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

Question

I'm really new to programming, and I can't figure out how to use a Numpy array to be manipulated in a function such as P**2.

import math
import numpy
span_x = numpy.array([0,250,500,750,1000])
P = span_x
example = P**2
span_y = [example for i in P]
y = float(input("Enter y: "))
bracket1 = min(span_y, lambda span_y: abs(span_y-y))
if bracket1 < y:
    for i in span_y:
        bracket2 = span_y[span_y.index(bracket1) + 1]
else:
    for i in span_y:
        bracket2 = span_y[span_y.index(bracket1) - 1]
print "Brackets: ", bracket1, bracket2

I've tried not using a Numpy array, but received a TypeError.

My main issue is that I have this array of x-values (span_x) that I want to put into a function like P**2 and get y-values (span_y) in an array. Then, the user inputs a y-value and I want to check which y-value in span_y is closest to this input, and that is bracket1. bracket2 is the second closest y-value. I would love some help!

Please, do a search on the error - see the 'Related' column to see how many other SO questions have the same subject line. — hpaulj
– hpaulj, Commented May 16, 2015 at 2:54
Thank you for the feedback. The error occurs at: bracket2 = span_y[span_y.index(bracket1) - 1] — theo1010
– theo1010, Commented May 16, 2015 at 15:50
As Alik noted bracket1 is not an array like one those in span_y. You need to test how you generate it. — hpaulj
– hpaulj, Commented May 16, 2015 at 15:59

Konstantin · Accepted Answer · 2015-05-16 02:36:57Z

1

span_y is a list of 1D-arrays so min doesn't work as you expect and returns a function. After that span_y.index(bracket1) raises an exception. span_y should be initialized like this

span_y = list(example)

Pass your key function (lambda) in min as a named parameter as said in documention.

bracket1 = min(span_y, key = lambda span_y: abs(span_y-y))

answered May 16, 2015 at 2:36

Konstantin

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Saullo G. P. Castro · Accepted Answer · 2015-05-16 02:58:56Z

1

In NumPy you can and should vectorize the operations like:

span_y = span_x**2
y = float(input("Enter y: "))
bracket1 = np.array((span_y, np.abs(span_y - y))).min(axis=0)
bracket2 = np.zeros_like(bracket1)
bracket2[ bracket1 < y ] = np.roll(span_y, 1)
bracket2[ bracket1 >= y] = np.roll(span_y, -1)

edited May 16, 2015 at 2:58

answered May 16, 2015 at 2:51

Saullo G. P. Castro

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2 Comments

theo1010 Over a year ago

I tried this, and it got rid of my Error Message, but when I print bracket1, bracket2 both outputs display long arrays when the answers should really just each be one value. Why is that please?

Saullo G. P. Castro Over a year ago

@theo1010 it was a bit unclear from your question to understand what you really wanted, so I built this example using vectors... I hope this example clarifies your doubts

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