15-110 Principles of Computing - SUMMER TWO 2014

Lab 2

Goals

• Start Python version 3 (python3)
• Use Python to evaluate formulas
• Properly name your Python program file (whatever.py)
• Import and use functions from pre-existing Python modules (import math and math.whatever)
• Define Python functions that implement mathematical formulas
• Start Python and direct it to read your program file (python3 -i whatever.py)
• Write Python code to return a value to other Python code
• Write Python code to print a value to the screen, and understand the difference between return and print
• Choose the proper Python notations to control the evaluation order of a mathematical formula
• Write Python comment lines (beginning with #) to explain the purpose of the code
• Command Python to call a function that you have defined

Files

1. force.py (demonstration)
2. circle_area.py (demonstration)
3. answers.txt
4. sphere_vol.py
5. my_func.py

Place these files in a lab2 folder. Before leaving lab, zip up this folder, and hand it in.

CA Demonstration

• creating a lab2 directory
• starting python3 within a terminal window
• creating and using functions: force(m,a) to calculate F = ma
  • setting gedit line numbers and tabs
  • using gedit to define a function in force.py
  • loading force.py: python3 -i force.py
  • force's parameters and return values
• math constants and print: circle_area(r) to calculate A = π r²
  • math.sqrt(x) and math.pi
  • returning a value from circle_area(r)
  • using the return value
  • printing a value instead of returning

Activities

1. Type each of the following expressions into python3. What value do each of the following Python expressions evaluate to? Is that value an integer or a floating point?

   1. 250
   2. 28 % 5
   3. 2.5e2
   4. 3e5
   5. 3 * 10**5
   6. 20 + 35 * 2
      Why is this different from (20 + 35) * 2?
   7. 2 / 3 * 3
   8. 2 // 3 * 3
      Why is this different from 2 / 3 * 3?
   9. 25 - 5 * 2 - 9
      Is this different from ((25 - 5) * 2) - 9 and/or 25 - ((5 * 2) - 9)? Why?

   Write your answers in the file answers.txt. (Review Unit 2A lecture slides if necessary.)

2. In sphere_vol.py, define a Python function sphere_vol(r) that calculates and returns the volume of a sphere with a radius r. This can be calculated using the formula:

   V = (4/3) π r³

   Place in answers.txt a copy of your interaction with python3 in which you call sphere_vol(r) to compute the volume of a sphere with a radius of 7, and python3 shows you the result.

3. Pick a mathematical formula that is used in your major. (For APEA students, pick any mathematical formula that interests you.) In a file called my_func.py, create a python3 function that calculates some value using that formula.

   At the beginning of your my_func.py include a comment (one or more lines of English text that start with "#" and are ignored by python3) that describes what the fomula the function computes and what units the parameters and results are in.