Inputting Other Data Types

Announcements

• Problem Set 2 due Friday!
  • I’m halfway through PS1 feedback, should be coming back tomorrow
• A reminder that you can contact your section leaders to ask questions as well!
  • Don’t wait until 11pm on Friday night!
• I’ll be updating the syllabus schedule, as we added a chapter to the book, but it shouldn’t make a huge difference outside of chapter numbering
  • I’ll also actually fix the old website schedule, since I realized I failed to update that originally
• Polling: rembold-class.ddns.net

Review Question

def func1(x):
    if x > 10:
        return 3 + x
    else:
        return 0

def func2(y):
    A = func1(y+1)
    if y % 2 == 0 or A ** 3 > 30:
        A -= 5
    return A + y

Shorting the Circuit

• Python evaluates and and or operators using a strategy called short-circuit mode
• Only evaluates the right operand if it actually needs to

  • Example: if n=0, then the x % n == 0 is never actually checked in the statement

    n != 0 and x % n == 0

    since n != 0 already is False and False and anything is always False

• Can use short-circuit to prevent errors: the above x % n == 0 statement would have erred out if n=0

Other Data Types

• Numbers are great, but what about other types of data?
• We will spend considerable time on the details of these data types later
• For now, let us introduce:
  • strings!
  • lists!

Lists

• A list in Python represents a sequence of any type of data
• Denote by bordering with square brackets ([, ]) with commas separating each element of the sequence
  • Each element could be any data type (even mixing from element to element!)
  • ['This', 'is', 'a', 'list']
  • ['Great', 4, 'storing', 5 * 10]
• There are many operations that we will see are possible on lists, but will start with only the basics

Sequences

• Both strings and lists are examples of a more general type called a sequence
  • Strings are sequences of characters
  • Lists are sequences of anything
• Sequences are ordered, so we can number off their elements, which we call their index
  • Counting in Python always starts with 0, so the first element of the sequence has index 0
• Python defines operations that work on all sequences
  • Selecting an individual element out of a sequence
  • Concatenating two sequences together
  • Determing the number of elements in a sequence

Selection

• You can select or “pluck out” just a single element from a sequence using square brackets [ ]
  • There are no commas between these square brackets, so they can’t be confused with a list
  • The square brackets come after the sequence (or variable name representing a sequence)
  • Inside the square brackets, you place the index number of the element you want to select

>>> A = [2, 4, 6, 8]
>>> print(A[1])
4

>>> B = "Spaghetti"
>>> print(B[6])
't'

Concatenation

• Concatenation is the act of taking two separate objects and bringing them together to create a single object
• For sequences, concatenation takes the contents of one sequence and add them to the end of another sequence
• The + operator concatenates sequences
  • This is why it is important to keep track of your variable types! + will add two integers, but will concatenate two strings

  >>> A = 'fish'
  >>> B = 'sticks'
  >>> print(A + B)
  'fishsticks'

  >>> A = [1, 'fish']
  >>> B = [2, 'fish']
  >>> print(A + B)
  [1, 'fish', 2, 'fish']

Lengths

• The number of elements in a sequence is commonly called its length, and can be given by the len( ) function

• Simply place the sequence you desire to know the length of between the parentheses:

  >>> len("spaghetti")
  9

• You can have sequences of 0 length as well!

  >>> A = ""
  >>> B = [ ]
  >>> print( len(A) + len(B) )
  0

Understanding Check

A = "hots"
B = ["fire", A + A]
C = A[3] + A[1]
B += [C + C[0]]
D = B[0] + B[1] + B[2]
print(len(D))

Input: input

• To retrieve data from a user, we can use Python’s built-in input() function

• The form will generally look like:

  variable = input(prompt_text)

  • variable is the variable name you want to assign the user’s typed input to
  • prompt_text is the string that will be displayed on the screen to communicate to the user what they should be doing

• The input() function always returns a string

  • If you want to get an integer from the user, you will need to convert it yourself after retrieving it

    num = int(input('Pick a number between 1 and 10: '))

Running a Program

• Python programs specify what part of the code is supposed to be executed when a program is run using a few special lines at the end of the program

  if __name__ == '__main__':
      function_to_run()

  • function_to_run is the name of whatever function you want to execute when the program is run directly

• Patterns of this sort are commonly called boilerplate

  • Less important to fully understand all the pieces of it now
  • Is important to understand what it is doing and how to implement it correctly

An adding program

# File: AddTwoIntegers.py

"""
This program adds two integers entered by the user.
"""

def add_two_integers():
    print("This program adds two integers.")
    n1 = int(input("Enter n1? "))
    n2 = int(input("Enter n2? "))
    total = n1 + n2
    print(f"The sum is: {total}")

# Startup boilerplate
if __name__ == '__main__':
    add_two_integers()

Common Patterns

• Many programs in programming can be approached similarly by using a particular idiom or pattern

Visiting the library(ies)

• A huge strength of Python is that it offers a very large number of code collections called libraries
  • Can save you the effort from writing that code yourself!
• Requires you to import that library, which can take several different forms

  • Most common is to use import to grab everything in a library

    import math

    • You must then access any function in that library using its fully qualified name, which includes the library name (var = math.sqrt(4))

  • Can also use from ... import to grab specific functions from the library

    from math import sqrt

    • You do not need to use the library name then when you call it (var = sqrt(4))

Useful math definitions