Announcement

Tutoring/TA-ing at Data Analytics Lab (South 321)

• Marcie Hogan (Tutor):
  1. Sunday, 2:00 PM--5:00 PM
  2. Wednesday, 12:30 PM--1:30 PM

• Andrew Mosbo (Tutor):
  1. Mondays, 4:00 PM--5:00 PM
  2. Wednesdays, 11:00 A.M.--noon
  3. Thursdays, 5:00 PM--6:00 PM

• Emine Morris (TA):
  1. Mondays and Wednesdays, 5:00 PM--6:30 PM
  2. Tuesdays and Thursdays, 3:00 PM--4:45 PM

Workflow

Shortcuts

• F9 runs a current line (where the blinking cursor bar is) or selected lines.

• Home/End moves the blinking cursor bar to the beginning/end of the line.

  • Fn + / works too.

• PgUp/PgDn moves the blinking cursor bar to the top/bottom line of the script on the screen.

  • Fn + / works too.

Workflow

Shortcuts

• # %% defines a coding block in Spyder IDE.

Workflow

More Shortcuts

• Ctrl (command for Mac Users) + Z undoes the previous action.
• Ctrl (command for Mac Users) + Shift + Z redoes when undo is executed.

• Ctrl (command for Mac Users) + F is useful when finding a phrase in the script.
• Ctrl (command for Mac Users) + R is useful when replacing a specific phrase with something in the script.

• Ctrl (command for Mac Users) + D deletes a current line.

Text Strings

• Data scientists often work with strings of text.

• Strings in Python are immutable.

  • We can’t change a string in place, but we can copy parts of strings to another string to get the same effect.

Text Strings

Create with Quotes

• We make a Python string by enclosing characters in matching single or double quotes:

'Business'
"Data"Copy Code

• The Console (interactive interpreter) echoes strings with a single quote, but all are treated exactly the same by Python.

Text Strings

Create with Quotes

• Why have two kinds of quote characters?
  • The main purpose is to create strings containing quote characters.
  • We can have single quotes inside double-quoted strings, or double quotes inside single-quoted strings.

"'Nay!' said the naysayer. 'Neigh?' said the horse."
'The rare double quote in captivity: ".'
'A "two by four" is actually 1 1⁄2" × 3 1⁄2".'
"'There's the man that shot my paw!' cried the limping hound."Copy Code

Text Strings

Create with Quotes

• We can also use three single quotes (''') or three double quotes ("""):

'''Hi!'''
"""Hello!"""Copy Code

• The most common use of triple quotes is to create multiline strings:

poem_bluebird = '''there’s a bluebird in my heart that
wants to get out
but I’m too tough for him,
I say, stay in there, I’m not going
to let anybody see
you.'''Copy Code

Text Strings

Create with str()

• We can make a string from another data type by using the str() function:

str(3.141592)
str(1.0e4)
str(True)Copy Code

Text Strings

Escape with \

• Python lets us escape the meaning of some characters within strings to achieve effects that would otherwise be difficult to express.

  • By preceding a character with a backslash (\), we give it a special meaning.

• The most common escape sequence is \n, which means to begin a new line.

  • With this we can create multiline strings from a one-line string:

palindrome = 'A man,\nA plan,\nA canal:\nPanama.'
print(palindrome)Copy Code

Text Strings

Escape with \

print('\tabc')
print('a\tbc')
print('ab\tc')
print('abc\t')Copy Code

testimony = "\"I did nothing!\" he said. \"Or that other thing.\""
testimony
print(testimony)Copy Code

speech = 'The backslash (\\) bends over backwards to please you.'
print(speech)Copy Code

info = r'Type a \n to get a new line in a normal string'
infoCopy Code

poem = r'''Boys and girls, come out to play.
The moon doth shine as bright as day.'''
poem
print(poem)Copy Code

Text Strings

Combine by Using +

• We can combine literal strings or string variables in Python by using the + operator:

'DANL 100: Programming for ' + 'Data Analytics'Copy Code

• We can also combine literal strings (not string variables) just by having one after the other:

'DANL 100: Programming for ' 'Data Analytics'Copy Code

Text Strings

Duplicate with *

• We use the * operator to duplicate a string.

start = 'Na ' * 4 + '\n'
middle = 'Hey ' * 3 + '\n'
end = 'Goodbye.'
print(start + start + middle + end)Copy Code

Text Strings

Get a Character with []

• To get a single character from a string, specify its offset inside square brackets after the string’s name.

  • The first (leftmost) offset is 0, the next is 1, and so on.
  • The last (rightmost) offset can be specified with –1, so you don’t have to count; going to the left are –2, –3, and so on:

Text Strings

Get a Character with []

• To get a single character from a string, specify its offset inside square brackets after the string’s name.

  • The first (leftmost) offset is 0, the next is 1, and so on.
  • The last (rightmost) offset can be specified with –1, so you don’t have to count; going to the left are –2, –3, and so on:

letters = 'abcdefghijklmnopqrstuvwxyz'
letters[0]
letters[1]
letters[-1]
letters[5]
letters[100]Copy Code

Text Strings

Get a Character with []

• Because strings are immutable, we can’t insert a character directly into one or change the character at a specific index.

name = 'Macintosh'
name[0] = 'P'Copy Code

• Instead, we need to use some combination of string functions such as replace() or a slice (which we look at in a moment):

name = 'Macintosh'
name.replace('M', 'P')
'P' + name[1:]Copy Code

Text Strings

Get a Substring with a Slice

• We can extract a substring (a part of a string) from a string by using a slice.

• We define a slice by using square brackets ([]), a start offset, an end offset, and an optional step count between them.

  • We can omit some of these.

• The slice will include characters from offset start to one before end:

Text Strings

Get a Substring with a Slice

letters = 'abcdefghijklmnopqrstuvwxyz'
letters[:]Copy Code

letters = 'abcdefghijklmnopqrstuvwxyz'
letters[20:]
letters[10:]
letters[-3:]
letters[-50:]Copy Code

letters = 'abcdefghijklmnopqrstuvwxyz'
letters[:3]
letters[:-3]
letters[:70]Copy Code

letters = 'abcdefghijklmnopqrstuvwxyz'
letters[12:15]
letters[-51:-50]
letters[70:71]Copy Code

letters = 'abcdefghijklmnopqrstuvwxyz'
letters[4:10:3]   # From offset 4 to 9, by steps of 3 characters
letters[::7]      # From the start to the end, in steps of 7 characters
letters[19::4]    # From offset 19 to the end, by 4
letters[:21:5]    # From the start to offset 20 by 5:
letters[-1::-1]   # Starts at the end and ends at the start
letters[::-1]Copy Code

Text Strings

String-related Functions

len(letters)
empty = ""
len(empty)Copy Code

tasks = 'get gloves,get mask,give cat vitamins,call ambulance'
tasks.split(',')
tasks.split()Copy Code

crypto_list = ['Yeti', 'Bigfoot', 'Loch Ness Monster']
crypto_string = ', '.join(crypto_list)
print('Found and signing book deals:', crypto_string)Copy Code

setup = "a duck goes into a bar..."
setup.replace('duck', 'marmoset')
setup
setup.replace('a ', 'a famous ', 100) # Change up to 100 of them
setup.replace('a', 'a famous', 100) # If we're unsure the exact substringCopy Code

world = " earth "
world.strip()
world.lstrip()
world.rstrip()Copy Code

world = " earth "
world.strip(' ')
world.strip('!')
blurt = "What the...!!?"
blurt.strip('.?!')Copy Code

poem = '''All that doth flow we cannot liquid name
Or else would fire and water be the same;
But that is liquid which is moist and wet
Fire that property can never get.'''
word = 'the'
poem.count(word)Copy Code

Text Strings

Case and Allignment

• We have more built-in functions for case and allignment:

setup = 'a duck goes into a bar...'
setup.capitalize()    # Capitalize the first word
setup.title()         # Capitalize all the words
setup.upper()         # Convert all characters to uppercase
setup.lower()         # Convert all characters to lowercase
setup.swapcase()      # Swap uppercase and lowercase
setup.center(30)    # Center the string within 30 spaces
setup.ljust(30)     # Left justify
setup.rjust(30)     # Right justifyCopy Code

Text Strings

Formatting

• We’ve seen that we can concatenate strings by using +.

• Let’s look at how to interpolate data values into strings using f-strings formats.

  • f-strings appeared in Python 3.6, and are now the recommended way of formatting strings.

  • There are (1) an old style of string formatting with % and (2) a new style of string formatting with {}, for which we may not discuss here.

Text Strings

Formatting

• To make an f-string:

  • Type the letter f or F directly before the initial quote.
  • Include variable names or expressions within curly brackets ({}) to get their values into the string.

thing = 'wereduck'
place = 'werepond'
f'The {thing} is in the {place}'
f'The {thing.capitalize()} is in the {place.rjust(20)}'Copy Code

Loop with while and for

• Sometimes, we need to do something more than once.
  • We need a loop, and Python gives us two choices: while and for.

Repeat with while

count = 1        
while count <= 5:
    print(count)
    count += 1Copy Code

Repeat with while

Asking the user for input

• Sometimes we would like to take the value for a variable from the user via their keyboard.
  • The input() function gets input from the keyboard.
  • When the input() is called, the program stops and waits for the user to type something on Console (interactive Python interpreter).
  • When the user presses Return or Enter on Console, the program resumes and input returns what the user typed as a string.

stuff = input()
# Type something and press Return/Enter on Console 
# before running print(stuff)
print(stuff)Copy Code

Repeat with while

Cancel with break

• While loop is used to execute a block of code repeatedly until given boolean condition evaluated to False.
  • while True loop will run forever unless we write it with a break statement.

• If we want to loop until something occurs, but we’re not sure when that might happen, we can use an infinite loop with a break statement.

while True:
    stuff = input("String to capitalize [type q to quit]: ")
    if stuff == "q":
        break
    print(stuff.capitalize())Copy Code

Repeat with while

Skip Ahead with continue

• Sometimes, we don’t want to break out of a loop but just want to skip ahead to the next iteration for some reason.

• The continue statement is used to skip the rest of the code inside a loop for the current iteration only.

while True:
    value = input("Integer, please [q to quit]: ")
    if value == 'q': # quit
        break
    number = int(value)
    if number % 2 == 0: # an even number
        continue
    print(number, "squared is", number*number)Copy Code

Repeat with while

Check break Use with else

• We can consider using while with else when we’ve coded a while loop to check for something, and breaking as soon as it’s found.

numbers = [1, 3, 5]
position = 0
while position < len(numbers):
    number = numbers[position]
    if number % 2 == 0:
        print('Found even number', number)
        break
    position += 1
else: # break not called
    print('No even number found')Copy Code

• Consider it a break checker.

Loop with while and for

Iterate with for and in

• Sometimes we want to loop through a set of things such as a string of text, a list of words or a list of numbers.

  • When we have a list of things to loop through, we can construct a for loop.

  • A for loop makes it possible for you to traverse data structures without knowing how large they are or how they are implemented.

Loop with while and for

Iterate with for and in

• Let's see two ways to walk through a string here:

word = 'thud'
offset = 0
while offset < len(word):
    print(word[offset])
    offset += 1Copy Code

word = 'thud'
for letter in word:
    print(letter)Copy Code

• Which one do you prefer?

Iterate with for and in

word = 'thud'
for letter in word:
    if letter == 'u':
        break
    print(letter)Copy Code

word = 'thud'
for letter in word:
    if letter == 'u':
        continue
    print(letter)Copy Code

Iterate with for and in

Check break Use with else

• Similar to while, for has an optional else that checks whether the for completed normally. If break was not called, the else statement is run.

  • This is useful when we want to verify that the previous for loop ran to completion instead of being stopped early with a break:

word = 'thud'
for letter in word:
    if letter == 'x':
        print("Eek! An 'x'!")
        break
    print(letter)
else:
    print("No 'x' in there.")Copy Code

Iterate with for and in

Generate Number Sequences with range()

for x in range(0, 3):
    print(x)
list( range(0, 3) )Copy Code

for x in range(2, -1, -1):
    print(x)
list( range(2, -1, -1) )Copy Code

for x in range(0, 11, 2):
    print(x)
list( range(0, 11, 2) )Copy Code

Loop with while and for

Class Exercises