The PTRARR function returns a pointer vector or array. The individual elements of the array are set to the Null Pointer.
Normally, PTRARR sets every element of the result to the null pointer. It you wish IDL to allocate heap variables for every element of the array instead, set the ALLOCATE_HEAP keyword. In this case, every element of the array will be initialized to point at an undefined heap variable.
If ALLOCATE_HEAP is not specified, PTRARR sets every element of the result to the null pointer. If NOZERO is nonzero, this initialization is not performed and PTRARR executes faster. NOZERO is ignored if ALLOCATE_HEAP is specified.