CS 111 f21 — Variables and Arithmetic

What are the two things the CPU does in our notional machine?

• Computer needs to make a decision at a particular point in time
• Warmup: sketch what the computer needs to know (input) and what the possible outcomes are (output)
• Simplified problem: difference between current and hot temperatore (i.e., should the app show the current temperature in red)
  • outline: get input data, calculate difference, output result
  • have the values we want: get_cur_temp() and 80° F
    • don't worry about exactly what's going on with get_cur_temp()
    • we'll get into it next week—for now, abstraction!
    • need to make the values available and give them useful names: assignment

• = and variables have different meaning than in math

• assignment statement: <variable name> = <expression>

  cur_temp = get_cur_temp()
  hot_temp = 80
  

• assignment copies value to memory and gives it a label (the variable name)
• variable names follow certain rules
  • name must begin with a letter or an underscore, can only contain letters, number, and underscores
  • no spaces
  • case sensitive (e.g., cur_temp and Cur_temp are different names)
• to make variable names easier to read, convention is to use underscores or capitalization
  • cur_temp or curTemp
  • snake case and camel case
  • I expect you to use one of these conventions and be consistent, but which you choose is up to personal preference
    • I tend to use snake case

• just got off the phone with Current Temperature Inc., get_cur_temp() is in C
• Python provides the standard mathematical operators
  • + (addition)
  • - (subtraction)
  • * (multiplication)
  • / (division)
  • % (remainder or modulo)
  • ** (exponent)
• Follows the order of operations you might have seen in math class
  • PEMDAS
  • parens, exponents, multiplication & division, addition & subtraction
  • minus signs happen after exponents, so -2**2 evaluates to -4
• POLL: 3 * 5 - 10**2 % 9
• First step: what's the formula we need to implement in Python code?
  • \(C = \frac{5}{9}(F - 32)\)
• Python version: cur_temp_F = (cur_temp - 32) * 5 / 9
  • when a variable appears on the right side of an =, it is interpreted as the value it labels in memory
  • when a variable appears on the left side of an =, it is the label where the value computed on the right side will be stored
• quick check: write Python code to compute a lower bound on hours I spent watching Fellowship of the Ring (2 hours 58 minutes) when it came out on VHS (I watched it once per day for a week), the final result should be assigned to a variable
• Calculate the diffence: difference = hot_temp - cur_temp_F

from temperature import get_cur_temp

cur_temp = get_cur_temp()
hot_temp_F = 80
hot_temp_C = hot_temp_F - 32 * 5 / 9 # missing parentheses

• output result

from temperature import get_cur_temp

cur_temp = get_cur_temp()
hot_temp_F = 80
hot_temp_C = (hot_temp_F - 32) * 5 / 9
print("hot_temp_C =", hot_temp_c, "degrees Celsius
print("difference from current temp is", cur_temp - hot_temp_C)

• Three mistakes
  • Forgetting a closing " around text causes a SyntaxError
  • anatomy of the error message
    • File where the error occurred, line where the error occurred, and the error type and message
  • it may feel like it's Python yelling at you for your mistakes
  • really Python catching on fire and saying I don't understand, aaaaaaaahhhhhhhhhh.
  • Forgetting a closing ) also causes a SyntaxError, but often on a line after the line missing a parenthesis
  • Mispelling a variable name results in a NameError
• How would you demonstrate to me this is correct (or incorrect)
  • print everything, check for known results

• Compare to one-line version:

  print("difference between hot and current temp in Celsius:", (80 - 32) / 9 * 5 - get_cur_temp())
  

• any line that starts with a # is a comment
• ignored by Python, useful for explaining and documenting your code