Texas Instruments TI89 Developer Guide - Page 219

Chapter 15: Expressions and The Expression Stack 177 15.6.2. Estack Calculations Now lets modify the example to perform the calculation on the estack rather than in BCD16 variables. This extension will be necessary if the function must handle arguments other than floating-point numbers. If the arguments to a function can be floats, rationals, symbolic constants, variables, expressions, or lists of any of these, then the computations are best done on the estack. The changes use the following features. • EStackIndexes are declared to point to the arguments and temporary results. • The system function push_arg_plus_1 is used to add one to an argument. • The system function push_exponentiate is used to raise a value to a power. • The system function push_product is used to multiply two values. • The system function delete_between is used to delete temporary results. void fv (void) { EStackIndex pv, ir, np, tmp; /* argument pointers */ /* point to the present value argument */ pv = top_estack; /* point to the interest rate argument */ ir = next_expression_index (pv); /* point to the number of periods argument */ np = next_expression_index (ir); /* perform the future value calculation */ /* add 1 to the interest rate */ push_arg_plus_1 (ir); /* point to the temporary result */ tmp = top_estack; /* raise (ir + 1) to the np power */ push_exponentiate (tmp, np); /* point to the temporary result */ tmp = top_estack; /* multiply by the present value */ push_product (tmp, pv); /* now the future value is on top of the estack */ /* delete intermediate results */ delete_between (pv, tmp); } This version of the example is longer and more complicated than either of the previous versions. Thus, an obvious question is "what has been gained?" The answer is a great deal of power and flexibility. This latest version does not care about the types of the arguments. If all of the arguments are rational numbers, the result will be a rational number. If the arguments are symbolic, the result will be symbolic. If the arguments are of valid but differing types, they will be combined in an appropriate way. If the arguments are not valid for the specified calculation, an appropriate error will be reported. TI-89 / TI-92 Plus Developer Guide Not for Distribution Beta Version January 26, 2001