What am I doing wrong to calculate V from numpy array? I can not get array with “V” values

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()

Answer

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)