Find the shortest path fast

Question

I want to make the shortest path between many points.

I generate an 8x8 matrix, with random values like:

[[ 0 31 33  0 43 10  0  0]
 [31  0 30  0  0 13  0  0]
 [33 30  0 11 12  5  6  0]
 [ 0  0 11  0 15  0 38 11]
 [43  0 12 15  0 39  0  0]
 [10 13  5  0 39  0  3 49]
 [ 0  0  6 38  0  3  0 35]
 [ 0  0  0 11  0 49 35  0]]

Now I want to take the first list and see which is the smaller number. The see where it is in the list and take its position. Next I clear the first list to forget the first point. And put the next position in a new list of path. Then it will do the same for the new point. And at the final when all points are in my list of path it shows me the shortest way.

indm=0
lenm=[]
prochain=max(matrixF[indm])
chemin=[]
long=len(chemin)

while long != n:
    for i in range (n):
        if matrixF[indm,i] <= prochain and matrixF[indm,i]!=0:
            pluspetit=matrixF[indm,i]   
            
    prochainpoint=np.where(matrixF == pluspetit)
       
    chemin.append(prochainpoint)
    
    indm=prochainpoint
    
    for i in range (n):
        matrixF[indm,i]=0
    long=len(chemin)   
    
print(chemin)
print(matrixF)

But I got this error:

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

Does [this](https://stackoverflow.com/questions/10062954/valueerror-the-truth-value-of-an-array-with-more-than-one-element-is-ambiguous) answer your question? — kenntnisse, May 11 '22 at 15:15
@kenntnisse: That's useful background, but I doubt it answers their question; they've got code that inadvertently shifts from using scalars to using arrays partway through without them realizing it. I doubt they wanted to have arrays in the place the error occurs in any event. — ShadowRanger, May 11 '22 at 15:20
Oh I thought it was related because of the `and` instead of `&` and the same error. — kenntnisse, May 11 '22 at 15:26

score 1 · Answer 1 · answered May 11 '22 at 15:19

This line computes all the indices where matrixF == pluspetit:

prochainpoint=np.where(matrixF == pluspetit)

Problem is, there's more than one, so on your first pass through, prochainpoint ends up as (array([0, 5]), array([5, 0])). You then set indm to prochainpoint, so on your next pass, instead of getting a single value with matrixF[indm,i], you retrieve a 2x2 array of (repeated) values, as if you'd done:

np.array([[matrixF[0,1], matrixF[5,1]]
          [matrixF[5,1], matrixF[0,1]])

Comparing this to prochain (still a scalar value) produces a 2x2 array of boolean results, which you then try to test for truthiness, but numpy doesn't want to guess at whether you mean "are they all true?" or "are any of them true?", so it dumps that decision back on you with the error message.

I'm assuming the problem is with prochainpoint=np.where(matrixF == pluspetit), where you get many results when you presumably only want one, but I'm not clear what the real intent of the line is, so you'll have to figure out what you really intended to do there and replace it with something that consistently computes a single value.

Find the shortest path fast

1 Answers1