98 Section 8: Program Branching and Controls
Looping
Looping is an application of branching which uses a t instruction to
repeat a portion of the program. A loop can continue indefinitely, or may
be conditional. A loop is frequently used to repeat a calculation with
different variables. At the same time, a counter, which increments with
each loop, may be included to keep track of loop iterations. This counter
can then be checked with a conditional test to determine when to exit the
loop. (This is shown in the example on page 112.)
Conditional Branching
There are two general applications for conditional branching. One is to
control loops, as explained above. A conditional test can check for either
a certain calculated value or a certain loop count.
The other major use is to test for options and pursue one. For example, if
a salesperson made a variable commission depending on the amount of
sale, you could write a program which takes the amount of sale, compares
it to a test value, and then calculates a specific commission depending on
whether the sale is less than or greater than the test value.
Tests. A conditional test takes what is in the X-register (“x”) and
compares it either to zero (such as ~) or to “y”, that is, what is in the
Y-register (such as £). For an x:y comparison, therefore, you must
have the x- and y-values juxtaposed in the X- and Y-registers. This might
require that you store a test value and then recall it (bringing it into the X-
register). Or, the value might be in the stack and be moved, as necessary,
using ®, ), or (.
Tests With Complex Numbers and Matrix Descriptors. Four of the
conditional tests also work with complex numbers and matrix descriptors:
~, T 0 (x ≠ 0), T 5 (x = y), and T 6 (x ≠ y). Refer to
Sections 11 and 12 for more information.
Flags
As a conditional test can be used to pick an option by comparing
two numbers in a program, a flag can be used to pick an option externally.
Usually, a flag is set or cleared first thing in a program by choosing a
different starting point (using different labels) depending on the
condition or mode you want (refer to the example on page 95).
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