Lookup function

[rajaa1510]

Hi there,
I would like to know how I can writte an equation which include IF function and Lookup togather, I mean how can express the case (IF X =0 then Y=0, otherwise y= Graph(x1,y1), (x2,y2)...etc.)
It is simmply you can say add the value (0,0) to the graph, but, actually, this is not accceptable in my case.

Kind regards
Rajaa

[Administrator]

Something like

Code: Select all

y = IF THEN ELSE ( x =0 , 0, lookup graph(x))

[tomfid]

See also:

http://www.vensim.com/documentation/usr08.htm
http://www.vensim.com/documentation/fn_if_then_else.htm

[rajaa1510]

Hi,
Thanks for your response.
Actually, what I need to know how to write this equation in the equation editor. Because If I wrote in the current case (IF THEN ELSE ( x =0 , 0, lookup graph(x))), Vensim would ask to identify the "lookup graph" as a variable!!

Regards
Rajaa

[Administrator]

If you read through
http://www.vensim.com/documentation/ind ... ?usr08.htm

It will show you how to create a lookup (and how to use it).

[rajaa1510]

Hi,
Many thanks for your response.
I've already create the lookup graph. I have no problem in that. My problem is in how to include the lookup graph in the IF function.
For example,
effect of competition on fee= if then else (number of competitors=0, 1, Lookup graph(number of competitors))
where, effect of competition on fee= graph(2,1.33), (3, 1.2) .... (6,1)
I insert the values to get the lookup function as usual and I get the grap, but I do not know how to build IF function including this lookup function.
When I insert the equation in the equation editor as oyu told me above (y=if then else (x=0,0, lookup graph (x)), it considered "lookup graph" statemnet an indicator for an absent variable that I need to add it to my SD daigram as I recieved this message (do you want to add "lookup graph" to the model) instead of considering the "lookup graph" as a part of the IF fucntion.

I hope that I made myself clear
Many thanks for being patient

Kind regards
Rajaa

[Administrator]

Are you using PLE? If yes, just draw an arrow from your lookup to the variable where you need to use it.

[rajaa1510]

Hi,
I am using DSS. I already drew this link and I wrot the equation in the way you told me but it does not work
Please find attached a graph ilustrating the link and equation
Regards
Rajaa

Graph.docx

(121.39 KiB) Downloaded 173 times

[tomfid]

You're missing parentheses around the lookup argument. In other words, you have

IF THEN ELSE( a=b, x, lookup"y" )

and you need

IF THEN ELSE( a=b, x, lookup("y"))