HP 15c Manual

 Section 8: Program Branching and Controls 91 
 
of this loop can be  controlled by a  conditional  branch, an ¦ instruction 
(written  into  the  loop),  or  simply  by  pressing  any  key  during  execution 
(which stops the program). 
 
Conditional Tests 
Another way to alter the  sequence of program execution is by a conditional 
test, a  true/false  test  which  compares  the  number  in  the  X-register  either  to 
zero  or  to  the  number  in  the  Y-register.  The  HP-15C  provides  12...

92 Section 8: Program Branching and Controls 
 
Following  a  conditional  test,  program  execution  follows  the  Do  if  True 
Rule:  it  proceeds sequentially  if  the  condition  is true,  and  it skips  one 
instruction if the condition is false. A t instruction is often placed right 
after  a  conditional  test,  making  it  a conditional  branch; that  is,  the t 
branch is executed only if the test condition is met. 
 
Flags 
Another  conditional  test  for  programming  is  a flag test.  A...

 Section 8: Program Branching and Controls 93 
 
Examples 
Example: Branching and Looping 
A  radiobiology  lab  wants  to  predict  the 
diminishing  radioactivity  of  a  test  amount  of 131I,  a  radioisotope.  Write  a  program  to  figure 
the  radioactivity  at  3-day  intervals  until  a 
given limit is reached. The formula for Nt, the 
amount of radioisotope remaining after t days, 
is 
Nt = No (2-t/k), 
where k = 8 days, the half-life of 131I, and N0 is the initial amount. 
The  following...

94 Section 8: Program Branching and Controls 
 
 
Keystrokes Display  
l * 1 010-45,20, 1 Recall multiplication with the contents of R1 (N0), yielding Nt, 
the mci of 131I remaining after t days 
´© 011-   42 31 Pauses to display Nt. 
l 2 012-   45  2 Recalls limit value to X-register. 
| T 9 013-43,30, 9 x ≥ y ? Tests whether limit value 
(in X) meets or exceeds Nt  (in Y). 
| n 014-   43 32 If so, program ends. 
3 015-       3 If not, program continues. 
O+ 0 016-44,40, 0 Adds 3 days to t in R0. 
tA...

 Section 8: Program Branching and Controls 95 
 
Example: Flags 
Calculations  on  debts  or investments  can  be  calculated  in  two  ways:  for 
payments  made  in  advance  (at  the  beginning  of  a  given  period)  and  for 
payments  made  in  arrears  (at  the  end  of  a  given  period).  If  you  write  a 
program  to  calculate  the  value  (or  ―present  value‖)  of  a  debt  or  investment 
with  periodic  interest  and  periodic  payments,  you  can  use  a  flag  as  a  status 
indicator...

96 Section 8: Program Branching and Controls 
 
 
Keystrokes Display  
| ¥ 000- Program mode. 
´ bB 001-42,21,12 Start at B if payments to be 
made at the beginning. 
|  0 002-43, 5, 0 Flag 0 clear (false); indicates advance payments. 
t 1 003-   22  1 Go to main routine. 
´ b E 004-42,21,15 Start at E if payments to be 
made at the end. 
| F 0 005-43, 4, 0 Flag 0 set (true); indicates 
payment in arrears. 
´ b 1 006-42,21, 1 Routine 1 (main routine). 
O1 007-   44  1 Stores i (from X-register). 
1 008-...

 Section 8: Program Branching and Controls 97 
 
Now  run  the  program  to  find  the  total  amount  needed  in  an  account  from 
which  you  want  to  take  $250/month  for  48  months.  Enter  the  periodic 
interest  rate  as  a  decimal  fraction,  that  is,  0.005  per  month.  First  find  the 
sum  needed  if  payments  will  be  made  at  the  beginning  of  the  month 
(payments  in  advance),  then  calculate  the  sum  needed  if  payments  will  be 
made at the end of the month (in...

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...

 Section 8: Program Branching and Controls 99 
 
In this way, a program can accommodate two different modes of input, such 
as  degrees  and  radians,  and  make  the  correct  calculation  for  the  mode 
chosen.  You  set  a  flag  if  a  conversion  needs  to  be  made,  for  instance,  and 
clear it if no conversion is needed. 
Suppose you had an equation requiring temperature input in degrees Kelvin, 
although sometimes your data might be in degrees Celsius. You could use a 
program with a flag to...

100 Section 8: Program Branching and Controls 
 
Flag  9. An  overflow  condition  (described  on  page  61)  automatically  sets 
flag  9.  Flag  9  causes  the  display  to  blink  or,  if  a  program  is  running,  waits 
until execution is complete and then starts blinking the display. 
Flag 9 may be cleared in three ways: 
 Press |  9 (the common procedure for clearing flags). 
 Press −. This will only clear flag 9 and stop the blinking—it will 
not clear the display. 
 Turn the calculator off....