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 as argument results in a RANK ERROR:

A RANK ERROR can be caused when arguments do not conform because they have differing ranks:

It may also be caused when an array's rank is too small for a function. Windowed Reduction is not defined on scalars:

A RANK ERROR may be caused when argument ranks are incompatible in some other way. In languages with major cell search such as Dyalog APL and J, the right argument rank of Index Of 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: