Make your Swift code expressive: addition operator use cases

I like reading short and expressive code. Because developer spends more time reading code than writing, expressiveness often is obligatory.

Unless shortness does not obscure the intent, I favor concise expressions over longer ones. For example:

import Foundation

let greeting = "Hello, "  
let who = "World"

let message1 = greeting + who  
let message2 = greeting.appending(who)  
print(message1) // => "Hello, World"  
print(message2) // => "Hello, World"  

The sample shows 2 options to concatenate strings: using addition operator + or appending(_) string method.
What option do you like more? I guess the concise one greeting + who.

The operator overloading in Swift enables to write short expressions. Many types like Int, String, Array overload addition (+) and addition assignment (+=) operators. It makes the manipulation of the corresponding types more intuitive.

Would you like to write expressive code? I'm sure you do! So let's continue with an interesting list of types that support operator overloading for + and +=.
The alternative methods with the same behavior are also presented, for comparison purposes.

1. Sum numbers

Obviously the regular usage of addition operator is meant to perform arithmetic addition on numbers. For instance, 4 + 8 is evaluated to 12.

Because Swift is type-safe, you can apply + and += operators when both operands are exactly the same type (Int + Int, but not UInt + Int or Float + Int).

All Swift number types Int, Float, Double and others support addition operators. Let's see a sample:

var x = 5  
let y = 3  
print(x + y) // => 8  
x += y  
print(x)     // => 8  

x + y performs an arithmetic addition of two integers. Plain and simple.
The expression x += y mutates x variable by appending y to it (same as x = x + y). During this operation x is mutated, so it must declared as a variable with var.

The equivalent methods of addition operators are adding(_:) and the mutating add(_:). These methods are available for Float and Double, but not for Int.
Let's see them in action:

var p = 5.0  
let r = 3.0  
print(p.adding(r)) // => 8.0  
p.add(r)  
print(p)           // => 8.0  

p and r are Double type.
The invocation of p.adding(r) is the same as p + r. Respectively p.add(r) mutates p and is equivalent to p += r.

2. Concatenate strings

The addition and addition assignment operators can perform strings concatenation. For instance, "abc" + "def" creates a string "abcdef".

Let's see an example:

var message = "Hello "  
let name = "Batman"  
print(message + name) // => "Hello Batman"  
message += name  
print(message)        // => "Hello Batman"  

message + name concatenates two strings.
The statement message += name also performs a concatenation. It modifies message in place by appending to its end name string.

You can also use equivalent methods appending(_:) and mutating append(_:), which are more verbose. Let's transform the above example:

var message = "Hello "  
let name = "Batman"  
print(message.appending(name)) // => "Hello Batman"  
message.append(name)  
print(message)                 // => "Hello Batman"  

The invocation message.appending(name) concatenates message and name, returning the result. message variable is not modified.
The invocation message.append(name) is mutating the message variable and appends to its end name string.

3. Concatenate arrays

Addition operators are useful to concatenate arrays. [val1, val2] + [val3] creates [val1, val2, val3].

The concatenated arrays must have elements of the same type.

var colors = ["white"]  
let darkColors = ["black", "gray"]  
print(colors + darkColors) // => ["white", "black", "gray"]  
colors += darkColors  
print(colors)              // => ["white", "black", "gray"]  

The expression colors + darkColors creates a new array that contains elements from colors followed by elements from darkColors.
colors += darkColors mutates the colors array in place, by adding to its tail elements from darkColors.

Alternatively you can use the mutating append(contentsOf:_), which is an equivalent of addition assignment operator (+=). Transforming the above example:

var colors = ["white"]  
let darkColors = ["black", "gray"]  
colors.append(contentsOf: darkColors)  
print(colors) // => ["white", "black", "gray"]  

The invocation of colors.append(contentsOf: darkColors) modifies colors in place and appends the elements of darkColors.

4. Add time interval to a date

The addition operator enables expressively to add intervals to a Date. The expression date + timeInterval creates a new Date with a specified amount of time added to it.
The addition assignment modifies the date in place date += timeInterval.

Let's see how it can be done:

import Foundation

let interval: TimeInterval = 60 * 60 * 24  
let dateFormatter = DateFormatter()  
dateFormatter.dateFormat = "yyyy-MM-dd"

if let date = dateFormatter.date(from: "2017-02-15") {  
   let dayAfter = date + interval   
   print(dateFormatter.string(from: dayAfter)) // => 2017-02-16
}

oneDay is an interval that contains the number of seconds in 24 hours. dateFormatter creates a date for 2017-02-15.
The expression date + oneDay evaluates to a new date dayAfterDate that is created from date with oneDay time interval added to it.

If you want to modify date directly, make it a variable and use addition assignment operator +=:

/* ... */
if var date = dateFormatter.date(from: "2017-02-15") {  
   date += interval   
   print(dateFormatter.string(from: date)) // => 2017-02-16
}

date =+ oneDay mutates date by adding oneDay seconds to it.

The Date methods that provide the same behavior are addingTimeInterval(_:) and the mutating addTimeInterval(_:).

Tip about calendar

The provided way to modify a date adjusts absolute values. You may have unexpected results when adding longer time intervals like weeks or months.

Most of the times Calendar usage is preferable. It provides accurate date modifications according to daylight saving time, months with different numbers of days, and so on.

Let's update the above example and use a calendar instance Calendar.current:

import Foundation

let interval = 60 * 60 * 24  
let dateFormatter = DateFormatter()  
dateFormatter.dateFormat = "yyyy-MM-dd"

if let date = dateFormatter.date(from: "2017-02-15") {  
   let calendar = Calendar.current
   let dayAfter = 
     calendar.date(byAdding: .second, value: interval, to: date)!
   print(dateFormatter.string(from: dayAfter)) // => 2017-02-16
}

5. Sum measurements

A recent Foundation update introduced measurements and units. It allows to represent distances (for instance 10 miles, 12 kilometers), weights (8 kg) and more.

The good part is that Measurement structure overloads + operator (and additionally * - / < ==). This makes measurement manipulations easy and concise.

Let's sum two distances in kilometers:

import Foundation

let morningRun = Measurement(value: 3, unit: UnitLength.kilometers)  
let eveningRun = Measurement(value: 5, unit: UnitLength.kilometers)  
let dailyRun = morningRun + eveningRun  
print(dailyRun) // => '8000.0 m'  

morningRun and eveningRun describe the distance someone ran in the morning and evening.
Plain and simple the addition operator is used to find the daily run distance: morningRun + eveningRun.

The addition operation must sum measurements that describe the same type of physical units (length, mass, speed and more).

For instance, it doesn't make sense to sum speed and mass values. In such case Swift triggers an error:

import Foundation

let turtleSpeed = Measurement(value: 3, unit: UnitLength.kilometers)  
let turtleWeight = Measurement(value: 100, unit: UnitMass.grams)  
print(turtleSpeed + turtleWeight)  
// Error: binary operator '+' cannot be applied to operands of 
// type 'Measurement<UnitLength>' and 'Measurement<UnitMass>'

turtleSpeed and turtleWeight are measurements that holds different type of units: UnitLength.kilometers and UnitMass.grams. These are not compatible, and as result Swift triggers an error.

Measurement structure does not provide methods for manipulation. In this case you have to use operators only.
In my opinion it's a nice decision, because operators fits good with measurements.

6. Conclusion

As seen, the addition and addition assignment operators provide short and concise syntax.

Generally these are used to sum numbers and concatenate strings.

You can also benefit from a concise syntax when concatenating arrays, manipulating dates and sum measurements.

Do you know other Swift types that implement addition operator overloading? Feel free to write a comment bellow!

You might be interested to read:
Mastering Swift: essential details about strings
Coding like Shakespeare: practical function naming conventions
Useful details about underscore keyword in Swift

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