Updating array values using higher order functions in Swift

Question

I’m struggling with using the new map, flatMap, filter, reduce and zip functions. Consider the following: You have two arrays, A and B, containing different objects. For each object in A, you need to find the corresponding object in B (by their id property), and update some of the properties of the object from B. This could be done the old way, using two for cycles, like so:

private func update(statuses: [JobStatus], forJobs jobs: [JobBookPayload]) {
    for jobStatus in statuses {
        for job in jobs {
            if jobStatus.jobId == job.jobId {
                job.status = jobStatus.status!
                job.option = jobStatus.option!
            }
        }
    }
}

Can this be done using the new functions, to make the code more "Swifty" and improve readability?

That is clear, readable code. – You'll always need a nested loop (explicit or hidden in a filter) unless you change the data structures. — Martin R
– Martin R, Commented Jul 25, 2017 at 9:23
It's not bad, I was just wondering if it could be achieved with the new functions in Swift. — damjandd
– damjandd, Commented Jul 25, 2017 at 9:24

user3581248 · Accepted Answer · 2017-07-25 09:41:53Z

2

I suppose that since you're using classes, you don't actually want to create a new object from the old one. I believe this could be achieved using map and first methods:

@discardableResult
func update(statuses: [JobStatus], forJobs jobs: [JobBookPayload]) -> [JobBookPayload] {
    return jobs.map({ (payload) -> JobBookPayload in
        if let status = statuses.first(where: { payload.jobId == $0.jobId }) {
            payload.status = status.status
            payload.option = status.option
        }
        return payload
    })
}

Using @discardableResult will allow you to use this function and ignore the resulting array - the objects inside the initial array were mutated anyway.

answered Jul 25, 2017 at 9:41

user3581248

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3 Comments

damjandd Over a year ago

If you are discarding the result anyway, why make the function return anything? Could it not be done like so: _ = jobs.map({ ..., and have the function without a return value?

user3581248 Over a year ago

@damjandd can't argue with that, but is this approach actually better in any way?

damjandd Over a year ago

Not really, I was just curious how it would be done using the new techniques and if it would make the code more readable. Thanks for your input.

dfrib · Accepted Answer · 2017-07-25 10:25:04Z

1

Replacing for .. in .. { if conditional { ... } } with for .. in .. where conditional { ... }

You could make a minor re-factoring by introducing a where clause in the inner for loop to replace the single if statement of its body:

private func update(statuses: [JobStatus], forJobs jobs: [JobBookPayload]) {
    for jobStatus in statuses {
        for job in jobs where jobStatus.jobId == job.jobId {
            job.status = jobStatus.status!
            job.option = jobStatus.option!
        }
    }
}

edited Jul 25, 2017 at 10:25

answered Jul 25, 2017 at 10:18

dfrib

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Comments

S2dent · Accepted Answer · 2017-07-25 09:36:33Z

0

The idea in the "new" (functional) way, is to create a new object from the previous one:

private func update(statuses: [JobStatus], for jobs: [JobBookPayload]) -> [JobBookPayload] {
    return jobs.reduce([]) { newJobs, job in
        return newJobs + statuses.flatMap { status in
            return status.jobId == job.jobId ?
                //Construct a new object
                JobBookPayload(status: status.status, option: status.option) : nil
        }
    }
}

Assuming there is one status object corresponding to a job:

private func update(statuses: [JobStatus], for jobs: [JobBookPayload]) -> [JobBookPayload] {
    return jobs.reduce([]) { newJobs, job in
        return newJobs + statuses.lazy.filter { $0.jobId == job.jobId }.first.flatMap { 
            JobBookPayload(status: $0.status, option: $0.option) // Construct the object
        }
    }
}

Avoiding reduce:

private func update(statuses: [JobStatus], for jobs: [JobBookPayload]) -> [JobBookPayload] {
    return jobs.flatMap { job in
        return statuses.flatMap { status in
            return status.jobId == job.jobId ?
                //Construct a new object
                JobBookPayload(status: status.status, option: status.option) : nil
        }.last
    }
}

edited Jul 25, 2017 at 9:36

answered Jul 25, 2017 at 9:29

S2dent

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4 Comments

Martin R Over a year ago

It looks as if you are using reduce as a "map replacement", which is extremely inefficient because lots of intermediate arrays are created. Compare airspeedvelocity.net/2015/08/03/…

S2dent Over a year ago

yeah, right, I could use flatMap here too, I prefer reduce for readability at times, plus afaik there's been another article recently where the exponential part of reduce has been removed, and it was taking linearly 1.5 times of a map

Martin R Over a year ago

Do you have a link to that article? – Also this github.com/apple/swift-evolution/blob/master/proposals/… should improve can help in Swift 4, when implemented.

OOPer Over a year ago

Your second code does not compile. Created new JobBookPayload needs to keep jobId and other properties unchanged. You code returns a new Array where some elements are removed, which is completely wrong. The original code never deletes any elements from the Array of JobBookPayload.

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