Add explainer for arrays of unequal rank

data-apis · rgommers · Aug 11, 2020 · Jul 30, 2020 · Jul 30, 2020 · Jul 30, 2020
commit be2a6367ffe845c48d05efe93b15b22f9e3ff56f
diff --git a/spec/API_specification/broadcasting.md b/spec/API_specification/broadcasting.md
@@ -14,6 +14,8 @@ Broadcasting facilitates user ergonomics by encouraging users to avoid unnecessa
 
 Given an element-wise operation involving two compatible arrays, an array having a singleton dimension (i.e., a dimension whose size is one) is broadcast (i.e., virtually repeated) across an array having a corresponding non-singleton dimension.
 
+If two arrays are of unequal rank, the array having a lower rank is promoted to a higher rank by (virtually) prepending singleton dimensions until the number of dimensions matches that of the array having a higher rank.
+
 The results of the element-wise operation must be stored in an array having a shape determined by the following algorithm.
 
 1.  Let `A` and `B` both be arrays.

diff --git a/spec/purpose_and_scope.md b/spec/purpose_and_scope.md
@@ -62,10 +62,18 @@ two arrays whose dimensions are compatible (i.e., where the size of each dimensi
 
 an operation performed element-by-element, in which individual array elements are considered in isolation and independently of other elements within the same array.
 
+### rank
+
+number of array dimensions (not to be confused with the number of linearly independent columns of a matrix).
+
 ### shape
 
 a tuple of `N` non-negative integers that specify the sizes of each dimension and where `N` corresponds to the number of dimensions.
 
+### singleton dimension
+
+a dimension whose size is one.
+
 * * *
 
 ## Normative References