Reminders/notes to help recall variable meanings in equations



#13

Several equations I want to store in my 35s contain, when written in books, variables using the same letter with different subscripts. (See the 3rd post here for an example of what I mean: http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/forum.cgi?read=211648#211648 ) Of course the 35s doesn't support subscripts so a letter can only be used once when storing an equation. This means other (non-meaningful) letters must be used in the stored version of the equation. I can see this being a problem after not using one for a while (I'll forget what values go where in the equation.)

I do know that setting flag 10 in a program allows prompting for variables using strings of descriptive characters. This works very well when using the equation as it is programmed, and would seem to avoid the problem described above. However, when a variable within an equation is needed to be solved for, SOLVE won't work for equations in programs unless the program uses the INPUT command to receive variables - and the INPUT command prompts for variable entry using single letters only.

So, despite the usefulness of flag 10, programs that don't use INPUT can't be used when I'll need to solve for different variables within equations. When this ability is necessary I'll have to use the equation list, and the original problem in the first paragraph comes back.

Is there a solution to this dilemma (other than writing a program for every way an equation can be solved) ?

Or if not, is there an intuitive way to save notes somewhere to help remind me which letters represent which variables for equations in the equation list?

Edit: I'm using this calculator because it's the best option on NCEES's approved calc list, so switching to another calculator isn't an option.

Edited: 14 Feb 2012, 9:44 p.m. after one or more responses were posted


#14

Quote:
... is there an intuitive way to save notes somewhere to help remind me which letters represent which variables for equations in the equation list?

Perhaps I misunderstand your question? You are putting the (probably easy?) equations into your calculator so you don't have to remember them, and then making notes so you don't have to remember the variables in the equations you don't remember?

At some point you'll need to remember something, right?

May I make two recommendations:
1) Rather than spend time typing formulas into calculators, spend the same amount of time studying the physical units behind the variables and how to derive them from first principles, then you won't need to a mnemonic to decipher equations you can't remember. (e.g. learning -vs- memorizing -vs- using technology as a crutch)

2) You might consider an upgrade to a HP 48/49/50G and write a SOLVER routine. You can capture notes along with the benefits of making a nice GUI. It takes longer to write, but in the process you'll remember it better and have a cool navigation function. The TVM SOLVER is a good example. Or Page 4 of this HP guide


#15

I can assure you I understand the material and the equations. I think you may not understand the reason for my question. Thanks anyway.

#16

Quote:
Or if not, is there an intuitive way to save notes somewhere to help remind me which letters represent which variables for equations in the equation list?

You may enter the original equation just above the equation to solve the problem:

NC=(H1-H2A)/(H1-H2S)
N=(A-B)/(A-C)

Kind regards

Thomas


#17

That's exactly what I was going to say. It is a "dummy equation". Heck, I even put a phone number in mine once, when I didn't have a piece of paper and a pencil (how crazy!).

#18

This seems good. It's probably just about as good of a solution as there is. Thank you very much.

#19

During the execution of a program, you can display a reminder prior to INPUT.

This listing displays the program name CROSS PRODUCT and then prompts for the input of VECTOR 1.

    A001 LBL A
    A002 SF 10
    A003 CROSS PRODUCT
    A004 PSE
    A005 VECTOR 1
    A006 PSE
    A007 CF 10
    A008 INPUT I
    A009 INPUT J
    A010 INPUT K
    .
    .
    .

You can also use a dummy equation in the Equation List as a reminder.

Example:

If you are taking the FE, I'd reccomend sticking to the more common equations used for the discipline specific portion of the exam. (e.g. If you are taking the Civil specific, I'd go with Manning's eqn, Hazen-Williams, Darcy-Weisbach, nominal moment capacity for concrete beams, depth of compressive stress block for concrete, etc.)

Also, knowing how to use the STAT, nCr, and nPr functions will save you a lot of time during the morning session.


#20

Using dummy equations as labels above equations is a good idea. I'll definitely do that. Thanks! And I am planning on taking the FE, so thanks for the heads-up on using the Stat, nCr, and nPr. I'll have to look into them.

I believe, though, that using reminder text before INPUT statements in programs doesn't help when using SOLVE on equations in programs (which is why I was looking for ideas on reminders for equation variables in the equation list). Won't the SOLVE function prompt for the INPUT statement variables without showing the reminder lines first? Or am I wrong?

Edited: 15 Feb 2012, 5:33 p.m.


#21

Quote:
Won't the SOLVE function prompt for the INPUT statement variables without showing the reminder lines first? Or am I wrong?

In a program, SOLVE does not prompt for inputs unless Flag 11 is set. However, Flag 11 is default clear, and also cleared after every SOLVE operation. See chapter 15 of the manual.


Thus while Flag 11 is clear, you have to write the prompts for inputs into your program (see the example for the Gas Law equation on p15-3).

Regards,
Bart
#22

Here you go!


This program uses SOLVE without requiring the use of an extra label, the only caveat is that you must start the program at line 004, i.e. with "XEQ A004".

Notes:

  • This one skips input for the variable required to solve, but stores whatever is in the X-register at the time into the variable's register. It's OK in a simple equation as this, but for other situations you may wish to enter initial guesses (see manual p7-8).
  • Indirect registers -1 to -26 refer to storage registers A to Z. Thus SOLVE(-1) = SOLVE A etc., the result being stored back into that register.
  • The program does not end in a return, but a stop and a goto. This allows easy re-run of the program by pressing R/S.
  • For "x<>y?", the "<>" is the "not equal" sign (equal with a strike through it) on the calculator.


Program listing:
LABEL  STEP                    COMMENT
A001 LBL A These first three lines
A002 A=(B-C)/(B-D) define the function for SOLVE
A003 RTN
A004 SF 10 the program actually starts here
A005 TEST EQUATION information text briefly displayed with pause
A006 PSE
A007 NC=(H1-H2A)/(H1-H2S)
A008 PSE
A009 CHOOSE 1 TO 4
A010 PSE
A011 TO SOLVE FOR
A012 PSE
A013 1NC 2H1 3H2A 4H2S waits for input (use right arrow to scroll for more text)
A014 +/-
A015 STO I
A016 -1
A017 x<>y? this tests if it is the variable to solve for
A018 ENTER NC and will skip if it is
A019 STO A
A020 RCL I
A021 -2
A022 x<>y?
A023 ENTER H1
A024 STO B
A025 RCL I
A026 -3
A027 x<>y?
A028 ENTER H2A
A029 STO C
A030 RCL I
A031 -4
A032 x<>y?
A033 ENTER H2S
A034 STO D
A035 CF 10
A036 FN= A calls the function in label A (which returns after A003)
A037 SOLVE(I) see second bullet point above
A038 CLSTK just to get rid of intermediate entries before recalling result
A039 RCL(I)
A040 STOP program stops, but can continue with R/S
A041 GTO A004 restarts program


Regards,
Bart

Edited: 16 Feb 2012, 9:25 a.m.


#23

Extremely cool. Thanks again, very much, Bart! I won't be able to enter it for a few days, but it is at the top of my to-do list, and asap.

#24

Follow on:


After some experimenting, i managed to get more than 1 equation for "FN=" in a label space. It seems that this command will ignore anything that is not a valid equation. This allows for the use of conditional statements to select the required equation.


Example program:

A001   LBL A
A002 FS? 0
A003 GTO A005
A004 GTO A007
A005 A=B/(B+C)
A006 RTN
A007 A=B*(B+C)
A008 RTN
A009 CF 0
A010 SF 10
A011 USE EQN 1 OR 2
A012 1
A013 x=y?
A014 SF 0
A015 A=
A016 STO A
A017 C=
A018 STO C
A019 CF 10
A020 FN= A
A021 SOLVE B
A022 RCL B
A023 CF 0
A024 STOP
A025 GTO A009


This program would be started by "XEQ A009"

In this case I used one of the 5 user flags, but i guess other types of conditionals would work too.

Edit: Removed a stray "h" that crept in on line A009.


Edited: 17 Feb 2012, 6:39 a.m.


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