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Data from multiple entities into array for math ops

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Hello,

I have multiple entities of a type which I would like to extract all of the elements of a certain field into an array and then perform a math operation onto it. An example is if I had multiple homes each with a field 'price' and I would like to determine the max price. I expect either a fetch or or calling allObjects and converting the target field to an array operation followed by calling .max() on the array. Thank you

3      

You can use NSExpression to create a fetch request for the following functions: sum, count, min, max, and average.

Some sample code (from Group by, Count and Sum in CoreData | Cocoanetics):

let keypathExp1 = NSExpression(forKeyPath: "col")
let expression = NSExpression(forFunction: "sum:", arguments: [keypathExp1])
let sumDesc = NSExpressionDescription()
sumDesc.expression = expression
sumDesc.name = "sum"
sumDesc.expressionResultType = .integer64AttributeType

let request = NSFetchRequest(entityName: "Record")
request.returnsObjectsAsFaults = false
request.propertiesToFetch = [sumDesc]
request.resultType = .dictionaryResultType

Another example, from objc.io's book Core Data (pp82-83):

let request = NSFetchRequest<NSManagedObject>(entityName: "Employee") 
request.resultType = .dictionaryResultType

let salaryExp = NSExpressionDescription() 
salaryExp.expressionResultType = .doubleAttributeType
salaryExp.expression = NSExpression(forFunction: "average:", arguments: [NSExpression(forKeyPath: "salary")])
salaryExp.name = "avgSalary" 

request.propertiesToGroupBy = ["type"]
request.propertiesToFetch = ["type", salaryExp] 
_ = try! context.fetch(request)

Hopefully these can get you started. And maybe someone else on here has an example from their own code they can share.

3      

thanks for posting, I believe the first example is exactly what I am looking for. Will try it out

3      

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hmm, I must be missing something.

let's take a simple department, employee example, in which I would like to find the sum of employee computers. The following expected code does not work for me. What am I doing wrong? results is always invalid. Thank you


let keypathExp1 = NSExpression(forKeyPath: "computer")
let expression = NSExpression(forFunction: "sum:", arguments: [keypathExp1])
let sumDesc = NSExpressionDescription()
sumDesc.expression = expression
sumDesc.name = "sum"
sumDesc.expressionResultType = .integer64AttributeType

let request = NSFetchRequest(entityName: "department")
request.returnsObjectsAsFaults = false
request.propertiesToFetch = [sumDesc]
request.resultType = .dictionaryResultType

let fetch: NSFetchRequest = department.fetchRequest()
do{
    let results = try? fetch.execute()
    print(results ?? "invalid?")  // results is now an array
}
catch {
   NSLog("Error fetching entity: %@", error.localizedDescription)
}

Edit: put source in code block for clean formatting

3      

First things first...

When posting code to these forums, place three backticks ``` on the line before your code and three backticks ``` on the line after your code so that it will be formatted properly. You can also highlight an entire code block and click the </> button on the toolbar to wrap the block for you.

This makes it far easier to read and also makes it easier for other posters to copy/paste the code in order to test solutions and such.

Doing this will ensure that you end up with something like this:

let keypathExp1 = NSExpression(forKeyPath: "computer") 
let expression = NSExpression(forFunction: "sum:", arguments: [keypathExp1]) 
let sumDesc = NSExpressionDescription() 
sumDesc.expression = expression 
sumDesc.name = "sum" 
sumDesc.expressionResultType = .integer64AttributeType

let request = NSFetchRequest(entityName: "department") 
request.returnsObjectsAsFaults = false 
request.propertiesToFetch = [sumDesc] 
request.resultType = .dictionaryResultType

let fetch: NSFetchRequest = department.fetchRequest() 
do{ 
    let results = try? fetch.execute() 
    print(results ?? "invalid?") // results is now an array 
} catch { 
    NSLog("Error fetching entity: %@", error.localizedDescription) 
}

instead of this:

let keypathExp1 = NSExpression(forKeyPath: "computer") let expression = NSExpression(forFunction: "sum:", arguments: [keypathExp1]) let sumDesc = NSExpressionDescription() sumDesc.expression = expression sumDesc.name = "sum" sumDesc.expressionResultType = .integer64AttributeType

let request = NSFetchRequest(entityName: "department") request.returnsObjectsAsFaults = false request.propertiesToFetch = [sumDesc] request.resultType = .dictionaryResultType

let fetch: NSFetchRequest = department.fetchRequest() do{ let results = try? fetch.execute() print(results ?? "invalid?") // results is now an array } catch { NSLog("Error fetching entity: %@", error.localizedDescription) }

4      

bump

3      

Sorry it's taken me so long to respond; real life got in the way for a while.

Anyway, check out this simple sample project I uploaded to github: roosterboy/CoreDataFunctions

Hopefully, this will give you a good idea how to do what you are looking to do.

4      

thanks so much for the reply @roosterboy. Taking a look at this solution now. Will reply back

3      

sorry that it has been this long, but I have finally been able to download and run this code. And it seems to suit my purpose, thanks for putting it together, especially the sweet preview readme file. I am going to dig in and add this to my project. Stay tuned!

3      

Hello,

In response to https://www.hackingwithswift.com/forums/swift/data-from-multiple-entities-into-array-for-math-ops/18754/18881 above, I have cloned and tested that project and it works great!

I have adapted it to my project, and it works in some capacity there too. My project (weight lifting data logger) has core data for a number of exercises and each exercise has one or more sets. Sets contain a weight lifted and the number of repetitions completed. I am able to fetch the maximum weight lifted across sets of all exercises (bench press, squat, etc), which is good, however I would like to fetch the max weight of all sets limited to a given exercise. How can I accomplish this? I expect that an NSPredicate may be part of the solution, but I am not able to find something working in a search. Alternatively, I believe it could be by exercising the relationship from set to exercise (which is an inverse relationship in my data). Any help is appreciated.

The function that I have and is working now to get max of all sets in the data:

    func maxSetWeight() -> Float {
        let request = NSFetchRequest<NSDictionary>(entityName: "Set")
        request.resultType = .dictionaryResultType

        let maxWeightExpression = NSExpressionDescription()
        maxWeightExpression.expressionResultType = .floatAttributeType
        // max, min, sum, average
        maxWeightExpression.expression = NSExpression(forFunction: "max:", arguments: [NSExpression(forKeyPath: "weight")])
        maxWeightExpression.name = "maxSetWeight"

        request.propertiesToFetch = [maxWeightExpression]

        let result = try! container.viewContext.fetch(request) as! [[String: AnyObject]]
        let maxSetWeight = result[0]["maxSetWeight"] as? Float ?? 0.0
        return maxSetWeight
    }

My data model:

Exercise {String name}
Set {UUID id, Float weight, Int16 reps} has relationship {exercises} Destination {Exercise} Inverse {sets}

3      

bump

3      

bump, still looking for suggestions on this! thanks

2      

You can achieve this by fetching the entities and extracting the 'price' field into an array. Here's a quick example in Python:

# Assuming 'homes' is your collection of entities
prices = [home['price'] for home in homes]

# Now, find the max price
max_price = max(prices)

# 'max_price' holds the maximum price value

Feel free to adapt it to your specific implementation.

2      

https://www.hackingwithswift.com/forums/swift/data-from-multiple-entities-into-array-for-math-ops/18754/25156 is a correct python representation of the info I am trying to extract. But I do not see how to use the NSFetch to do the equivalent. Substituting the entities for my case:

weights = [exercises['weights'] for 'Bench Press' in exercises]

max_weight = max(weights) // this is specific to the exercise 'Bench Press'

2      

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