Weird NaN print issue

[dbor...@willamette.edu]

Hey there, Steve/Bruce!

One of my students ran into an interesting issue in Web VPython. 

Here's the minimal working code:

Web VPython 3.2

print(((-b+sqrt((b**2)-4*a*c))/(2*a)))
a = 3
b = 5
c = -2

If you run this in Web VPython, the print statement prints "NaN." If you run this in raw Python, it throws and error since a, b, and c aren't defined at the point that the print statement runs. In Web VPython, if you comment out or delete one of the variable definitions, then a "ReferenceError: [variable] is not defined" error is actually thrown. Not sure if this is a known issue (or if it is fixable or even something you would want to fix) but I figured I would point it out in case it matters.

Best,

Daniel

[Steve Spicklemire]

This is a consequence of the design of WebVPython. It transpiles python into javascript and runs the javascript in your browser. In javascript variables that are not yet assigned literal values, but are declared as “var” are initially assigned “undefined”. This is what’s causing the “NaN”. In python, variables that are not yet assigned are not defined as anything, so you’ll get NameError.

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