iOS Core Data and standard deviation NSExpression crashes

Question

I'm trying to compute a standard deviation with a Core Data fetch request

This works:

let numbers = [1, 2, 3, 4, 4, 5, 9, 11]
let statsExpression = NSExpression(forFunction:"stddev:", arguments:[NSExpression(forConstantValue: numbers)])
let statsValue = statsExpression.expressionValue(with: nil, context: nil) as? Double

This also works:

    let request = NSFetchRequest<NSFetchRequestResult>(entityName: Result.entityName(), context: context)
    request.resultType = .dictionaryResultType
    let avgED = NSExpressionDescription()
    avgED.name = "average"
    avgED.expression = NSExpression(forFunction: "average:", arguments: [NSExpression(forKeyPath: "amount")])
    avgED.expressionResultType = .decimalAttributeType

But changing the NSExpressionDescription in the above code with this crashes:

    let stdED = NSExpressionDescription()
    stdED.name = "stdDev"
    stdED.expression = NSExpression(forFunction: "stddev:", arguments: [NSExpression(forKeyPath: "amount")])
    stdED.expressionResultType = .decimalAttributeType

Here is the error I get:

Invalid keypath (request for aggregate operation on a toOne-only keypath): amount

iOS Core Data and standard deviation NSExpression crashes

0 Answers0