Get value from CDT array

Question

I'm facing an issue. 
 I'm calling an API that returns this response: 
 Engine: Dictionary
 Fuel: Dictionary
 Code: "100003"
 Name: "D"
 NameEx: "Diesel"
 Layout: Dictionary
 Name: "l"
 Power: List of Dictionary: 2 items
 Dictionary
 Value: "88.00"
 Unit: "KW"
 Dictionary
 Value: "120.00"
 Unit: "HP"
 Displacement: Dictionary
 Value: "1560.00"
 Unit: "ccm"
 CylindersNumber: "4"
 ValvesPerCylinder: "2" 
 I need to get the Power Value when Unit is equal to "HP". 
 
 I created this expression rule (and many others): 
 a!forEach(
 ri!Power,
 if(
 ri!Power[fv!index].Unit = "HP",
 ri!Power[fv!index].Value,
 {}
 )
) 
 But when I use it inside a process I get this error: 
 An error occurred while evaluating expression: power:rule!SBX_GetVehiclePowerValue(pv!response.eurotaxIdentificationResponse.VehicleList.Vehicle.Engine.Power) (Expression evaluation error in rule 'sbx_getvehiclepowervalue' at function a!forEach: Cannot index "Power" because it is an array type (List of SBX_Unit_Value_Type). Only fields with scalar types can be indexed from an array.) (Data Outputs) 
 
 When I test my expression rule I use this data and it works: 
 {
 {
 "Value": "88.00",
 "Unit": "KW"
 },
 {
 "Value": "120.00",
 "Unit": "HP"
 }
} 
 
 Any lead to resolve this issue ? Thanks ! 
 
 UPDATE: 
 Well, it was a problem from the response I get. I had to create a rule to retrieve data from vehicle list 
 =load(
 a!forEach(
 items: ri!vehiculeList.Vehicle,
 expression: 
 fv!item.Engine.Power
 
 )
) 
 Thanks for your help !

Dave Lewis · Accepted Answer

Looking at your error, it appears to be farther upstream. It looks like you haven't included the entirety of your response. If your response has a Vehicle list, and each in a list of Vehicles has an Engine with a list of Power, you've got nested arrays. You can't index nested arrays all at once. If you had 10 Vehicles, you'd be trying to index 10 sets of 2 Powers. It can do a set of 2, or 10 things, but it can't do 10 sets of 2 all at once. 
 The solution is index all the vehicles one at a time in a larger parent rule with a!forEach. Call a helper rule on each vehicle. To that rule's perspective, you have only 1 Vehicle (10 times it thinks you only have 1 Vehicle), so you can pull the array of Powers out of just one vehicle. Then you can get the one "HP" based value and have the helper rule return that, which will return one at a time in the larger a!forEach, resulting in a list of HP values.

Dave Lewis · Answer

The error states there's a vehicleList. The error explicitly states you're not able to index an array member from an array of CDT. You have to have 2 arrays at minimum in order to generate the error. 
 Perhaps it would be best to look at both how you call it in the rule and how you call it in the process model. 
 It might be something as simple as accidentally hitting the check box that says a PV is multiple. Even if it is only 1 item in that case, it's a 1 length list and won't let you index.

Get value from CDT array

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