Function to create Multiple "IF...ELSE" statements

[oludot5]

Hi,
I am building a model with a variable, say X, with a function to take a value of various multipliers depending on the range of its input.e.g if X input 0.8-1, X output = 1,if X input is 0.6 - 0.79, X output = 0.8...so this is more or less like a "switch" statement.Can I do this in Vensim please.I tried the "IF" function but it only takes an argument..
Thanks

[Rafael]

You should use a lookup. Look at the attached model.
Rafael

[Administrator]

There are a number of ways.

1. You can nest the IF THEN ELSE statements

Code: Select all

result = IF THEN ELSE ( a > 0 , b , IF THEN ELSE ( a > 0.5 , c , 0 ) )

I personally don't like nesting IF THEN ELSE statements, it makes it difficult to read and debug what is happening.

2. Add IF THEN ELSE statements

Code: Select all

result = 
IF THEN ELSE ( a >= 0 :AND: a < 0.5 , b , 0 )
+ IF THEN ELSE ( a >= 0.5 :AND: a < 1 , c , 0 )
+ IF THEN ELSE ( a >= 1 :AND: a < 1.5 , d , 0 )

This is my preferred method if you have to do something like this.

3. Use a lookup if b, c or d are constants.

[tomfid]

Also, for most kinds of strategic simulation, you'll be dealing with systems that are not well-described by discrete logic like IF THEN ELSE. Continuous functions will be more desirable. There's a nice article about this somewhere, but I can't think of the citation at the moment. Forrester wrote about it in Industrial Dynamics, I think.

[oludot5]

cheers guys, the lookup function does it well....

[oludot5]

hi guys,

So, i have a variable "age"as an input into the lookup variable that accepts the age variables and produces other figures depending on the age value entered. As I have completed the lookup values in the equation box, there is no place to represent "age" as the input in the equation box and vensim notifies me of an error as it is an expected input but I have not used the input "age" in the equation.

Thanks

[tomfid]

That happens in a separate equation.

First, define your lookup shape (sounds like you've already done that).

Then, use the lookup in an equation, e.g.

price of cheese = normal cheese price * cheese inventory effect on price lookup( cheese inventory/normal inventory )

[oludot5]

Hi,
Still not clear, so in the attached example, the variable "lookup" is suppose to accept input Design quality an output"average manpower to maintain test suite". The relationship between these two has been described in the lookup.Where does the "Design quality" fit into the equation box of the "lookup variable"?Are you saying I create another variable?

Thanks

[Administrator]

See the attached.

Tony.

[oludot5]

cheers Tony