RANK ERROR
A RANK ERROR is an error message which indicates that an array had an incorrect rank for the way it was used. For example, in most APLs, Iota only accepts a vector or scalar, so giving it a higher-rank array results in a RANK ERROR: <source=apl>
⍳2 3⍴⍳6
RANK ERROR
⍳2 3⍴⍳6
∧
</source>
A RANK ERROR can be caused when arguments do not conform because they have differing ranks: <source=apl>
(2 3⍴2) + ⍳6
RANK ERROR: Mismatched left and right argument ranks
(2 3⍴2)+⍳6
∧
</source>
It may also be caused when an array's rank is too small for a function. Windowed Reduction is not defined on scalars: <source=apl>
2 +/ 0.5
RANK ERROR
2+/0.5
∧
</source>
A RANK ERROR may be caused when argument ranks are incompatible in some other way. In languages with high-rank set functions such as Dyalog APL and J, the right argument rank must be at least the left argument rank minus one. Calling it with a matrix left argument and a scalar right argument, which has a rank two smaller, gives a RANK ERROR: <source=apl>
(⍳3 4) ⍳ ⊂1 3
RANK ERROR
(⍳3 4)⍳⊂1 3
∧
</source>