python - how to make a float to float function behave elementwise on lists?

Question

I have the following code:

def tau_r(u):
    return (u_1 < u < u_2) * (T1 - T2) + T2
if __name__ == "__main__":
    figure()
    plot(u, tau_r(u))
    show()

I get the following error when I run it:

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

I suppose it's because it is seeing u as a list while it should perform the task on each element of the list separately. How can I fix this?

Can you just use a list comprehension: `return [(u_1 < v < u_2) ... for v in u]`? Note the `...` is just a placeholder for the rest of your equation and should not be in your code. — Engineero, Nov 17 '17 at 15:27
Are u_1 and u_2 constants? Should tau_r return T2 if u is not between u_1 and u_2? — bphi, Nov 17 '17 at 15:28
@miradulo I don't think this is a duplicate, as he seems to *want* the array of element-wise comparisons. — Engineero, Nov 17 '17 at 15:28
@Engineero And there's no reason OP cannot get it if they use the solution from the linked duplicate. An approach that dips into element-wise comparison without leveraging NumPy is wasteful. — miradulo, Nov 17 '17 at 15:30

MatsLindh · Answer 1 · 2017-11-17T15:34:10.323

1

You can apply the same function to all elements of a list by using map. It'll return a new list with each returned value from the function.

plot(u, list(map(tau_r, u)))

edited Nov 17 '17 at 15:34

answered Nov 17 '17 at 15:25

MatsLindh

1

You should change your code to list(map(tau_r, u)) because matplotlib does not support generators as input – yukashima huksay Nov 17 '17 at 15:28
This is going to be very slow for any lengthy `u`. – miradulo Nov 17 '17 at 15:35

1 Answers1