Parallax BASIC Stamp 1, Руководство по программированию

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BASIC Stamp Architecture – Constants and Expressions
Page 58
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BASIC Stamp Programming Manual 2.0b
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www.parallaxinc.com
help, but you might accidentally change some 3s that weren’t numbers of
cheers, too. However, if you made smart use of a named constant, all you
would have to do is change 3 to 5 in one place, the constant's declaration:
SYMBOL Cheers = 5 ‘ Number of cheers.
-- or --
Cheers CON 5 ‘ Number of cheers.
Now, assuming that you used the constant cheers wherever your
program needed ‘the number of cheers,’ your upgrade would be done.
On the BS2, BS2e, BS2sx and BS2p, you can take this idea a step further by
defining constants with expressions; groups of math and/or logic
operations that the editor software solves (evaluates) at compile time (the
time right after you start the download and before the BASIC Stamp starts
running your program). For example, suppose the “Cheers” program also
controls a pump to fill glasses with champagne. Perhaps the number of
glasses to fill is always twice the number of cheers, minus 1 (another
constant):
Cheers CON 5 ‘ # of cheers.
Glasses CON Cheers*2-1 ‘ # of glasses.
As you can see, one constant can be defined in terms of another. That is,
the number glasses depends on the number cheers.
The expressions used to define constants must be kept fairly simple. The
editor software solves them from left to right, and doesn’t allow you to use
parentheses to change the order of evaluation. The operators that are
allowed in constant expressions are shown in Table 4.4.
Operator Symbol Description
+ Add
- Subtract
* Multiply
/ Divide
<< Shift Left
>> Shift Right
& Logical AND
| Logical OR
^ Logical XOR
1
2
e
2
sx
2
p
2
2
e
2
sx
2
p
2
Table 4.4: BS2, BS2e, BS2sx and
BS2p operators allowed in constant
expressions.
2
e
2
sx
2
p
2