How to make the sum of only part of array in Swift? - swift

Question

I'm currently trying to build my first app in Swift, and am looking to find the total sum of part of an array relative to the current month. Here is my code:
struct Hour {
var date: String?
var time: String?
init(date: String?, time: String?) {
self.date = date
self.time = time
}
}
let hoursData = [
Hour(date: "Nov 29, 2015", time: "7"),
Hour(date: "Dec 12, 2015", time: "7"),
Hour(date: "Dec 14, 2015", time: "7"),
Hour(date: "Dec 25, 2015", time: "7") ]
I was wondering how I'd go about making a variable which contains the sum of the time data for the current month? Or any month for that matter? Any help you guys can give would be really appreciated.

Answer 1

First I'd rewrite your struct a bit,
both the properties can be immutable.
Instead of a String for date, I use a NSDate, and a double for the duration
struct Hour {
let date: NSDate
let duration: Double
init(date: NSDate, duration: Double) {
self.date = date
self.duration = duration
}
}
I create the hours like
let hoursData = [
Hour(date: { let c = NSDateComponents(); c.day = 29; c.month = 11; c.year = 2015; return NSCalendar.currentCalendar().dateFromComponents(c)!}(), duration: 7),
Hour(date: { let c = NSDateComponents(); c.day = 12; c.month = 12; c.year = 2015; return NSCalendar.currentCalendar().dateFromComponents(c)!}(), duration: 7),
Hour(date: { let c = NSDateComponents(); c.day = 14; c.month = 12; c.year = 2015; return NSCalendar.currentCalendar().dateFromComponents(c)!}(), duration: 7),
Hour(date: { let c = NSDateComponents(); c.day = 15; c.month = 12; c.year = 2015; return NSCalendar.currentCalendar().dateFromComponents(c)!}(), duration: 7)]
if you wonder about this syntax: I use implicit unnamed closures to created the NSDate parameters
now I filter for month and year an use the reduce method to sum up the filtered objects
let today = NSDate()
let totalDuration = hoursData.filter{
let objectsComps = NSCalendar.currentCalendar().components([.Month, .Year], fromDate: $0.date)
let todaysComps = NSCalendar.currentCalendar().components([.Month, .Year], fromDate: today)
return objectsComps.month == todaysComps.month && objectsComps.year == todaysComps.year
}.reduce(0) { (duration, hour) -> Double in
duration + hour.duration
}
Though it is a good answer, I want to point out that Rob's answer has a little flaw: The struct Hour hides the dependency it has that the date string need to be of a certain format. This violates the Dependency Inversion Principle, the D in the SOLID Principles. But this is easy to fix.
Just pass in the date formatter when creating the Hour instances.
struct Hour {
let date: NSDate
let duration: Double
init(date: NSDate, duration: Double) {
self.date = date
self.duration = duration
}
init (dateFormatter:NSDateFormatter, dateString:String, duration:Double) {
self.init(date: dateFormatter.dateFromString(dateString)!, duration:duration)
}
}
let dateFormatter = NSDateFormatter()
dateFormatter.dateFormat = "MMM d, y"
let hoursData = [
Hour(dateFormatter: dateFormatter, dateString: "Nov 29, 2015", duration: 7),
Hour(dateFormatter: dateFormatter, dateString: "Dec 12, 2015", duration: 7),
Hour(dateFormatter: dateFormatter, dateString: "Dec 14, 2015", duration: 7),
Hour(dateFormatter: dateFormatter, dateString: "Dec 25, 2015", duration: 7),
Hour(dateFormatter: dateFormatter, dateString: "Dec 25, 2017", duration: 7)
]
Now who ever uses Hour can define the format as needed, might be helpful in localized apps.
The filtering and reducing stays the same.
But now we have a new issue: NSDateformatter's dateFromString() might return nil. currently we force unwrap it with !, but this might be bad in a production app.
We should allow proper error handling, by allowing the convenience init to throw errors
enum HourError: ErrorType {
case InvalidDate
}
struct Hour {
let date: NSDate
let duration: Double
init(date: NSDate, duration: Double) {
self.date = date
self.duration = duration
}
init (dateFormatter:NSDateFormatter, dateString:String, duration:Double) throws {
let date = dateFormatter.dateFromString(dateString)
guard date != nil else { throw HourError.InvalidDate}
self.init(date: date!, duration:duration)
}
}
if we use it like
do {
let now = NSDate()
let dateFormatter = NSDateFormatter()
dateFormatter.dateFormat = "MMM d, y"
let hoursData = [
try Hour(dateFormatter: dateFormatter, dateString: "Nov 29, 2015", duration: 7),
try Hour(dateFormatter: dateFormatter, dateString: "Dec 12, 2015", duration: 7),
try Hour(dateFormatter: dateFormatter, dateString: "Dec 14, 2015", duration: 7),
try Hour(dateFormatter: dateFormatter, dateString: "Dec 25, 2015", duration: 7),
/*⛈*/ try Hour(dateFormatter: dateFormatter, dateString: " 25, 2017", duration: 7)
]
let totalDuration = hoursData.filter{
let objectsComps = NSCalendar.currentCalendar().components([.Month, .Year], fromDate: $0.date)
let todaysComps = NSCalendar.currentCalendar().components([.Month, .Year], fromDate: now)
return objectsComps.month == todaysComps.month && objectsComps.year == todaysComps.year
}.reduce(0) {
$0 + $1.duration
}
print(totalDuration)
} catch HourError.InvalidDate{
print("one or more datestrings must be wrong")
}
the error will be caught.
The full code

Answer 2

I might do something like:
// get prefix for current month
let formatter = NSDateFormatter()
formatter.dateFormat = "MMM"
let monthString = formatter.stringFromDate(NSDate())
// now add up the time values for that month
let results = hoursData.filter { $0.date?.hasPrefix(monthString) ?? false }
.reduce(0) { $0 + (Int($1.time ?? "0") ?? 0) }
Or, if you wanted to add a check for year, too:
let formatter = NSDateFormatter()
formatter.dateFormat = "MMM"
let monthString = formatter.stringFromDate(NSDate())
formatter.dateFormat = "yyyy"
let yearString = formatter.stringFromDate(NSDate())
let results = hoursData.filter { $0.date?.hasPrefix(monthString) ?? false && $0.date?.hasSuffix(yearString) ?? false }
.reduce(0) { $0 + (Int($1.time ?? "0") ?? 0) }
print(results)
Note, in both of the above, since you're using optionals, I'm using ?? to handle when the values are nil (as well as if there was a non-numeric string in time).
Personally, I'd suggest that Hour use NSDate and Float rather than String, not use optionals unless you really need them, and use let instead of var:
struct Hour {
let date: NSDate
let time: Float
init(dateString: String, time: Float) {
let formatter = NSDateFormatter()
formatter.dateFormat = "MMM, d, y"
self.date = formatter.dateFromString(dateString)!
self.time = time
}
}
Then the code becomes:
let hoursData = [
Hour(dateString: "Nov 29, 2015", time: 7),
Hour(dateString: "Dec 12, 2015", time: 7),
Hour(dateString: "Dec 14, 2015", time: 7),
Hour(dateString: "Dec 25, 2015", time: 7),
Hour(dateString: "Dec 25, 2017", time: 7)
]
let calendar = NSCalendar.currentCalendar()
let currentComponents = calendar.components([.Month, .Year], fromDate: NSDate())
let results = hoursData.filter {
let components = calendar.components([.Month, .Year], fromDate: $0.date)
return components.month == currentComponents.month && components.year == currentComponents.year
}.reduce(0) { return $0 + $1.time }
print(results)
There are further optimizations possible (e.g. not repeated instantiate NSDateFormatter), but it illustrates the idea that using NSDate objects lets you use calendrical calculations, rather than looking for substrings.

Answer 3

You could also do it functionally
let sum = hoursData
.filter { $0.date?.hasPrefix("Dec") ?? false } // Only get dates in the correct month
.flatMap { $0.time } // Map to an array of non nil times
.flatMap { Int($0) } // Convert strings to ints
.reduce(0) { $0 + $1 } // Sum up the times
This can be a lot simpler if you store non optional Ints instead of optional Strings. Or you could just use NSDateComponents.