Thanks! As I recently posted in a followup message,

--------

Here's an example of what goes wrong:

t = var('t')

L(t) = t*725.5%360.0

This produces the following error message:

...

TypeError: unsupported operand type(s) for %:

'sage.symbolic.expression.Expression' and

'sage.symbolic.expression.Expression'

--------

So it looks like I've got a Sage-specific problem. I still don't know how to fix it, but at least I understand the problem better!

Many thanks,

Kim

________________________________

From: Steven D'Aprano <

[email protected]>

To: (e-mail address removed)

Sent: Thursday, April 10, 2014 1:01 AM

Subject: Re: Plotting the integer-and-fraction remainder of a function value modulo 360

How can I get Python to represent a value of a function in degrees,

i.e., with values between 0 and 360, by taking the (non-integer)

function expression mod 360?

That is, I have a function with non-integer values, called Longitude,

which is defined in terms of the variable t. I just want to plot

Longitude modulo 360 for a range of values of t: that is, for every

value of t, plot the integer-AND-fraction remainder after dividing

Longitude by 360.

But Python (in Sage) apparently won't let me use the int function or the

// operator on functions defined in terms of a variable: I get a "cannot

evaluate symbolic expression numerically" TypeError.

That's an issue with Sage itself, probably sympy. Python the language

doesn't natively understand symbolic expressions, that's added by sympy,

so you may need to asksome sympy experts.

It may help if you show the actual code you are using, and the full

traceback generated by the error. When I try something similar, I get a

different error message:

py> from sympy.abc import x

py> (x + 1) % 60

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

TypeError: unsupported operand type(s) for %:'Add' and 'int'

which could mean I'm doing something different, orjust a difference in

version numbers.

How do I do this?

There must be a simple way to tell Python that I want it to compute the

value of Longitude for a given value of t and then take the

integer-and-fraction remainder from dividing by 360.

That's simple with numeric types: use the divmod function or % operator.

But you're using some sort of symbolic library, so we need to know what

the library is.