Python Syntax

Claimed by Benjamin Thorne for Spring 2018

The Main Idea

VPython is an extension of the Python programming language that contains a 3D graphics module called Visual, which allows users to create simple simulations of physics problems. Its primary use is for educational purposes, although it has been used in research before in the past. Although it is slightly different from the actual Python program, it is similar enough for the purpose of this class. Most of the labs done this year will be done using the vPython program.

Downloading vPython

In all cases you will need to have Python 2.7.9 installed. Future versions may also work, but be warned that it may cause VPython to behave erratically.

For Windows

For Mac

For Linux

Note for Linux: You can only install Classic VPython, which is no longer supported and may be missing some features and bug fixes that are present in the current version of VPython.

Alternatively, you can use GlowScript, a virtual environment that allows you to execute VPython code in your browser. You can create an account at http://www.glowscript.org/.

Python Basics

It's always helpful to have a background knowledge of Python, however basic, before proceeding with the applications more relevant to this course. Almost any question can be answered on websites like www.stackoverflow.com. Resources like this are great if you are trying to self teach yourself, or simply need a quick question answered.

   # Comments start with a number symbol. A comment is writing that does not impact the code in any way. These are essential so that both you, or anyone else doesn't get lost in the code and can   
   #easily locate where a bug may be happening. It is encouraged to use these often.
   # We're always going to start our programs with the following headers to ensure that math works correctly.
   from__future__ import division
   from visual import*

   # Math works as you would expect.
   1 + 1  # -> 2
   2 - 1  # -> 1
   3 * 2  # -> 6
   8 / 4  # -> 4
   (1 + 3) * 2  # -> 8
   1 + (3 * 2)  # -> 7

   # That is, until you need to do an exponent. Instead of a "^", you simply do a double asterisk"**". But you shouldn't encounter anything much weirder than that math-wise.
   2 ** 3  # -> 8

   # There's also various math functions you can use, such as for finding the square root.
   sqrt(4) # -> 2

   # However, these math operations are useless if we don't store the values anywhere. A variable can be any combination of letters. Storing values to a variable allows for said value to be accessed  
   #again in the future very easily.
   x = 1 + 1
   # Now if you try to access x in the future, it will have the value of '2'.

   # Printing something writes the input in the shell so that you can see the results of the output. You can print your variables like this:
   print(x)
   # You can also add in some text with them like this:
   print("The value of x is ", x)

   # Sometimes we only want to execute something based on a certain condition.
   if x == 2:
       y = 3
   # The '==' means that you're directly comparing two values. If they're equal, whatever is on the inside of the if-statement will be executed.
   # Now is a good time to note that whitespace is very important in Python.
   # 'y = 3' will always execute if it isn't tabbed, so even if x is equal to 7, y will still become 3.
   # If you ever see me use whitespace in this tutorial, make sure you do as well.
   # Some other useful comparators: != (not equal), >, >= (greater than or equal to), <, <= (less than or equal to), 'and', 'or', 'not'
   # You can add onto the above if-statement by tacking on:
   elif x == 3:
       y = 4
   else:
       y = 5
   # With elif meaning "else if" for a second possible condition, and "else" for something that you want to execute if none of the earlier conditions are met.

   # You can also have a loop that executes code for a certain amount of time or based on a certain condition. There are two kinds of loops. The first is a while loop that keeps looping
   # until the terminating condition is met.
   while x < 5:
       print("x is ", x)
       x = x + 1
   # This will output the value of x up until it reaches 4.

   # The second type of loop is a for loop. This type of loop sets the amount of loops the program will run before hand. 
   

Thus concludes our very, very brief foray into Python. That was simply the bare minimum required to understand what will be needed for this class. More concepts will be introduced as needed, whether that be in this tutorial or in a lab. For a more complete understanding of Python, you can look up other Python tutorials elsewhere on the Internet.

VPython Basics

In this section we will analyze some of the objects utilized by VPython. Each object is more or less self-explanatory with equally clear fields within.

   # The scene has a width and a height; you can set them like so.
   scene.width = 1024
   scene.height = 760

   # Beyond that, we have vectors.
   # You can access each component with pos.x, pos.y, or pos.z depending on which component you want.
   # Now would be a good time to mention that you access an object's field with a period following the variable name.
   pos = vector(1, 2, 3)
   xComponent = pos.x  #This value would be 1
   yComponent = pos.y  #This value would be 2
   zComponent = pos.z. #This value would be 3

   # You can add two or more vectors together and you can multiply a vector by a scalar,
   # Adding
   A = vector(1, 2, 3)
   B = vector(4, 5, 6)
   C = A + B  # The value of C is now (5, 7, 9)
   # Multiplying
   D = 5 * C. # The value of D is now (25, 35, 45)
   # but you can't add a scalar to a vector.
   # To get the magnitude of or to normalize a vector, use the appropriate function.
   initialPos = vector(10, 3, 0)
   finalPos = vector(30, 15, 0)
   deltaR = finalPos - initialPos # -> vector(20, 12, 0)
   rMag = mag(finalPos)
   rHat = norm(finalPos)
   errorDemo = magR + finalPos # -> error; causes your program to crash

   # There are a number of different shapes you can create for visual reference in the vPython program. These will be used quite frequently in lab to observe things like momentum, force, etc.

   # If you were to make, say, a ball, you would want to draw a sphere.
   # It has a position field, a radius field, and a color field.
   # When creating a new instance of an object, each field is comma separated.
   # You access each field the same way as we did with the position vector.
   ball = sphere(pos = vector(10, 10, 10), radius = 9e2, color = color.red)
   ballColor = ball.color
   ballPos = ball.pos
   ballRadius = ball.radius

   # You can draw arrows.
   # These also have a color and position field, but instead of a radius, they have an axis that determines their length.
   # These arrows can be used to represent the magnitude of the force or the momentum or anything else the lab asks you to make. 
   velocityArrow = arrow(color = color.blue, pos = vector(-10, -10, -10), axis = velocity * vScale)

   # There are also boxes.
   # They have a position vector and a size vector.
   myBox = box(pos = vector(50, 50, 50), size = vector(20, 15, 12))

   # You can draw helixes, which are useful for depicting springs.
   # They have a position field, a color field, a thickness field, a radius field, and a field for the number of coils.
   spring = helix(pos=ceiling.pos, color=color.cyan, thickness=.003, coils=40, radius=0.015)

   # If you want to trace a moving object, you can have a curve follow it by adding to it every time step. This will show the entire path the object has traveled during the simulation.
   posTrace = curve(color = color.yellow)
   trail.append(ball.pos)

   # You also have the option of making graphs as your program progresses; this is particularly useful for potential vs kinetic energy.
   Kgraph = gcurve(color = color.cyan)
   Ugraph = gcurve(color = color.yellow)
   KplusUgraph = gcurve(color = color.red)
   # For each time step...
   Kgraph.plot(pos = (t, Kenergy))
   Ugraph.plot(pos = (t, Uenergy))
   KplusUgraph.plot(pos = (t, Kenergy + Uenergy))

By updating the fields of whatever objects you decide to use and by using any intermediate variables necessary, you're able to draw and compute what you'll need for this course.