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[[category:Evaluation Functions]]
[[Category:Doc Status C]] <!-- For Lumina use, do not change -->
 
==Whatif(expr, v, x)==

Returns the value of expression «expr», after assigning value «x» to variable «v». 

'''Whatif''' helps you explore the effects of a change to a variable, without making it permanent. After evaluation, it returns «v» back to its original definition, and so makes no permanent change to the model. 

[[Syntax]]: 
:'''Whatif'''(expr: Expression; v: Variable; x ''; preserve : optional boolean'')

==WhatIf(expr, v, x, i)==
''(requires [[Analytica 5.0]])''

Returns the result of evaluating «expr» after altering one slice at a time of «v» along «i», setting that slice to the corresponding slice of «x» along «i». This results in «expr» being evaluated once for every element of «I», plus one addition time for the base value. Any number of indexes can be listed, for example <code>[[WhatIf]](expr, v, x, i, j, k)</code>, in which case it re-evaluates «expr» for every combination of indexes. The result will have all the listed indexes.

Use [[WhatIf]] with one or more indexes when you want to generate a tornado diagram or other local sensitivity analyses when you want to see the effect on «expr» of changing one slice at a time of the input «v», assigning the slice to the corresponding slice in «x».

==WhatifAll(expr, vars, newVals)==
Returns an array of values indexed by «vars», a  list of variables, obtained by evaluating expression «expr», after assigning to each variable in «vars» the corresponding value from «newVals». Usually, «newVals» is an array indexed by «vars». 

Use '''WhatifAll''' for generating tornado diagrams and other sensitivity analyses when you want to see the effect on «expr» of changing each variable one-at-a-time.  '''Whatif''' helps you explore the effects of a change to a variable, without making it permanent. After evaluation, it returns the variables in «vars» back to their original definition, and so makes no permanent change to the model. 

[[Syntax]]: 
:'''Whatif'''(expr: Expression; vars: Variable; newVals ''; preserve : optional Boolean'')

== Preserve Computed Values ==

By default, [[Whatif]] and [[WhatIfAll]] preserve the initial value of «V», and any variables that depend on it, and restore these values after evaluation. This avoids having to recompute «V»  if the results are required later, inclduing Monte Carlo samples (which do not need to be resampled).  Two related '''WhatIf''''s descending from the same varying variable don't invalidate each other.

There is a downside to preserving previously computed values -- it increases memory consumption.  In a large model that uses most available memory,  preserving these values during '''WhatIf''' or '''WhatIfAll''' might exhaust memory.  In this case, you can ask  '''WhatIf''' or '''WhatIfAll''' not to preserve these values by setting the optional «preserve» parameter to ''false'', e.g.:

:<code>WhatIf(expr, v, x, preserve: false)</code>

== Changing two or more input variables ==

If you need to change two or more inputs at the same time, then you should nest calls to [[WhatIf]]. For example, to set variable <code>x1</code> to 10 while simultaneously setting <code>x2</code> to 20 while evaluating <code>y</code>, use:

:<code>[[WhatIf]]( [[WhatIf]]( y, x1, 10 ), x2, 20 )</code> 

You should take note of the distinction between WhatIfAll and nested WhatIf, which both change multiple inputs. The difference is that WhatIfAll changes ''one variable at a time'', whereas nested WhatIfs cause all variables to be changed ''at the same time''.

== History ==

* Prior to Analytica 4.1, use of '''WhatIf''' or '''WhatIfAll''' caused previously computed results downstream of the varied variable(s) to be invalidated, requiring a recomputation of these results the next time they are requested.  Starting with Analytica 4.2, this behavior has changed -- '''WhatIf''' and '''WhatIfAll''' now preserve previously computed values.
* The optional repeated index parameter was introduced in [[Analytica 5.0]].

== See Also ==
* [[If-Then-Else]]
* [[DyDx]]
* [[Elasticity]]
* [[Analytica_User_Group/Past_Topics#Tornado_Charts|Tornado Charts webinar]], recorded at: [http://AnalyticaOnline.com/WebinarArchive/2008-03-20-Tornado-Charts.wmv Tornado-Charts.wmv]