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

I am trying to use a mask to mark the elements in the vector nodes that are in the matrix e. For that I am using a mask for the vector nodes, and trying to set to false the corresponding position, so that at the end I can extract all elements in nodes that are left with a TRUE in the corresponding mask array

def SortedInteriorNodes(p,e,t):
    # e is a matrix with two rows, p a matrix with 2 rows, t a matrix with 3 rows
    nn=p.shape[1]  # number of nodes
    nt=t.shape[1]  # number of triangles
    ne=e.shape[1]  # number of edges = number of boundary nodes
    nodes = np.arange(start=0, stop=nn) # 0 1 2 3 ....np-1
    mask = np.full((1, nn), True)
    for col in range(ne):
        for row in range(2):
            k = e[row, col]
            if (mask[k] == True):
                mask[e[row,col]]= False
    # I end up with a vector of boolean where the true indicate the position of Interior Nodes
    InteriorNodes = nodes[mask]
    return InteriorNodes

I get this error "ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()"in the line "if (mask[k] == True):"

I ran the debugger and saw that mask is an array of booleans so mask[k] is a single value not an array so I can't make sense of the error, why would I have to use any or all if I am working with one element at a time not with an array? Any help will be appreciated

CodePudding user response：

This code will work, provided that your matrix 'e' only has integers and all integers are nonnegative and less than nn:

def SortedInteriorNodes(p,e,t):
# e is a matrix with two rows, p a matrix with 2 rows, t a matrix with 3 rows
nn=p.shape[1]  # number of nodes
nt=t.shape[1]  # number of triangles
ne=e.shape[1]  # number of edges = number of boundary nodes
nodes = np.arange(start=0, stop=nn) # 0 1 2 3 ....np-1
mask = [True] * nn
for col in range(ne):
    for row in range(2):
        k = e[row, col]
        if (mask[k] == True):
            mask[e[row,col]]= False
# I end up with a vector of boolean where the true indicate the position of Interior Nodes
InteriorNodes = nodes[mask]
return InteriorNodes

I changed "mask = np.full((1, nn), True)" to "mask = [True] * nn". The reason for the error is that you gave the mask variable two dimensions, (1 row with nn columns) rather than 1 dimension. In other words, if nn is 3, then you should have made a mask = [True, True, True] and instead you made it [[True, True, True]]. So when you ask if mask[0] == True you are asking it if the first element of mask is equal to True. If mask = [True, True, True] like I made it, the first element is True, which is what you wanted. But if mask = [[True, True, True]] like you made it, then the first element is the first row: [True, True, True], which contains three terms. This confuses your program, because the == operator compares two elements, and you just tried to compare a list of THREE elements to True, which is not allowed.