Plotting the integer-and-fraction remainder of a function valuemodulo 360

[Kim Plofker]

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. 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.



Many thanks,
Kim

 

[Steven D'Aprano]

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 ask some 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, or just 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.

 

[Kim Plofker]

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.