Tue Sep 10

Any questions about anything?

This week :

• get python running (if you haven't already)
• read & practice the chapter 2 "writing simple programs"
• read & practice the chapter 3 "computing with numbers"

Tutoring!

• Nick is doing CS tutoring from 6:30 - 8:30 on Mon & Thu in Sci 217.

running python

I'll do a few demos.

• IDLE (python.org's all-in-one application)
• command line with nano (terminal editor)
• jupyter.marlboro.college

I can give access to jupyter.marlboro.college , including a shell, to anyone who wants that - let me know.

submitting homework

Depending on the assignment, typically you should turn in

• answers to questions
• source code (including docs, comments, tests - more on that to come)
• what running that code looks like (cut'n'paste session preferred; screenshot if graphics)

The best way to submit work is through the course website - I'll do a demo of how this works.

If that isn't working, email me (mahoney@marlboro.edu) with your code as attachments.

Full credit for a homework assignment is "1.0". For a submission that only does some of what was assigned, especially if I can't see evidence of a real effort, you'll get a fraction of that. There's one term letter grade (A=excellent,B=good,C=fair) for all the homework together.

the big picture

• computing ... managing complexity
  • computer science
  • software engineering
  • information systems & technology
  • web development
• programming paradigms :
  • imperative (do this, then do that) : C
  • object oriented (data abstraction & associated methods): java
  • functional (math-like data flow definitions): haskell

Python programs can be written in several styles including imperative, object oriented, or functional. This term we'll be doing mostly the first two ... and a little of the third.

chapter 2

Some important ideas:

• functions
• function arguments (i.e. inputs)
• built-in print function (i.e. show on terminal)

# an example of a function with an input
# Jim M | Sep 3 2018

def echo(message):
  print("you said '", message, "'.")

echo('Hello there')

Run this and discuss it.

software development cycle

• understand problem (can be harder than you think)
• create specification : inputs? outputs? (be very specific)
• choose algorithm: what is the recipe ?
• implement (write the code)
• test and debug (again, can be harder than you expect)
• maintain (if used over time, needs will usually change)

variables

variables - named "boxes" to put data in

• legal names : x, yy23 the_data, this_is_a_long_name
• NOT legal : 3x (starts with number), x-y (no dashes), x.y (no periods)
• good names : principal, count, first_name
• not so good names: h (usually short), hpltz (too cryptic)

assignment statement (i.e. how to put data into a variable)

name = "Jim Mahoney"
age = 57
data = [2.3, 3.2, 16.23]

The part on the RIGHT is evaluated first. Then that is put INTO the thing on the left. Consider

x = x + 1

What is going on? Run this pythontutor.com to see.

example 1 : temperature convert

# convert.py
# A program to convert Celsius temps to Fahrenheit # by: Susan Computewell

def main():
    celsius = eval(input("What is the Celsius temperature? ")) 
    fahrenheit = 9/5 * celsius + 32
    print("The temperature is", farenheit, "degrees Fahrenheit.") 

main ()

• What is eval() ?
• What is input() ?
• Can you see how these two are nested inside each other?
• This program has a bug in in ...

example 2 : future value

# futval.py
# A program to compute the value of an investment 
# carried 10 years into the future

def main() : 
    print("This program calculates the future value") 
    print("of a 10-year investment.")
    principal = eval(input("Enter the initial principal: ")) 
    apr = eval(input("Enter the annual percent interest rate : "))
    for i in range(10):
        principal = principal * (1 + apr/100)
    print("The value in 10 years is: ", principal)

other stuff to discuss

• python, blocks, and whitespace
• loops
• flow charts
• eval() is used to convert strings ("1.2") to numbers (1.2) in the text.
  • ... which works.
  • ... but has security and performance issues.
  • There's a better way which we'll see later.

coming next

• for Thursday : chap 3 - data types
• The assignment due next Tuesday (chap 2 & 3) will be posted by the end of the day.