While Loops

Write a Program to Print out the numbers 1 through 5

Motivation for Loops

A program to count from 1 to 5

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)

Motivation for Loops Continued

Um. That was kind of tedious. Can't we just tell the computer to repeat those two lines of code?

YES! …using loops

n = 1
end = 5
delta = 1

while n <= end:
	print(n)
	n = n + delta

Iteration and Loops

Some formal definitions:

• iteration - repeated execution of a set of programming statements.
• loop - the construct that allows us to repeatedly execute a statement or a group of statements until a terminating condition is satisfied.
• sometimes these words are used interchangeably

While Loops

Using a while loop allows us to:

• repeat a block of code
• …as long as a condition is met
• stops as soon as the condition is no longer true
• looks similar to an if statement

While Loop Syntax

A template:

# while <some sort of condition>:
#	<do stuff here>

Some real code:

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

What does this output? →

100
100
100

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

Figuring Out How to Write a While Loop

Before you write your while loop, you should probably first determine…

• if your code needs a while loop at all (is there repeated code)
• if there's data that affects whether or not the loop will continue running
• what condition will make your loop repeat
• how your loop will end

Count From 2 Through 8 By 2's

How would you implement this?→

count = 2
while count <= 8:
	print(count)
	count = count + 2

Stepping Through Counting By 2's

count = 2
while count <= 8:
	print(count)
	count = count + 2

1. count is 2
2. condition is true because count (2) is less than 8
3. print count
4. add 2 to count… count is now 4
5. condition is true because count(4) is less than 8..
6. goes on until count gets to 10, at which point condition is no longer true

Check out the fancy step through

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

Do You Want Cake (Again)

Repeatedy ask if user wants cake until user says yes or yeah. How would you implement this?→

Do you want cake?
> no
Do you want cake?
> No
Do you want cake?
> yeah
Have some cake!

answer = 'no'
while answer != 'yes' and answer != 'yeah':
	answer = input("Do you want cake?\n> ")
print("Have some cake!")

Stepping Through Cake

Let's make an assumption that the user enters "no" first, and then "yeah" second.

answer = 'no'
while answer != 'yes' and answer != 'yeah':
	answer = input("Do you want cake?\n> ")
print("Have some cake!")

1. answer is set to no by default
2. condition is true, answer (no) is not 'yes' or 'yeah'
3. answer is set to user input of 'no'
4. condition is true, answer (no) is not 'yes' or 'yeah'
5. answer is set to user input of 'yes'
6. condition is false, answer != 'yeah' is now false!
7. have some cake is printed

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))

A Difficult One…

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
• 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))

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

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