Death by Subscript

[geo_curious]

Hi,
As usual, I have no idea where to find coherent documentation on subscripts in the Vensim manual. I am trying to use the SUM() function for the equation shown. Pretty straightforward in reality. In Vensim it is like pulling teeth.

picture.png (2.25 KiB) Viewed 132 times

Shown is what I have input into the variable:

picture2.png (7.31 KiB) Viewed 132 times

Shown is the function editor for the ground function:

picture3.png (503.32 KiB) Viewed 132 times

The SUBSCRIPTS error reads:
1. Unspecified subscript equation for -h
2. Subscript 1 of variable -ground function- is a member of the -h- family when it should be a member of the -pressure number- family.

Any thoughts on this? Please don't bother responding "impossible to tell without looking at the model."

[tomfid]

Looking at the model may not be required, but I'm not sure an answer can be given from the material here. What you're showing seems syntactically valid, so the problem may be somewhere else.

What are the definitions of h and pressure number?

What is on the left side of the sum expression?

Is h the equivalent of n in the equation png, which presumably defines the ground function? In that case why does it not appear on the right side of the equation?

[geo_curious]

I need to counter this by asking what the SUM() function is. It does not appear that it is a large operator, upper case sigma. If it was, it would not need subscripts at all. h does nothing. It is a formality in an attempt to get SUM() to work like a sum. In the manual and in the program I cannot simply have SUM(ground function), I need to have SUM(ground function[pointless exclamation!]).

So, what does SUM() actually do as a mathematical use case?

[Administrator]

The SUM function is used over a subscript range, and you can think of subscripts as an array.

Looking at the equation you are trying to implement, there is a sum over "n". So you would define the subscript range as
n : (n1-n2). But I cannot see "n" in the equation you've entered in Vensim, so I'm not sure what "n" actually is.

[LAUJJL]

do the errors disappear if you suppress the sum function in the model, and is the ground function calculated? In that case the ground function does not look like depending on the subscript n and you should replace the sum function by ground function * elmcount'n). the answer to your question is in the model and you should show it!

[tomfid]

SUM is definitely the summation operator Sigma. The exclamation point is not useless; it is necessary in cases where you'd like to sum over a subset of the dimensions belonging to the right side.

I would write the equation in your image with something like the following:

n : n1, n2
s : <something>
amp : <something>

number[n] = INITIAL( n )
The subscript values for n will be an integer sequence starting with 1; you could also write something like,
number[n] = 1,2
to initialize to arbitrary constant values.

F[s,amp] = SUM( EXP(-z*SQRT(number[n!]*pi/stuff)*T[s,amp,n!] )