Looping through a numpy matrix

Question

Why doesn't the following code work?

import numpy

grid = numpy.matrix([[1,0,1,1],[1,1,0,0],[1,0,1,0],[0,0,0,1]])
i = 0
for line in grid:
    for block in line:
        if block == 1:
            i += 1    
print("Grid has " + str(i) + " times number 1")

I thought it's gonna first cycle through every line and then every element of the line and compare it with 1, but I get this error:

Traceback (most recent call last):
File "python", line 7, in <module>
ValueError: The truth value of an array with more than one element is 
ambiguous. Use a.any() or a.all()

If you had used numpy.array instead of numpy.matrix, you wouldn't have had this problem. Seriously, numpy.matrix isn't worth it. — user2357112
– user2357112, Commented Dec 12, 2017 at 19:31

user2357112 · Accepted Answer · 2017-12-12 20:25:59Z

1

Iterating through a numpy.matrix matrix produces 1-row numpy.matrix matrices, one for each row.

Iterating over a 1-row numpy.matrix matrix produces a 1-row numpy.matrix matrix, not the individual cells.

Don't use numpy.matrix. It's not at all worth it. Also, don't loop over NumPy objects at all if you can help it:

grid = numpy.array([[1,0,1,1],[1,1,0,0],[1,0,1,0],[0,0,0,1]])
i = grid.sum()

edited Dec 12, 2017 at 20:25

answered Dec 12, 2017 at 19:33

user2357112

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