Piecewise functions

Question

I'm fairly new to python and would like to know why I can't graph a piecewise function like below, and why the error message is popping up?

def get_data(self, frame):
    self.ydata =np.piecewise(self.xdata, [self.xdata<.2, self.xdata>=.2][ np.sin((self.frequency) * self.xdata +
                                                                frame), np.sin((5*self.frequency) * self.xdata + frame)])

It says my error is

  line 72, in get_data
    frame), np.sin((5*self.frequency) * self.xdata + frame)])
TypeError: list indices must be integers or slices, not tuple

I guess my question can also be what would I bind the piecewise function to? This is an animation and I'm doing this piecewise for a variable for my second go at it, but I don't know what i should put as 'x' if I'm wanting x<.2 because my x values are in an arange set-up.

@Bryan Oakley ```line 72, in get_data frame), np.sin((5*self.frequency) * self.xdata + frame)]) TypeError: list indices must be integers or slices, not tuple``` — VanillaceIce, Nov 16 '19 at 14:08
@VanillaceIce Please [edit] to put the error message in the question — wjandrea, Nov 16 '19 at 16:10

Tls Chris · Accepted Answer · 2019-11-19T18:42:30.247

I'm not sure what self is in the question. My code below works as a function but can be changed to a method if required.

Your error message is caused by there not being a comma between the condition list and the function list.

def get_data(self, frame):        
    self.ydata =np.piecewise(self.xdata, [self.xdata<.2, self.xdata>=.2]  # Must be a comma here
        [ np.sin((self.frequency) * self.xdata + frame), 
          np.sin((5*self.frequency) * self.xdata + frame) ] )

Another problem is that the function lists must be functions, not the result of functions.

import numpy as np    

def get_data(xdata, frame):
    def f_lt(x):
        return np.sin( frequency * x + frame)

    def f_ge(x):                                                                                              
        return np.sin( 5*frequency * x + frame)  

    return np.piecewise(xdata, [xdata<.2, xdata>=.2], [ f_lt, f_ge ])

frequency = 1.0

xdata = np.arange(1,26)/25
get_data(xdata, 0.1) 
# array([ 0.13954311,  0.17902957,  0.21822962,  0.25708055,  0.89120736,
#         0.96355819,  0.99749499,  0.99166481,  0.94630009,  0.86320937,
#         0.74570521,  0.59847214,  0.42737988,  0.23924933,  0.04158066,
#        -0.15774569, -0.35078323, -0.52983614, -0.68776616, -0.81827711,
#        -0.91616594, -0.97753012, -0.99992326, -0.98245261, -0.92581468])

I'm not sure what you're trying to do so I can't test the result but perhaps this helps.

Edit: Response to a comment.

get_data(x, frame) can be called as often as required.

It may be better to expand the params list to make it more flexible. It really depends on what you're trying to do.

def get_data(xdata, frame, frequency, step_point=0.2 ):
    def f_lt(x):
        return np.sin( frequency * x + frame)

    def f_ge(x):                                                                                              
        return np.sin( 5*frequency * x + frame)  

    return np.piecewise(xdata, 
                       [xdata<step_point, xdata>=step_point],
                       [ f_lt, f_ge ])

None of the parameters in the function are now hardcoded. They can all be set each time the function is called.

for frame in range(100):
    frame = frame / 100
    y = get_data(xdata, frame, frequency, 0.2)
    do_plot(xdata, y)

Does this answer your comment?

@Tis Chris, thank you! I think I'm going to use this as a basis for the change of the variable rather the entire equation, but this really helped — VanillaceIce, Nov 17 '19 at 21:13
@ Tis Chis, sorry one more question- is there any way to assign a single name to the equation overall? — VanillaceIce, Nov 18 '19 at 13:47
@VanillaceIce. I'm not sure what you mean. The functions can be named with any valid python name. You could give the list of functions a variable name, `name = [ f_lt, f_ge ]'. — Tls Chris, Nov 19 '19 at 11:36
@Tis Chris, I probably misworded it, but I was wondering if I could refer to it all under one name for the rest of the code. I f I wanted to call back the final result to plot it, or would I have to say both individually? — VanillaceIce, Nov 19 '19 at 13:09

Piecewise functions

1 Answers1