sum(X) on a matrix with one row

Fortran fixes rank at declaration. sum(a, dim=1) on a rank-2 array is rank 1 whatever the extents, one row included. The bug here is choosing the dimension from the extents. Pass the dimension every time, in the helpers too.

max has a second output that makes this worse. On a matrix, [m, i] = max(X) gives, for each column, the row where the maximum sits. On a one-row X it gives a single maximum and i becomes a column index. Same variable, different meaning, and if the next line uses i to pick an observation it now picks a feature. [m, i] = max(X, [], 1) keeps i a row index, so for one row it's a vector of ones, one per column.

Julia made the opposite call. sum(X; dims=1) always returns a one-row matrix, even when X has one row, so the shape survives at the price of a vec call when you wanted a plain vector.

So the shape of the result is decided by whatever data the caller sends. Lovely, a function whose return type depends on luck. Fair enough, though, sum(X, 1) is three more characters and the docs say plainly which dimension gets picked.

Right about max, and the same function had a different problem in it. By analogy with sum(X, 1) it said max(X, 1). That's not a dimension. That's the element-wise maximum with 1, so everything below one got pulled up to 1. No error, no warning, same shape. The right call is max(X, [], 1), with the empty placeholder for the second argument. A search through the code for max( and min( with two arguments found three places. Two were real element-wise comparisons and one was the same mistake.

Sounds like one small test would have caught both. An input with one row and values below one, then a check on the shape and the values of the result. max(X, 1) shows up on it right away, and so does sum(X). An input with zero rows could go in too, once you decide what the function should return for it. For a start the one-row case probably covers both of the bugs found so far.