Problem 3. Find the sum of all the numbers of the input vector

If x is 3x3 (or 3x3x3) or similarly shaped vector, sum(x) doesn't automatically add up all of the numbers in the entire vector. You need a loop to go through each dimension of x to get the sum of every element.

James, a vector traditionally denotes a 1xd or dx1 matrix (with d > 1). If d = 1, we tend to call that a scalar.