i in range(0,10):
     if(i % 3 == 0):
        continue
    print(i, end = ' ')

Choices

  • 0 1 2 3 4 5 6 7 8 9
  • 0 1 2
  • 0 1 2 4 5 7 8 9
  • 0 1 2 4 5 7 8

Question

What is the output of the following?

for i in range(1,10): 
    if(i % 3 == 0): 
        break
    print(i, end = ' ')

Choices

  • 1 2 3 4 5 6 7 8 9
  • 1 2
  • 1 2 4 5 7 8 9
  • 1 2 4 5 6 7 8

Nested Loops

  • If we write a loop inside a loop it is a nested loop.
  • Look at the pattern below and write a code to generate this pattern.
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5 
1 2 3 4 5

If I want to print 1 to 5, how will I write the code?

for i in range(1,6):
    print(1, end = " ")

Now if I want to do this 5 times will I do this?

for i in range(1,6):
    print(1, end = " ")
print()
for i in range(1,6):
    print(1, end = " ")
print()
for i in range(1,6):
    print(1, end = " ")
print()
for i in range(1,6):
    print(1, end = " ")
print()
for i in range(1,6):
    print(1, end = " ")
print()
  • No right? What principle it is not following?
  • DRY (Do not repeat yourself).
  • I will use a nested loop.
for _ in range(5):
    for i in range(1,6):
        print(1, end = " ")
    print()
  • Single for loop gives 1D data, 2 loops nested will give 2D, and so on.
  • Similarly we can write nested while loop

Difference b/w For and While Loop

For While
It is simple to use. Initialization, condition, and updation in a single line. Complex to use
Only for simple iteration Complex tasks such as scrapper and Scissors can be performed

Note: Never update the iteration variable in the for loop.

Pattern Printing Problems

* * * * *
* * * * *
* * * * *
* * * * *
* * * * *
Write a code to print this pattern.

Warning

Please take some time to think about the solution on your own before reading further.....

Solution 1

for i in range(5):
    for j in range(5):
        print("*", end = " ")
    print()
Solution 2

We are using string to integer multiplication. The statement print("* "*5) will generate a line with 5 stars and a space in between.

for _ in range(5):
    print("* " * 5)

Staircase Binding

*
* *
* * *
* * * *
* * * * *

Warning

Please take some time to think about the solution on your own before reading further.....

Solution 1

When i is 1 we print 1 star, when i is 2 we print 2 star, and so on. 

n = int(input())
for i in range(1, n + 1):
    print("* " * i)

Solution 2

We can do it with the nested loop as well

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

Reverse Staircase

         *
       * *
     * * * 
   * * * *
 * * * * *
  • Can you guys do it or should I give you a hint?

No spaces between stars

  • Assuming that there is no space between starts. What we are doing is 4 spaces 1 star, 3 spaces 2 stars, 2 spaces 3 stars and so on.
for i in range(1,6):
    spaces = " " * (n - i)
    stars = "*" * (i)
    print(spaces + stars)

Homework Problem

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