C3_W2 lab1 Why the error?

Course Q&A Machine Learning Specialization Unsupervised Learning, Recommenders, Reinforcement
1

typeError: unsupported format string passed to numpy.ndarray.format

I passed the test by checking the hint reference. The error comes from b_ = b[:,j] when I need to pick b value for each user’s cost function. Instead, I tried b_ = [0,j] and then it worked. I hope to know why my original code function incorrectly.

2

b_ = b[:,j]
This one puts all the values of b i.e in the jth column to b_ variable.
b_ = b[0,j]
this one puts the value [0,j] in the b_ variable. this will give a single value
b_ = [0,j]
this one creates a list containing 0,j