Another Look at While Loops (It's the Sequel!)

What's a While Loop Again?

Describe what a while loop is… →

while condition:
	print("execute this block of code")

Great. So. Why Even?

Sales Commission Program

Starting Out with Python has a program that:

1. asks for a salesperson's number of sales and their rate
2. …and then calculates their sales commission.

# get a salesperson's sales and commission rate.
sales = float(input('Enter the amount of sales: '))
rate = float(input('Enter the commission rate: '))

# calculate the commission
commission = sales * rate

# display the commission
print("the commission is $" + str(commission))

Commission for 10, Please?

Is it possible to change the code in the previous slide so that the program requests data and calculates commissions for 10 salespersons … WITHOUT USING A WHILE LOOP? →

sales = float(input('Enter the amount of sales: '))
rate = float(input('Enter the commission rate: '))
commission = sales * rate
print("the commission is $" + str(commission))

sales = float(input('Enter the amount of sales: '))
rate = float(input('Enter the commission rate: '))
commission = sales * rate
print("the commission is $" + str(commission))

# and keep on going...

What About Counting?

Can you write a program to count from 1 to 5 by printing out each number on a new line WITHOUT USING A LOOP (we saw this previously) →

n, delta = 1, 1
print(n)
n = n + delta
print(n)
n = n + delta
print(n)
n = n + delta
print(n)
n = n + delta
print(n)

Counting Without a While Loop

• again, very tedious
• there's a repeating pattern of code
• …but it's totally possible!

Last Example for Motivating Loops, I Swear

Continue? (Yes/No)

WITHOUT USING A WHILE LOOP, is it possible to write a program that: →

• asks a user if they want to continue
• if they say yes, the program will ask them if they want to continue again
• this goes on until they don't say Yes

Continue? (Yes keeps on going)
> Yes
Continue? (Yes keeps on going)
> Yes
Continue? (Yes keeps on going)
> No

Continue? (Yes/No)

We could try using consecutive nested if statements, but we don't know how deep the nesting could go.

answer = input('Continue? (Yes keeps on going) \n')

if answer == 'Yes':
	answer = input('Continue? (Yes keeps on going) \n')

	if answer == 'Yes':
		answer = input('Continue? (Yes keeps on going) \n')

		if answer == 'Yes':
			answer = input('Continue? (Yes keeps on going) \n')

Functions!?

• There's actually a way to do this using a function definition that calls itself.
• We'll take a look at recursion later this semester.

def ask():
    answer = input('Continue? (Yes keeps on going) \n')
    if answer != 'Yes':
        ask()

ask()

Loops Will Make All of Those Previous Programs Easier to Write!

Iteration and Loops

Some formal definitions:

• iteration - repeated execution of a set of programming statements
• loop (or a repetition structure) - the actual structure that allows repeated execution of a statement or a group of statements. The statements are repeated either:
  • a specific number of times or …
  • as long as a certain condition is true

Two Types of Loops

There are two broad categories of loops:

• condition controlled - condition determines number of repetitions
• count controlled - loop for a specific number of times
  • (it turns out that you can simulate count controlled loops in condition controlled loops)
• guess which one a while loops is?

While Loops

Condition Controlled Loops

• a condition controlled loop causes a set of statements to be repeated as long as a condition is true
• again, in Python, the while statement allows us to write a condition controlled loop
• … or more succinctly: while a condition is true, do some stuff
• a while loop consists of two parts:
  • a condition (a boolean expression) that is continually tested for a True or False value
  • a block of statements to repeat

While Loop Logic

A flow chart representing a while loop:

While Loop Logic Continued

What happens if the condition is True?

Compared to an If Statement

The flow chart for an if statement looks pretty similar to a while loop. What do you think the difference would be from the diagram showing while loop logic? →

If Statement Flow Chart

While Loop Syntax

A template:

while condition: # the book calls this the while clause
	statement
	statement

Some real code:

a = 100
while a > -1:
	print(a)

What does this output? →

100
100
100

Basic While Loop Examples

Trivial Cases, Again

What do these snippets of code print out?→

while True:
	print("I'm true!")

while False:
	print("I'm false!")

I'm true!
I'm true!
I'm true!

# nothing printed here

Let's Step Through True

while True:
	print("I'm true!")

1. condition is true
2. print "I'm true!"
3. go back to top
4. condition is true
5. print "I'm true!"
6. go back to top
7. you know the deal

Let's Step Through False

while False:
	print("I'm false!")

1. condition is false

We never even get into the body of the loop!

Slightly More Complicated

What does this print out? →

keep_on_going = True
while keep_on_going:
	print("I'm going!)"

I'm going
I'm going
I'm going
.
.
I'm going

Slightly More Complicated Continued

Let's add one line. What does this print out? →

keep_on_going = True
while keep_on_going:
	print("I'm going!")
	keep_on_going = False

I'm going

Slightly More Complicated Continued Continued

Going through each iteration

keep_on_going = True
while keep_on_going:
	print("I'm going!")
	keep_on_going = False

• condition (keep_on_going) is true
• print "I'm going!"
• set keep_on_going to false
• condition (keep_on_going) is false

Loop ends after one iteration.

What Happened There?

• we had some state outside of the loop
• within the loop we mutated / changed that state to eventually get out of the condition
• consequently, the loop terminates
• sometimes we call these kinds of variables sentinel variables
• they only let certain conditions in!

Affecting the Outcome of the Condition

To change the outcome of your conditional:

• maintain state
  • declare a variable outside of the loop!
• mutate that state on loop iteration
  • change that variable inside the loop!
  • this will eventually cause the conditional to fail
• examples:
  • using operators to change a variable in your condition
  • using input to change a variable in your condition

Some Practical While Loop Examples

Handling Input, Continue Again

First an easy one. Let's go back to the continue program that we discussed earlier:

• ask a user if they want to continue
• if they say yes, the program will ask them if they want to continue again
• this goes on until they don't say Yes

Continue? (Yes keeps on going)
> Yes
Continue? (Yes keeps on going)
> Yes
Continue? (Yes keeps on going)
> No

Continue Program, Some Questions

• what are we repeating?
• how long are we repeating it, or what's the condition to keep on repeating it?
• is there any data that we're keeping track of?

Continue Program Implementation

answer = input('Continue? (Yes keeps on going) \n')

while answer == 'Yes':
	answer = input('Continue? (Yes keeps on going) \n')

Notice that, like an if statement, variables declared outside of the body of the while loop can be accessed (and even changed) within the body of the while loop.

Another Continue Program Implementation

answer = 'Yes'

while answer == 'Yes':
	answer = input('Continue? (Yes keeps on going) \n')

• in order to go into the while loop, just start off with something that makes the condition true!
• what are some benefits to using this pattern? →

A More Flexible Continue

Let's modify the previous program slightly so that it accepts either yes or yeah.

• ask a user if they want to continue
• if they say yes or yeah, the program will ask them if they want to continue again
• this goes on until they do not say Yes or Yeah

Continue? (Yes or Yeah will keep going)
Yeah
Continue? (Yes or Yeah will keep going)
Yes
Continue? (Yes or Yeah will keep going)
Nope

Flexible Continue Program, Some Questions

• what are we repeating?
• how long are we repeating it, or what's the condition to keep on repeating it?
• is there any data that we're keeping track of?

Flexible Continue Program, Implementation

To implement this, modify the condition appropriately by adding or answer == 'Yeah'

answer = input('Continue? (Yes or Yeah will keep going) \n')
while answer == 'Yes' or answer == 'Yeah':
	answer = input('Continue? (Yes or Yeah will keep going) \n')

Designing How Your While Loop Works

So, I Heard You Want to Repeat Some Code…

If you have some code that requires repetition, what questions should you ask?

We've Seen These Before… Some More Examples

Odd Numbers Except 13

Write a program that… →

• prints all of the odd numbers from 1-99
• skips 13

There are a few ways to do this! What are some general strategies for solving this problem?→

Possible Solutions for Odd Numbers Except 13

Increment by 2's

n = 1
while n <= 99:
    if n != 13:
        print(n)
    n = n + 2

Using modulo to determine odds

n = 1
while n <= 99:
    if n % 2 == 1 and n != 13:
        print(n)
    n = n + 1

Accumulating Values

Write a program that will: →

• continually ask the user for a number (forever)
• add that number to a running total
• print out the running total

Give me a number to add
> 10
Current total is 10
Give me a number to add
> 15
Current total is 25
Give me a number to add
> 5
Current total is 30
Give me a number to add
> 

Potential Solution for Accumulating Values

total = 0
while True:
    n = int(input("Give me a number to add\n> "))
    total = total + n
    print("Current total is " + str(total))

Average of Input

Write a program that continually asks the user for numbers, and asks them if they'd like to keep going. In the end, it should output the average of all of the numbers entered→

I'll calculate the average for you!
Current total: 0
Numbers summed: 0
Please enter a number to add
> 10
Do you want to continue adding numbers (yes/no)?
> yes
Current total: 10
Numbers summed: 1
Please enter a number to add
> 12
Do you want to continue adding numbers (yes/no)?
> no
The average is 11.0

Some Hints, Please?

Let's try keeping track of multiple variables:

• a user's answer to whether or not the program should continue
• (possibly) the number that the user has just entered
• the total (sum) of the numbers that a user has entered
• the count of numbers input

An Average Solution

total = 0
count = 0
answer = 'yes'
print("I'll calculate the average for you!")
while answer == 'yes':
        print("Current total: " + str(total))
        print("Numbers summed: " + str(count))
        total = total + int(input("Please enter a number to add\n> "))
        count = count + 1
        answer = input("Do you want to continue adding numbers (yes/no)?\n> ")
print("The average is "+ str(total / count))

Syntactic Sugar

Increment / Decrement

We've used the following syntax to increment or decrement a variable

n = 0
n = n + 1

n = 100
n = n - 1

Slightly tedious…

Increment / Decrement Continued

There's some syntactic sugar that makes doing this less verbose: use += or -=

• n += 1 is the same as n = n + 1
• n -= 1 is the same as n = n - 1

n = 0
# adds one to n and binds the resulting value to n
n +=  1

n = 100
# subtracts one to n and binds the resulting value to n
n -= 1

More Syntactic Sugar

This works for other operators too. What does this code print out? →

n = 2
n *= 2
n *= 2
print(n)

n = 64
n /= 2
n /= 2
print(n)

8
16.0

What About Strings?

Also works with strings…. What does this code print out? →

s = "h"
s += "e"
s += "y"
s *= 3
print(s)

heyheyhey

Some More Exercises!

Powers of Two

Continually print out the next power of two, only if you say 'y' →

1
next power?
> y
2
next power?
> y
4
next power?
> n

Squares

Continually ask for a number, print out the square, and ask to continue →

number
> 5
25
continue?
> y
number
> 2
4
continue?
> n

Other Exercises

• count the number of digits in an int
  • repeatedly use integer division
  • …until the original number becomes 0
  • keep count of divisions
• continually add exclamation points to a word
  • ask for a word
  • ask for x number of exclamation points
  • print out resulting word and exclamation points
  • continue asking for another word and exclamation points

…And For Loops