exercise11b.md

Excercise 11 b (While Loop)

This is a continuation of exercise 11 a (For Loop)

While loop

A while loop executes its body until a condition is met or as far as a condition is still valid.

Structure of a while loop

while condition:
    # do something

Reference Exercise 10 (Conditions) here.

condition can be the result of a relational or boolean expression.

Example 1

Let's print hello world three times

i = 0

while i < 3:
    print("Hello world")
    i += 1

Example 2

Printing numbers from 1 to 10

# start from 0
number = 0

while number < 10:
    number += 1
    print(number)


# start from 1
number = 1

while number < 11:
    print(number)
    number += 1

Example 3

Let's look at another snippet that prints the string, <number> is even if the number is even else it prints the number.

number = 0

while number < 20:
    if number % 2 == 0:
        print(f"{number} is even")
    else:
        print(number)

    number += 1

What do you think would happen when we comment out number += 1?

Example 4

A snippet for finding the average of five numbers.

s = 0
i = 0
end = 5

while i < end:
    n = int(input("Enter a positive number: "))

    s += n
    i += 1

avg = s / end

print(f"Average: {avg}")

Structure looping

We shall be looping through an iterable using a while loop. We shall be implementing the same code in the for loop.

Example 1

This snippet prints out each character of the string on a new line.

# looping through `name` using index
name = "John Doe"

i = 0
end = len(name)

while i < end:
    print(name[i])
    i += 1

Example 2

In this snippet, we will take a long space-separated string and print out the words and their corresponding number of characters.

# we will not be using any function
text = "looping through name without index"

size = 0
word = ""
start = 0
end = len(text)  # `len` to find the length of the string

while start < end:
    ch = text[start]

    if ch != " ":
        size += 1
        word += ch
    else:
        print(f"{word} = {size}")
        size = 0
        word = ""

    start += 1

In the snippet above we use the len function to find the length of the text. Let's see another implementation where the number of lines is less.

text = "looping through name without index"

words = text.split()
end = len(words)
start = 0

while start < end:
    word = words[start]
    print(f"{word} = {len(word)}")

    start += 1

Note

• In the above code, without start += 1, the loop becomes an infinite loop. A loop that would never terminate and we would be print only the first element.
• Also assume we just say, while True: instead of while start < end:, will also not terminate.

Break and Continue

Break

break when used in a loop breaks out of the loop.

Example 1

text = "looping through name without index"

words = text.split()
end = len(words)
start = 0

while True:
    if start < end:
        word = words[start]
        print(f"{word} = {len(word)}")

        start += 1
    else:
        break

In the above snippet, we introduced if and else whereby in the else block, we break from the loop.

Example 2

The snippet below takes in several inputs, counts them, finds the sum and the average. When the input id done, the number of input, sum and average is printed out as the loop breaks.

s = 0
i = 0

while True:
    n = input("Enter a positive number: ")

    if n == "done":
        break

    s += int(n)
    i += 1

avg = s / i

print(f"Freq: {i}, Sum: {s}, Average: {avg}")

Continue

continue when used in a loop skips the current iteration. Unlike the break, do not break out of the loop.

Example

a program that prints all the numbers between 1 to 100 inclusive, excepts those that are a multiple of 4 and 7. We shall use loop and continue here.

for i in range(1, 101):
    if i % 4 == 0 or i % 7 == 0:
        continue
    print(i)

Modify this snippet, using a while loop.

Practicals

• write a lift-off program using a loop.

  Enter liftoff time: 3
  lift off in
  3 2 1 0 - liftoff!

• Given a string of alphabets, from a - z, use a loop to print out the vowels ( Tips: string of vowels, loop, condition )
• A snail is at the bottom of a 30 foot well. Every hour the snail can climb up 3 feet, then immediately slide back down 2 feet. Write a program to find how many hours it takes for the snail to get out of the well?

Summary

• A loop is used for repetition
• There are two types of loops, for and while loop
• for loop is best used when we know the range of execution
• while loop is best when the repetition is based on a condition
• we can nest one loop into another
• break terminates the loop and continue skips the current iteration