WhatIf

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 WhatIf(expr, v, x, i, j, k), 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)

Preservation of Computed Values

Whatif and WhatIfAll restores the value of «V», and of all values downstream of «V» that had been computed before beforehand. This saves computation time is saved if the results are required later, Monte Carlo samples are preserved so that the specific sample points are retained, and two related WhatIf's (both descending from the same varying variable) don't invalidate each other.

There is one downside to preserving previously computed values -- increased memory consumption. In some cases, WhatIf is used on models that consume nearly all available memory. Without dropping previously computed values, the memory required to remember the previous values is not recovered during the WhatIf evaluation. If you find yourself this close to exhausting available memory resources and need to drop previously computed values when evaluating WhatIf or WhatIfAll, then the optional «preserve» parameter can be specified as false, e.g.:

WhatIf(expr, v, x, preserve: false)

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 x1 to 10 while simultaneously setting x2 to 20 while evaluating y, use:

WhatIf( WhatIf( y, x1, 10 ), x2, 20 )

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
• Tornado Charts webinar, recorded at: Tornado-Charts.wmv