Texas Instruments TI89 Developer Guide - Page 220

178 Chapter 15: Expressions and The Expression Stack 15.7. Working With Lists This section describes some of the routine ways of working with lists and tails. A list is represented as a LIST_TAG on top of a tail. A tail is a sequence of expressions on top of an END_TAG. For example, the list {a, 1, ln(x)} takes the tokenized form END_TAG X_VAR_TAG LN_TAG 1 1 NONNEGATIVE_INTEGER_TAG A_VAR_TAG LIST_TAG where the END_TAG is at the lowest address and the LIST_TAG is at the highest address. The system routines that implement calculator functions understand and correctly handle lists. For example, push_ln automatically computes the natural logarithm of each element of a list. push_sum automatically adds two lists, element by element, and throws an appropriate error if the lists do not have the same number of elements. push_negate changes the sign of each element of a list, and so on. Thus, depending upon the operations involved, it is often possible to write code that does not need to check whether its input arguments are lists. The last future value function of the previous section is an example. Since each of the called system routines understands lists, the resulting future value function correctly handles lists. Sometimes new code must be written to perform some new process on lists. These new processes generally fall into two categories based on their result. Either they create a new version of the list or they do not. The functions mentioned in the previous paragraph create new lists from input lists. Here are examples that do not create new lists. is_constant determines whether every element of the list is a constant value and returns a Boolean result. push_sumlist returns an expression that represents the sum of all the elements of the list. push_dimension returns the number of elements in the list. Functions that do not create new lists generally use a loop to walk through the elements of the list. Here is a function that returns the number of elements in a list. unsigned short number_of_elements (EStackIndex i) { unsigned short count = 0; /* initialize counter */ --i; /* move index from LIST_TAG to first list element */ while (END_TAG != ESTACK(i)) /* while not at end of list */ { ++count; /* increment counter */ i = next_expression_index (i); /* step to next element */ } return count; } TI-89 / TI-92 Plus Developer Guide Not for Distribution Beta Version January 26, 2001