Lab3, week 1, Gradient checking error

Course Q&A Deep Learning Specialization Improving Deep Neural Networks: Hyperparameter tun
1

I have a question concerning Lab3, week 1, Gradient checking
Cannot understand, why expected output “difference” should be a vector.
As follows from the formula $$ difference = \frac {| grad - gradapprox |_2}{| grad |_2 + | gradapprox |_2 }$$ it should be a scalar.
Could you please clarify?
My best regards, Vasyl.

2

“Difference” is a scalar.

Where you read that?