lab 3: for and while loops - home | university of pittsburghpitt.edu/~naraehan/ling1330/lab3.pdflab...

Lab 3: for and while Loops

Ling 1330/2330: Intro to Computational Linguistics

Na-Rae Han

Objectives

Loops

for loop

while loop

Tips

How to program efficiently

Ctrl + c for terminating a process

Indenting and de-indenting a block

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Speed-coding fox_in_sox.py

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Video of me completing the script, in 5x speed!

http://www.pitt.edu/~naraehan/ling1330/code_in_shell.mp4

Watch closely! This may well be the most important video yet… It will change your life.




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The rookie way:

You are doing all your coding in the script window, blindly.

You only use the IDLE shell to verify the output of your script.


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The seasoned Python programmer way:

After a script is run, all the variables and the objects in it are still available in IDLE shell for you to tinker with.

Experimenting in SHELL is much quicker – instant feedback!

You should go back and forth between SCRIPT and SHELL windows, successively building up your script.

for x in SEQ : iterates through a sequence (list, str, tuple) for doing

something to each element

Iterates through every element s of the simpsons list and prints the value of s followed by 'is a Simpson.' .

for loop

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>>>

>>>

Homer is a Simpson.

Marge is a Simpson.

Bart is a Simpson.

Lisa is a Simpson.

Maggie is a Simpson.

>>>

simpsons = ['Homer', 'Marge', 'Bart', 'Lisa', 'Maggie']

for s in simpsons :

print(s, 'is a Simpson.')

Press ENTER at the end

of each line

for loop over a string

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>>> for l in 'victory' :

print('Give me a', l)

Give me a v

Give me a i

Give me a c

Give me a t

Give me a o

Give me a r

Give me a y

>>>

for l in list('victory') : produces the exact

same result.

In a loop environment, string is interpreted as a sequence of characters

for loop over a list

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>>> for n in [75, 80, 95, 55, 80]:

print(n)

75

80

95

55

80

>>>

Looping though a list of numbers

How to calculate a total?

Updating value while looping

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>>> total = 0

>>> for n in [75, 80, 95, 55, 80]:

total += n

print(n, 'Total is now', total)

75 Total is now 75

80 Total is now 155

95 Total is now 250

55 Total is now 305

80 Total is now 385

>>> total

385

>>>

New variable created with initial value of 0

The final value of total

Add each n to total while looping

for loop over words

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>>> chom = 'Colorless green ideas sleep furiously'

>>> for w in chom.split() :

print('"'+w+'"', 'is', len(w), 'characters long.')

"Colorless" is 9 characters long.

"green" is 5 characters long.

"ideas" is 5 characters long.

"sleep" is 5 characters long.

"furiously" is 9 characters long.

>>>

for loop over words

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>>> chom = 'Colorless green ideas sleep furiously'

>>> for w in chom.split() :

print('"'+w+'"', 'is', len(w), 'characters long.')

"Colorless" is 9 characters long.

"green" is 5 characters long.

"ideas" is 5 characters long.

"sleep" is 5 characters long.

"furiously" is 9 characters long.

>>>

chom string must be first turned into a list of words through

.split()

>>> chom.split() ['Colorless', 'green', 'ideas', 'sleep', 'furiously']

Practice

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Write a script that takes a sentence from keyboard input, and then prints out:

each word and its length

2 minutes

>>>

================================ RESTART ============

Give me a sentence: Colorless green ideas sleep furiously

Colorless 9

green 5

ideas 5

sleep 5

furiously 9

>>>

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# word_length.py

# Demonstrates for loop and summation

sent = input('Give me a sentence: ')

words = sent.split() # tokenize sentence into word list

for w in words : # for every word

print(w, len(w)) # print out word and its length

?

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# word_length.py






Practice

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Write a script that takes a sentence from keyboard input, and then prints out:

each word and its length,

the average word length.

2 minutes

>>>

================================ RESTART ============

Give me a sentence: Colorless green ideas sleep furiously

Colorless 9

green 5

ideas 5

sleep 5

furiously 9

The average word length of this sentence is 6.6

>>>

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# word_length.py




total = 0 # total is 0 at first



total += len(w) # add its length to total

# average word length is the total number of characters

# divided by the number of words

avg = total/len(words)

print('The average word length of this sentence is', avg)

2 minutes

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# word_length.py




total = 0 # total is 0 at first



total += len(w) # add its length to total

# average word length is the total number of characters

# divided by the number of words

avg = total/len(words)

print('The average word length of this sentence is', avg)

2 minutes

>>> wd = 'penguin'

>>> new = ''

>>> for x in wd :

new = x + new + x

>>> print(new)

niugneppenguin

for loop to build a new string

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2 minutes

>>> wd = 'penguin'

>>> new = ''

>>> for x in wd :

new = new + x + x

>>> print(new)

ppeenngguuiinn >>> wd = 'penguin'

>>> new = ''

>>> for x in wd :

new = x + new

>>> print(new)

niugnep

>>> wd = 'penguin'

>>> new = ''

>>> for x in wd :

new = x + new + x

>>> print(new)

niugneppenguin

for loop to build a new string

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2 minutes

>>> wd = 'penguin'

>>> new = ''

>>> for x in wd :

new = new + x + x

>>> print(new)

ppeenngguuiinn >>> wd = 'penguin'

>>> new = ''

>>> for x in wd :

new = x + new

>>> print(new)

niugnep

Works as a string-reversing

routine!

while loop

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We need a looping mechanism that keeps going until a certain condition holds:

"Keep counting until 10"

"Keep adding 5 as long as the sum is less than 100"

"Keep trying until you have the correct answer"

Before

Loop test

Loop body

After

False✘ True✔

while loop: count from 1 to 10

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number = 1

while number <= 10 :

␣␣␣␣print(number)

␣␣␣␣number += 1

print('Done!')

number = 1

number <= 10

print(number) number += 1

print('Done!‘)

False✘ True✔

initial condition

tests condition

changes condition

Terminating a process

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Aborting a process: Ctrl+c

Try this in IDLE shell:

Bad things can happen when you forget to include a condition-changing operation in the while loop.

Ctrl+c when things go

haywire.

Guess the secret animal

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Let's make it persistent:

keep prompting (loop back !) until correct .

secret = 'panda'

g = input("What's the secret animal? ")

if g == secret :

print('CORRECT!', secret, 'is the secret animal.')

else :

print('Wrong guess. Try again.')

================================ RESTART

What's the secret animal? fox

Wrong guess. Try again.

>>>

================================ RESTART

What's the secret animal? panda

CORRECT! panda is the secret animal.

>>>

secret.py

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================================ RESTART

What's the secret animal? fox


What's the secret animal? dog


What's the secret animal? cat


What's the secret animal? panda

CORRECT! panda is the secret animal.

>>>

Guess until correct!

Method 1: which part in loop?

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secret = 'panda'


if g == secret :


else :


1. Figure out which part should be repeated.

2. Put the part in the while loop block.

Method 1: which part in loop?

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secret = 'panda'


if g == secret :


else :




Method 1: while ... ?

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secret = 'panda'

while g != secret :


if g == secret :


else :




3. What's the condition for looping back?

?

Method 1: set initial value for g

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secret = 'panda'

g = ''

while g != secret :


if g == secret :


else :




3. What's the condition for looping back?

4. Before looping begins, the initial value of g must be set

Method 1: try it out

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secret = 'panda'

g = ''

while g != secret :


if g == secret :


else :


2 minutes

secret1.py

Copy/download the original script here and edit:

http://www.pitt.edu/~naraehan/ling1330/secret.py



Method 2: using Boolean variable

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secret = 'panda'

g = ''

while g != secret :


if g == secret :


else :


secret = 'panda'

correct = False

while not correct :

A new True/False variable which functions as the loop

condition


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secret = 'panda'

correct = False

while not correct :


if g == secret :


else :


But there's something wrong here. What is it?


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secret = 'panda'

correct = False

while not correct :


if g == secret :


correct = True

else :


Variable value must be explicitly set to True in loop body.

Otherwise the loop will never terminate!

Method 2: try it out

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secret = 'panda'

correct = False

while not correct :


if g == secret :


correct = True

else :


2 minutes

secret2.py

Summary: method 1

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secret = 'panda'

g = ''

while g != secret :


if g == secret :


else :


initial loop condition loop condition test

loop condition-changing operation

secret1.py

Summary: method 2

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secret = 'panda'

correct = False

while not correct :


if g == secret :


correct = True

else :


initial loop condition loop condition test

loop condition-changing operation

secret2.py

Indenting and de-indenting a block

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Select your lines, and then use the commands for changing indentation levels:

Indent Region: Ctrl + ]

Dedent Region: Ctrl + [

Utilizing screen shots

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Screen shots are handy. Good for note taking and asking questions.

Win/OS-X native solutions:

http://www.pitt.edu/~naraehan/python3/faq.html#Q-screen-shot

Third-party applications:

Greenshot (Windows only)

http://getgreenshot.org/

Jing (Windows & Mac)

https://www.techsmith.com/jing.html

They offer added convenience: annotate screenshots, video screencasts (Jing)











Wrap-up

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Next class:

More on Python

Homework 1

Character encoding, palindrome script

Recitation students: work on first script "Naïve Palindrome" before class tomorrow

Need help?

Monday is MLK day: Reed and Na-Rae will be answering emails. Na-Rae *may* be available to meet upon request (email by Sunday).

Friday: Na-Rae available 11:30am – 1pm, 2:20pm – 3 (in 2818 cathedral)

lab 3: for and while loops - home | university of pittsburghpitt.edu/~naraehan/ling1330/lab3.pdflab...

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