In M, subscripted variables identify elements in sparse arrays. Sparse arrays comprise existing subscripts and data nodes – no space is reserved for potential data nodes. These arrays generally serve logical, rather than mathematical, purposes.
M array subscripts are expressions, and are not restricted to numeric values.
The format for an M global or local variable is:
[^]name[(expr1[,...])]
The optional leading caret symbol (^) designates a global variable.
The name specifies a particular array.
The optional expressions specify the subscripts and must be enclosed in parentheses and separated by commas (,).
The body of the M standard places no restrictions on variable names. However, the portability section of the standard does suggest limits on the length of an individual subscript expression, and on the total length of a variable name. The measurement for the length of names includes the length of the global variable name itself, the sum of the lengths of all the evaluated subscripts, and an allowance for an overhead of two (2) times the number of subscripts. The total must not exceed 237. For globals, GT.M permits this total to be modified with GDE up to 255. For locals, GT.M limits the length of individual subscripts to the maximum string length of 32,767. GT.M restricts the number of subscripts for local or global variables to 32.