Texas Instruments TI89 Developer Guide - Page 209

Chapter 15: Expressions and The Expression Stack 167 15.2.5. 15.2.6. Strings are represented as a STR_DATA_TAG on top of the string data, which is delimited by a null character (0) on both ends. Thus, the string "hello" is represented as 0 h e l l o 0 STR_DATA_TAG. Since zero is also the value of VAR_TAG, this representation looks much like a variable name. However, variable names have a maximum length of eight characters and can contain only valid name characters. Strings are not limited in length and can contain any character except a null (0). One-argument Tags Tokenized Polish representation makes no distinction between functional expressions and operator expressions. Both are represented as an identifying tag value on top of its argument(s). Many built-in functions require exactly one argument, for example, sin(x), ln(x), abs(x), etc. A few of the operators also operate on exactly one operand, for example, Lx, x!, x%, etc. Expression Lx n! 20% sin(x) ln(abc) abs(ln(x)) Representation X_VAR_TAG CHS_TAG (change sign) N_VAR_TAG FACTORIAL_TAG 20 1 NONNEGATIVE_INTEGER_TAG PERCENT_TAG X_VAR_TAG SIN_TAG VAR_TAG a b c VAR_TAG LN_TAG X_VAR_TAG LN_TAG ABS_TAG Table 15.6: Examples of Single Argument Functions and Operators Two-argument Tags Many built-in functions require exactly two arguments, for example, zeros(ln(x),x), mod(a, b), nCr(m, n). Functions of two arguments are represented as the corresponding function tag on top of the first argument on top of the second argument. Expression zeros(ln(x),x) mod(a, b) nCr(m, n) Representation X_VAR_TAG X_VAR_TAG LN_TAG ZEROS_TAG B_VAR_TAG A_VAR_TAG MOD_TAG N_VAR_TAG M_VAR_TAG COMB_TAG (combinations) Table 15.7: Examples of Functions of Two Arguments TI-89 / TI-92 Plus Developer Guide Not for Distribution Beta Version January 26, 2001