View source for CumProduct

[[Category:Transforming functions]]
[[Category:Doc Status C]] <!-- For Lumina use, do not change -->

== CumProduct(X, I) ==
Returns an array with each element being the product of all of the elements of «X» along dimension «I» up to, and including, the corresponding element of «X».

[[Syntax]]:
:[[CumProduct]](X: Array[I]; I: Index, passNull: optional Boolean, reset: optional Array)

== Examples ==
:<code>I := </code>
::<code>[1, 2, 3, 4, 5, 6, 7]</code>
:<code>X := </code>
::<code>[[Array]](I, [8, 2, 1, 5, -3, 0, 4])</code>
:<code>CumProduct(X, I) &rarr;</code> 
::<code>[8, 16, 16, 80, -240, 0, 0]</code>    { indexed by <code>I</code>}

== Library ==
Array functions

== Optional Parameters ==
=== PassNull ===
«PassNull» is an optional boolean parameter that defaults to <code>False</code>. When it is omitted or explicitly set to <code>False</code>, [[CumProduct]] ignores [[Null]] values.  In that case they have essentially the same effect as a one, unless they happen to be the first value in «X», in which case they are passed since no numeric values are yet obtained.

When «passNull» is explicitly set to true, then [[Null]] values are passed through as [[Null]] in the result.

:<code>X := </code>
::<code>[«null», «null», 4, 2, «null», «null», 2, -1, 3, 2, «null»]</code>
:<code>CumProduct(X, I) &rarr;</code>
::<code>[«null», «null», 4, 8, 8, 8, 16, -16, -48, -96, -96]</code>
:<code>CumProduct(X, I, passNull: false) &rarr;</code>
::<code>[«null», «null», 4, 8, 8, 8,16, -16, -48, -96, -96]</code>
:<code>CumProduct(X, I, passNull: true) &rarr;</code>
::<code>[«null», «null», 4, 8, «null», «null», 16, -16, -48, -96, «null»]</code>

=== Reset ===
The optional «reset» parameter accepts an array of boolean values indexed by «I».  At the positions where «reset» is true, [[CumProduct]] starts over.  This sets the sum of all previous values to zero, so that the value at that position will be the same as the value in «X».

:<code>I := </code>
::<code>[1, 2, 3, 4, 5, 6, 7]</code>
:<code>X := </code>
::<code>[8, 2, 1, 5, -3, 7, 5]</code>
:<code>R := </code>
::<code> [0, 0, 1, 0, 0, 1, 0]</code>
:<code>CumProduct(X, I, reset: R) &rarr;</code> 
::<code>[8, 16, 1, 5, -15, 7, 35]</code>

«Reset» can be used to restart the cumulation each time some state change occurs.  In such a scenario, the «reset» parameter is set to <code>True</code> at the first instant (along «I») that the system is in the new state.   

Suppose <code>IntRate</code> is the monthly interest rate, indexed by <code>Month</code>.  We'll assume the elements of <code>Month</code> are numeric dates corresponding to the first of each month.  The following would compute the interest accrued up through the end of each month so far in the current year:
:<code>CumProduct((1 + IntRate), Month, Reset: DatePart(Month,"M") = 1 ) - 1</code>

== See Also ==
* [[Product]]
* [[Cumulate]]
* [[CumMin and CumMax]]