Simple Python Function with an if statement not working?

Question

Why is the function defined as shown below not working? I am getting the error msg ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

Thanks in advance.

import math
from math import sin, cos, exp, pi, sqrt
from matplotlib import pyplot as plt

def pulse(Amax,td,t):
    if t<=td:
        y = Amax*sin((pi/td)*t)
    else:
        y = 0
    return y

t = np.linspace(0,4*pi,100)
Amax=10
td=11/1000
plt.plot(t,pulse(Amax,td,t), 'r', label='pulse A=10,td=11')

Does this answer your question? [ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()](https://stackoverflow.com/questions/10062954/valueerror-the-truth-value-of-an-array-with-more-than-one-element-is-ambiguous) — TrebledJ, Apr 15 '20 at 10:28

score 2 · Answer 1 · answered Apr 15 '20 at 10:22

2

In t <= td you are directly comparing a scalar with an array. That type of operation is ambiguous because it can't determine what a truth value should be.

answered Apr 15 '20 at 10:22

Keith

42,110
11
57
76

Thanks. So what is the "standard" python way of defining such simple function? – JXB Apr 15 '20 at 10:50
That's actually from numpy, which is not standard Python. Check their docs. – Keith Apr 15 '20 at 17:17

JXB · Answer 2 · 2020-04-18T09:12:19.583

0

ugly but works

i=0
y=zeros(N0); ti=zeros(N0)

for t in np.arange(0,tend,dt):
    ti[i]=t
    if t <= td:
        y[i]=amax*np.sin(wf*t)
    else:
        y[i]=0
    i+=1

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Maybe a slightly better code

trg = np.arange(0,tend,dt)   # start,stop,step
for i,t in enumerate(trg):
    if t <= td:
        y[i]=amax*np.sin(wf*t)
    else:
        y[i]=0

edited Apr 18 '20 at 09:12

answered Apr 16 '20 at 08:05

JXB

29
5

score 0 · Answer 3 · answered Apr 16 '20 at 08:30

In plt.plot you give 2 arrays. its better to create array y beforehand using iterating over array t with suitable conditions or your function pulse should return an array. like

def pulse (Amax,td,t):
    y=[]
    for i in t:
        if i <= td:
            y.append(Amax*sin((pi/td)*i))
        else:
            y.append(0)
    return y

at last just use plt.plot(t,pulse(Amax,td,t), whatever you want here)

Simple Python Function with an if statement not working?

3 Answers3

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