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Chapter 15: Expressions and The Expression Stack
TI
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89 / TI
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92 Plus Developer Guide
Not for Distribution
Beta Version January 26, 2001
The Operating System has system variables that are reserved for special
purposes. These reserved variables are used in a variety of ways in graphing,
plotting, table generation, and statistical computations. They are numbered and
are represented as a SYSVAR_TAG on top of the corresponding system variable
number. For example, the function graphing variable xmin is represented by
SV_XMIN SYSVAR_TAG, and the statistics variable
Σ
x is represented by
SV_SIGMA_X SYSVAR_TAG.
Two special naming conventions are associated with the underscore character
‘_’. Variable names that end with an underscore are assumed to be complex
variables. Thus, the variable z is assumed to be real, but the variable z_ is
assumed to be complex. Variable names that begin with an underscore are
assumed to be unit names or the names of physical constants, which include a
unit expression. For example, the unit meter is named _m, and the unit kilogram
is named _kg. The physical constant for the speed of light is named _c and
evaluates to the unit expression 299792458.0 _m / _s.
15.2.4. Other Constants
Arbitrary real constants @1, @2, . . . , and arbitrary integer constants @n1,
@n2, . . . behave somewhat like variables and somewhat like constants. The
number following the @ symbol (1, 2, etc.) is referred to as the “suffix.” The
representation uses ARB_REAL_TAG or ARB_INT_TAG on top of one quantum
containing the identifying suffix. Thus, @25 is 25 ARB_REAL_TAG, and @n10 is
10 ARB_INT_TAG.
The system also implements the following symbolic constants.
Constant
Type Value Tag
Boolean TRUE TRUE_TAG
Boolean FALSE FALSE_TAG
Finite
π
PI_TAG
Finite
e
(base of the natural ln) E_TAG
Finite
i
(
√
(
L
1)) I_TAG
Transfinite
L
∞
MINUS_INFINITY_TAG
Transfinite
∞
PLUS_INFINITY_TAG
Transfinite
±∞
PLUS_OR_MINUS_INFINITY_TAG
Transfinite 0/0 (any real value) UNDEFINED_TAG
Table 15.5: Symbolic Constants
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