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.

“Difference” is a scalar.

Where you read that?