function to retain another variable at time T-1

[thrillz84]

Hi there,

Please is there any technique or function I can use to hold the value of a variable such that variable "A" at time T or T+1 will have the value of a variable "B" at that "T" or "T-1" respectively. Many thanks

Olu

[Administrator]

Take a look at SAMPLE IF TRUE or DELAY FIXED. One of these should do what you need.

Tony.

[thrillz84]

perhaps mixed it up..I need a function that can perform the opposite of the delay function i.e a function that will let a variable "A" at time T to hve a value of a variable "B" t time T+1

[tomfid]

Vensim (and reality) is causal - that is, no variable can use information about the future.

Can you say more about the reason for this approach?

[thrillz84]

cheers Tom, in the attache model for example, I need a function that will let variable "y" have values of 0.94, 0.909651...at times t= 1 and 2 ....theoretically i know it isnt right but I need it or some analysis...tnx

[Administrator]

There is no function that can do that. You can try using FORECAST or TREND. Otherwise, you would need to calculate the model a time step in the future.

[tomfid]

There is a trick you could do though - cumbersome in general, but feasible in a simple model. Create an auxiliary that calculates the value of the stocks ahead a unit of time (assuming Euler integration):

future bad code = bad code + roc*TIME STEP

Then use can use the "future" stocks to calculate other future variables.

[thrillz84]

cheers Tom!!!should have thought of that

[tomfid]

The other possibility is to just put minus signs in all your integrations, and run the whole model backwards.

[thrillz84]

splendid!!