Testing sequence for certain number of occurences of specific element

Question

How can I test whether an array mfselc is composed of zeroes or all zeroes except for a single value being 1.

Thus mfselc [0 0 1 0 0 0] is acceptable

Also mfselc [0 0 0 0 0 0] is acceptable

But mfselc [0 1 1 0 0 0] is not acceptable

Answer 1

(defun acceptable-p (zeros-and-ones-array)
  "Test ZEROS-AND-ONES-ARRAY for comprising at most one 1.
All elements are either 0 or 1."
  (<= (reduce #'+ zeros-and-ones-array) 1))

reduce applies the same function cumulatively to each element of a sequence. It's a solution that should leap to mind whenever you see a problem where you want a single-thing output from a collection of some kind. So what we're doing here is summing the elements of the array (by cumulatively applying the function called +) and then testing that the resulting sum is no more than 1.

Answer 2

You can get the number of occurrences of a certain element in a sequence using seq-count. You can check if all elements are the same using seq-every-p:

(let ((seqs '([0 0 0]
              [0 0 1]
              [0 1 1])))
  (mapcar (lambda (seq)
            (unless (> (seq-count (apply-partially #'= 1) seq) 1)
              (seq-every-p #'zerop (seq-remove (apply-partially #'= 1) seq))))
          seqs))