New Intrinsic Function PUTI()

Since their introduction way back in Release 3, the intrinsic functions put() and get() have rendered possible the implementation of many special tasks, that would otherwise not have been possible to implement. Introduced first as a simple means to avoid the recomputation of repeated terms in a single expression, with time their use has been extended to also allow storing and transporting of information from one expression to the next. Typical applications of the latter type are random number generation (see e.g. EMME2/NEWS 9, page 8) or the implementation of multiple linear regressions using the calculator modules (see e.g. macros NETREG.MAC and MATREG.MAC on the SHARE diskette).

The basic operation of the put() and get() intrinsics is quite simple: each call to put(X) will store the value X in the next word of a special stack, starting with stack index 1, each time an expression is evaluated, whereas a call to get(I) will return the I-th stack value. With this sequential way of writing to the stack, a small stack size of 20 is plenty, since the used stack size is limited to the maximum number of put() calls that can occur in a single expression.

In Release 8, the put() and get() are complemented by the new intrinsic function puti(), which allows defining explicitly a stack index for the following put() operations. Along with this new possibility of direct addressing when writing to the stack, the size of the stack is also increased from 20 to 250.

These enhancements open many possibilities for novel applications, such as:

• Lookup tables with a length of up to 250 words can now be implemented directly. The new function puti() provides means of loading the lookup table values directly.
• Various kind of histograms can be computed efficiently using expressions of the type put(get(puti(ClassIndex))+Weight).
• The number of independent variables in macros implementing multiple linear regressions is no longer limited to 3 or 4.
• The occurrence of various special or abnormal conditions during the evaluation of expressions (e.g. during the function evaluations of an assignment) can be detected and counted.

The usefulness of the put()/get() stack is even further increased by the fact that its contents can now also be accessed directly from macros.

The link speed histogram shown on the plot below has been computed in module 2.41 with the expression put(get(puti(@speed))+1) for all different volume-delay functions (which appear on the plot in different colors) and then displayed in module 2.43: