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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 |
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You can use Some sample code (from Group by, Count and Sum in CoreData | Cocoanetics):
Another example, from objc.io's book Core Data (pp82-83):
Hopefully these can get you started. And maybe someone else on here has an example from their own code they can share. |
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thanks for posting, I believe the first example is exactly what I am looking for. Will try it out |
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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
Edit: put source in code block for clean formatting |
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First things first... When posting code to these forums, place three backticks 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:
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) } |
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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. |
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thanks so much for the reply @roosterboy. Taking a look at this solution now. Will reply back |
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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! |
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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:
My data model:
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You can achieve this by fetching the entities and extracting the 'price' field into an array. Here's a quick example in Python:
Feel free to adapt it to your specific implementation. |
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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' |
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Pretty nice post. I just stumbled upon your weblog and wanted to say that I have really enjoyed browsing your blog posts. After all I’ll be subscribing to your feed and I hope you write again soon! |
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