Swift dates, switches and tuples

DatesInSwift

This app demonstrates a number of iOS techniques in Swift.

You can download the project from Github by clicking this link: DatesInSwift

It uses a date picker to let the user select a date. When the user taps the OK button, it displays a variety of messages depending on the date selected.

Extensions to NSDate

The program includes a file DateExtensions.swift that provides a number of useful utlities for working with dates.

Normally you can’t comare dates using ==, <, <=, >, or >=. The DateExtensions.swift extends NSDate to conform to the Equatable protocol, which allows the == comparison, and the Comparable protocol, which allows <, <=, >, or >= comparisions.

DateExtensions.swift defines a type alias mdyTuple which is a tuple containing the month, day, and year value for a date.

There are utilities that will convert NDate objects back and forth to mdyTuple tuples.

The NSDate instance method mdy() returns an mdyTuple.

The global function date(mdy: mdyTyple) takes an mdyTuple as input and returns an NSDate.

It turns out that you can’t match tuple constants in the cases of a switch statement by default.
In order to do that you have to implement the ~= (pattern match) operator for the tuple type.

For the mdyTuple type, the pattern match function looks like this:

/**
  This function makes it possible to use mdyTuple constants or variables in the cases
  of switch statements.
*/
func ~=(a: mdyTuple, b: mdyTuple) -&gt; Bool {
  return a.month ~= b.month &amp;&amp; a.year ~= b.year &amp;&amp; a.day ~= b.day
}

/**

This function makes it possible to use mdyTuple constants or variables in the cases

of switch statements.

func ~=(a: mdyTuple, b: mdyTuple) -> Bool {

return a.month ~= b.month && a.year ~= b.year && a.day ~= b.day

}

Exploring Swift switch statements

Swift has a very powerful version of the switch statement. You can match strings, you can match ranges of values, and you can match tuples.

The following a valid in Swift:

let aString = "foo"
switch aString
{
    case "bar":
        println("matched 'bar'")
    case "foo":
            println("matched 'foo'")
    case "foobar":
            println("matched 'foobar'")
    default:
            println("Didn't match anything")
}

let aString = "foo"

switch aString

{

case "bar":

println("matched 'bar'")

case "foo":

println("matched 'foo'")

case "foobar":

println("matched 'foobar'")

default:

println("Didn't match anything")

}

let a = 10
switch a
{
    case 1:                                 //match a soingle value
        println("value = 1")
    case 3...5:                             //Match a range
        println("value \(a) is between 3 and 5")
    case 7...9:                             //match another range
        println("value \(a) is between 7 and 9")
    case 10:
            println("value == 10")
    case let value where value % 2 == 0:    //wildcard match, boolean after "where" must be true
            println(""value \(a) is even")
    case _:                                 //match anything
    default:
            println("value \(a) didn't match any of our cases")
}

let a = 10

switch a

{

case 1: //match a soingle value

println("value = 1")

case 3...5: //Match a range

println("value \(a) is between 3 and 5")

case 7...9: //match another range

println("value \(a) is between 7 and 9")

case 10:

println("value == 10")

case let value where value % 2 == 0: //wildcard match, boolean after "where" must be true

println(""value \(a) is even")

case _: //match anything

default:

println("value \(a) didn't match any of our cases")

}

But some of the real power of the Swift Switch statment comes in when you use tuples:

typealias mixedTuple = (String, int, int)

let aTuple: mixedTuple = ()"Bob", 1, 3)

switch aTuple
{
    case ("Bob", 1, 3):
        println("A perfect match")

    case ("Bob", 1, _):
        println("matched all but last value")

    case (_, 1, 3):
        println("Values match, name does not.")
    case ("Bob", let middleValue, 3):       //Match on name and last value, 
                                            //save middle value to a local constant.
        println("Name and last value match, middle value \(middleValue) does not")

    case (_, 0...10, 3)
        println("Any name, middle value in range 0...10, last value match.")

    default: 
        println("No match.")
}

typealias mixedTuple = (String, int, int)

let aTuple: mixedTuple = ()"Bob", 1, 3)

switch aTuple

{

case ("Bob", 1, 3):

println("A perfect match")

case ("Bob", 1, _):

println("matched all but last value")

case (_, 1, 3):

println("Values match, name does not.")

case ("Bob", let middleValue, 3): //Match on name and last value,

//save middle value to a local constant.

println("Name and last value match, middle value \(middleValue) does not")

case (_, 0...10, 3)

println("Any name, middle value in range 0...10, last value match.")

default:

println("No match.")

}

The DatesInSwift demo converts the user-selected date to an mdyTuple, then uses it to demonstrate various types of switch cases.

First, define some constants:

let bobsBirthday =    (month: 11, day: 30, year: 1961)
let janesBirthday =   (month: 10, day: 14, year: 1987)
let fredsBirthday =   (month: 4, day: 3, year: 1973)
let patsBirthday =    (month: 7, day: 7, year: 1931)
let susansBirthday =  (month: 3, day: 8, year: 1933)

let bobsBirthday = (month: 11, day: 30, year: 1961)

let janesBirthday = (month: 10, day: 14, year: 1987)

let fredsBirthday = (month: 4, day: 3, year: 1973)

let patsBirthday = (month: 7, day: 7, year: 1931)

let susansBirthday = (month: 3, day: 8, year: 1933)

Then do some setup:

//Prefix will be used to build date sentences. 
//Is the user-selected date in the past, present, or future?
let prefix: String 

//I compare the mdy tuples for the dates so I can ignore the time and just compare dates.
if datePicker.date.mdy() == NSDate().mdy()
{
  prefix = "is"
}
else if datePicker.date &gt; NSDate()
{
  prefix = "will be"
}
else
{
  prefix = "was"
}

let todayMDY = NSDate().mdy()
let theMDY = datePicker.date.mdy()
let date = NSString(format: "%02d/%02d/%04d", theMDY.month, theMDY.day, theMDY.year)

//Prefix will be used to build date sentences.

//Is the user-selected date in the past, present, or future?

let prefix: String

//I compare the mdy tuples for the dates so I can ignore the time and just compare dates.

if datePicker.date.mdy() == NSDate().mdy()

{

prefix = "is"

}

else if datePicker.date > NSDate()

{

prefix = "will be"

}

else

{

prefix = "was"

}

let todayMDY = NSDate().mdy()

let theMDY = datePicker.date.mdy()

let date = NSString(format: "%02d/%02d/%04d", theMDY.month, theMDY.day, theMDY.year)

And finally the switch statement:

//The cases are evaluated top-to-bottom. 
//The code of first match is executed, then processing stops.
switch theMDY
{
case (3, 15, _):
  message = "Beware the Ides of March"

case (fredsBirthday):
  message = "\(date) \(prefix) the day Fred was born"

case (fredsBirthday.month, fredsBirthday.day, let year) where year &gt; fredsBirthday.year:
  message = "\(date) \(prefix) Fred's \(possessiveNumber(theMDY.year - fredsBirthday.year)) birthday"

case (janesBirthday):
  message = "\(date) \(prefix) the day Jane was born"

case (janesBirthday.month, janesBirthday.day, let year) where year &gt; janesBirthday.year:
  message = "\(date) \(prefix) Jane's \(possessiveNumber(year - janesBirthday.year)) birthday"

case (bobsBirthday):
  message = "\(date) \(prefix) the day Bob was born"

case (bobsBirthday.month, bobsBirthday.day, let year) where year &gt; bobsBirthday.year:
  message = "\(date) \(prefix) Bob's \(possessiveNumber(year - bobsBirthday.year)) birthday"

case (patsBirthday):
  message = "\(date) \(prefix) the day Pat was born"

case (patsBirthday.month, patsBirthday.day, let year) where year &gt; patsBirthday.year:
  message = "\(date) \(prefix) Pat's \(possessiveNumber(year - patsBirthday.year)) birthday"

case (susansBirthday):
  message = "\(date) \(prefix) the day Susan was born"

case (susansBirthday.month, susansBirthday.day, let year) where year &gt; susansBirthday.year:
  message = "\(date) \(prefix) Susan \(possessiveNumber(year - susansBirthday.year)) birthday"

//Match the month, day range 1...15, and copy the year to a constant.
case (5, 1...15, let year):
  message = "\(date) \(prefix) in the first half of May, \(year)"

case (5, 16...31, let year):
  message = "\(date) \(prefix) in the second half of May, \(year)"

//Match the month, copy the day value to a constant, and match any year.
case (2, let day, _)
  where day % 2 == 0:     //The optional where clause is a boolean expression which must be true
                          //for the case to match
  message = "\(date) \(prefix) an even day in February"

case (2, let day, _) where day % 2 == 1:
  message = "\(date) \(prefix) an odd day in February"

case todayMDY:
  message = "Today is \(date)"

//Match Any day in april, any year.
case (4, _, _):
  message = "April showers bring May flowers"

default:
  message = "The selected date \(prefix) \(date)"
}

//The cases are evaluated top-to-bottom.

//The code of first match is executed, then processing stops.

switch theMDY

{

case (3, 15, _):

message = "Beware the Ides of March"

case (fredsBirthday):

message = "\(date) \(prefix) the day Fred was born"

case (fredsBirthday.month, fredsBirthday.day, let year) where year > fredsBirthday.year:

message = "\(date) \(prefix) Fred's \(possessiveNumber(theMDY.year - fredsBirthday.year)) birthday"

case (janesBirthday):

message = "\(date) \(prefix) the day Jane was born"

case (janesBirthday.month, janesBirthday.day, let year) where year > janesBirthday.year:

message = "\(date) \(prefix) Jane's \(possessiveNumber(year - janesBirthday.year)) birthday"

case (bobsBirthday):

message = "\(date) \(prefix) the day Bob was born"

case (bobsBirthday.month, bobsBirthday.day, let year) where year > bobsBirthday.year:

message = "\(date) \(prefix) Bob's \(possessiveNumber(year - bobsBirthday.year)) birthday"

case (patsBirthday):

message = "\(date) \(prefix) the day Pat was born"

case (patsBirthday.month, patsBirthday.day, let year) where year > patsBirthday.year:

message = "\(date) \(prefix) Pat's \(possessiveNumber(year - patsBirthday.year)) birthday"

case (susansBirthday):

message = "\(date) \(prefix) the day Susan was born"

case (susansBirthday.month, susansBirthday.day, let year) where year > susansBirthday.year:

message = "\(date) \(prefix) Susan \(possessiveNumber(year - susansBirthday.year)) birthday"

//Match the month, day range 1...15, and copy the year to a constant.

case (5, 1...15, let year):

message = "\(date) \(prefix) in the first half of May, \(year)"

case (5, 16...31, let year):

message = "\(date) \(prefix) in the second half of May, \(year)"

//Match the month, copy the day value to a constant, and match any year.

case (2, let day, _)

where day % 2 == 0: //The optional where clause is a boolean expression which must be true

//for the case to match

message = "\(date) \(prefix) an even day in February"

case (2, let day, _) where day % 2 == 1:

message = "\(date) \(prefix) an odd day in February"

case todayMDY:

message = "Today is \(date)"

//Match Any day in april, any year.

case (4, _, _):

message = "April showers bring May flowers"

default:

message = "The selected date \(prefix) \(date)"

}