All calculator operations, from simple keyboard calculations to com-
plicated programs, involve evaluation. Some examples:
• When you key one or more objects into the command line and
the command line is translated into a program,
which is then evaluated.
• When you press a key on the USER menu in Immediate entry
mode, the corresponding name is evaluated.
• When you perform step-by-step differentiation, you press
evaluate the expression in level 1.
• When you use the Solver to find numerical solutions, the procedure
stored in the variable EQ is repeatedly evaluated.
It's easiest to understand calculator operations in terms of
Although the term "delaying eval-
uation" is new, the process is familiar: whenever you enter a quoted
name or an algebraic, the object's delimiters indicate that you want to
delay evaluation of the object-that you want the object to go on the
Delayed evaluation is the basis for programming on any computing
device, since otherwise a program would execute as soon as you
wrote it. The HP-28S extends the concept in a uniform way to allow
symbolic operations-you can use names and algebraics as data for
symbolic calculations. For example, you choose when, if ever, you
want to evaluate an expression. You can differentiate it, symbolically
solve it, make substitutions for variables in it, and so on. Of course,
you can also calculate its numerical value.