For Loops

Jake Shoudy

Sept 16, 2022

CSCI 110 - Lecture 13

Announcements

Project 1: Part 3

Due Sunday 9/18 11:59PM

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Using functions to improve our program!

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Last chance to get help at office hours is today

HW4

Due 2 days ago…

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Last day to turn in is Saturday at 11:59pm (using late days)

HW5

Posted last night

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For loops and simple lists (will learn Monday)

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Due 9/22 (Thursday) because it is posted a day late

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• 9/20
• Jubilee Hall: Appleton Room
• 5-7pm
• RSVP
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• PLEASE go if you signed up

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Didn’t like your exam 1 score? Just plain lost?

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For Loops - in

Why Loops?

Would you rather type and read this:

print(1) print(5) print(9)

print(2) print(6) print(10)

print(3) print(7) print(11)

print(4) print(8) print(12)

Or this:

Print from 1 to 12

Cases for Loops

Walk my dog 100 steps

Repeat a task a certain number of times

Cases for Loops

Walk my dog as many steps as it needs until it poops

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Repeat task until a condition is satisfied

Types of Loops

Repeat task a certain number of times

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Repeat task until a condition is satisfied

Strings and while loops

Iterate over, or traverse, characters in a string using a loop

name = 'cat'

index = 0

while index < len(name):

print(name[index])

index = index + 1

for loops

for loop: control structure that runs block of code repeatedly by iterating over elements in sequence

for letter in 'cat':

print(letter)

Tracing for loops

for letter in 'cat':

print(letter)

print('done')

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letter

Tracing for loops

for letter in 'cat':

print(letter)

print('done')

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'c'

letter

Tracing for loops

for letter in 'cat':

print(letter)

print('done')

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'c'

letter

c

Tracing for loops

for letter in 'cat':

print(letter)

print('done')

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'a'

letter

c

Tracing for loops

for letter in 'cat':

print(letter)

print('done')

​

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'a'

letter

c

a

Tracing for loops

for letter in 'cat':

print(letter)

print('done')

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't'

letter

c

a

Tracing for loops

for letter in 'cat':

print(letter)

print('done')

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't'

letter

c

a

t

Tracing for loops

for letter in 'cat':

print(letter)

print('done')

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't'

letter

c

a

t

No elements left!

Tracing for loops

for letter in 'cat':

print(letter)

print('done')

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't'

letter

c

a

t

done

for vs while

Similarity: repeating code in a loop

Difference:

while loop goes until condition is no longer met

for loop goes for a predetermined / known number of times

index = 0

while index < len(word):

print(word[index])

index = index + 1

print('All done!')

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for letter in word:

print(letter)

print('All done!')

Accumulator Pattern

word = 'friends'

result = ''

for char in word:

result = result + char + '-'

print(result)

Accumulator Pattern

word = 'friends'

result = ''

for char in word:

result = result + char + '-'

print(result)

1. initialize the accumulator

Accumulator Pattern

word = 'friends'

result = ''

for char in word:

result = result + char + '-'

print(result)

2. iterate over the string

Accumulator Pattern

word = 'friends'

result = ''

for char in word:

result = result + char + '-'

print(result)

3. update the result

Accumulator Pattern

word = 'friends'

result = ''

for char in word:

result = result + char + '-'

print(result)

f-r-i-e-n-d-s-

in operator

different than for x in ‘my_string’

in operator: checks if value exists in sequence, evaluates to boolean

word = 'banana'

if 'n' in word:

print('Found an n')

else:

print('Did not find a n')

Practice: in operator

'n' in 'Nashville, TN'

'N' in 'Nashville, TN'

True

False

True

False

char = ' '

char in 'Intro to Computer Science'

word = 'CS110'

char in word

For Loops - range

Baking Cookies

def bake_cookies():

print('Baked one batch of cookies!')

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Baking Cookies

# Bake 400 batches of cookies.

count = 0

while count < 400:

bake_cookies()

count = count + 1

Baking Cookies

# Bake 400 batches of cookies.

for i in ‘01234567890123456789…’:

bake_cookies()

Baking Cookies

# Bake 400 batches of cookies.

for i in range(0, 400):

bake_cookies()

range()

range(): returns sequence from start to end, incrementing by step (3rd parameter)

Use range() with for loops to repeat a loop when need numbers

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range(5, 8)

range(0, 5)

range(0, 5, 2)

range(5)

5 6 7

0 1 2 3 4

0 2 4

0 1 2 3 4

index = 0

while index < 10:

print(index)

index = index + 1

print('All done!')

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for i in range(0, 10):

print(i)

print('All done!')

Tracing for loops

for num in range(3, 5):

print(num)

print('exited loop')

num

Tracing for loops

for num in range(3, 5):

print(num)

print('exited loop')

3

num

Tracing for loops

for num in range(3, 5):

print(num)

print('exited loop')

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3

num

3

Tracing for loops

for num in range(3, 5):

print(num)

print('exited loop')

4

num

3

Tracing for loops

for num in range(3, 5):

print(num)

print('exited loop')

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4

num

3

4

Tracing for loops

for num in range(3, 5):

print(num)

print('exited loop')

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4

num

3

4

Reached 5. Nothing left!

Tracing for loops

for num in range(3, 5):

print(num)

print('exited loop')

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4

num

3

4

exited loop

for … in ...

for … in range(...)

for [variable] in range(...):

[code]

for [variable] in [sequence]:

[code]

Iterating over a sequence, like a string

for letter in 'dog'

letter will be each character in 'dog'

Iterating over numbers

for number in range(10)

number will be a number from 0 to 10

Let’s Code!

https://replit.com/team/csci110-01

For Loops

Review: For Loop

What is it?

Control structure that runs block of code repeatedly by iterating over elements in sequence

for [variable] in [sequence]:

[execute code]

Why is it important?

Write code once and run many times - do not need to copy and paste same code over and over

For vs While

for runs code predetermined / known number of times

while runs until certain condition is no longer met

Questions?