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import numpy as np
a = 3.9 #m
L = 7.8 #m

x = np.arange(0,L+0.1,0.1)

def fun_v():
    for value in x:
        if 0 < x < a :
            V= P*(1-(a/L))
        if a < x < L :
            V= -P*(a/L)
    return (V) 
V = fun_v()

print('V =',V)

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

DevLoverUmar
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Aivars
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3 Answers3

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You can do it with a fancy indexing technique of NumPy, which will be both quicker and easier to comprehend:

import numpy as np
a = 3.9 #m
L = 7.8 #m
P=1
x = np.arange(0,L+0.1,0.1)
x[(0<x)&(x>a)]=P*(1-(a/L))
x[(a<x)&(x>L)]=-P*(a/L)
x
ppwater
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Michael
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The issues are the following:

  • You are comparing an array instead of a value in a condition, which is ambiguous.
    Try 0 < value < a instead 0 < x < a

    To give an example of what you are doing:

    1. You are first assigning the x like [0, 0.1, 0.2, 0.3 ... ]
    2. Then you are comparing 0 < [0, 0.1, 0.2, 0.3 ... ] < 3.9 at line 0 < x < a

  • Also, the variable P is not defined. You will need to provide a value of P like the ones you provide for a and L.

  • To get an array of V values, you will need to push the calculated V values in an empty array or you can use the map function.

A working example:

import numpy as np
a = 3.9 
L = 7.8 
P = 10 # P is defined

x = np.arange(0,L+0.1,0.1)

def fun_v():
    arr = [] # empty array is initialized
    for value in x:
        if 0 < value < a : # value is compared instead of x
            V = P*(1-(a/L))
            arr.append(V) # V is pushed after calculation
        if a < value < L :
            V = -P*(a/L)
            arr.append(V)
    return arr # return the array of v values
V = fun_v()

print(V)
Nafiz Ahmed
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I think the comparison is wrong and you didn't declare what is p. I am actually not sure what you trying to do at the end as it wont return anything. but from your code, I believe it suppose to be like this:

import numpy as np
a = 3.9 #m
L = 7.8 #m
x = np.arange(0,L+0.1,0.1)

def fun_v(x_):
    for value in x_:
        P = 1 # declare your P value
        if 0 < value < a:
            V = P*(1-(a/L))
        elif a < value < L:
            V = -P*(a/L)
    return V

V = fun_v(x)
print(V)

But in this code, the V value will not be an array or list. it will just return a value. if you would like to calculate all the x value and separate it in different list or array, you might need an append.

theearlybird
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