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Latest revision as of 04:18, 10 November 2016
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So far we have looked at using variables and constants in Swift and also described the different data types. Being able to create variables is only part of the story however. The next step is to learn how to use these variables and constants in Swift code. The primary method for working with data is in the form of expressions.
Expression Syntax in Swift
The most basic Swift expression consists of an operator, two operands and an assignment. The following is an example of an expression:
var myresult = 1 + 2
In the above example, the (+) operator is used to add two operands (1 and 2) together. The assignment operator (=) subsequently assigns the result of the addition to a variable named myresult. The operands could just have easily been variables (or a mixture of constants and variables) instead of the actual numerical values used in the example.
In the remainder of this chapter we will look at the basic types of operators available in Swift.
The Basic Assignment Operator
We have already looked at the most basic of assignment operators, the = operator. This assignment operator simply assigns the result of an expression to a variable. In essence, the = assignment operator takes two operands. The left hand operand is the variable or constant to which a value is to be assigned and the right hand operand is the value to be assigned. The right hand operand is, more often than not, an expression which performs some type of arithmetic or logical evaluation, the result of which will be assigned to the variable or constant. The following examples are all valid uses of the assignment operator:
var x: Int? // Declare an optional Int variable var y = 10 // Declare and initialize a second Int variable x = 10 // Assign a value to x x = x! + y // Assign the result of x + y to x x = y // Assign the value of y to x
Swift Arithmetic Operators
Swift provides a range of operators for the purpose of creating mathematical expressions. These operators primarily fall into the category of binary operators in that they take two operands. The exception is the unary negative operator (-) which serves to indicate that a value is negative rather than positive. This contrasts with the subtraction operator (-) which takes two operands (i.e. one value to be subtracted from another). For example:
var x = -10 // Unary - operator used to assign -10 to variable x x = x - 5 // Subtraction operator. Subtracts 5 from x
The following table lists the primary Swift arithmetic operators:
| Operator | Description |
|---|---|
| -(unary) | Negates the value of a variable or expression |
| * | Multiplication |
| / | Division |
| + | Addition |
| - | Subtraction |
| % | Modulo |
Note that multiple operators may be used in a single expression.
For example:
x = y * 10 + z - 5 / 4
Compound Assignment Operators
In an earlier section we looked at the basic assignment operator (=). Swift provides a number of operators designed to combine an assignment with a mathematical or logical operation. These are primarily of use when performing an evaluation where the result is to be stored in one of the operands. For example, one might write an expression as follows:
x = x + y
The above expression adds the value contained in variable x to the value contained in variable y and stores the result in variable x. This can be simplified using the addition compound assignment operator:
x += y
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Numerous compound assignment operators are available in Swift. The most frequently used of which are outlined in the following table:
| Operator | Description |
|---|---|
| x += y | Add x to y and place result in x |
| x -= y | Subtract y from x and place result in x |
| x *= y | Multiply x by y and place result in x |
| x /= y | Divide x by y and place result in x |
| x %= y | Perform Modulo on x and y and place result in x |
Increment and Decrement Operators
Another useful shortcut can be achieved using the Swift increment and decrement operators (also referred to as unary operators because they operate on a single operand). Consider the code fragment below:
x = x + 1 // Increase value of variable x by 1 x = x - 1 // Decrease value of variable x by 1
These expressions increment and decrement the value of x by 1. Instead of using this approach, however, it is quicker to use the ++ and -- operators. The following examples perform exactly the same tasks as the examples above:
x++ // Increment x by 1 x-- // Decrement x by 1
These operators can be placed either before or after the variable name. If the operator is placed before the variable name, the increment or decrement operation is performed before any other operations are performed on the variable. For example, in the following code, x is incremented before it is assigned to y, leaving y with a value of 10:
var x = 9 var y = ++x
In the next example, however, the value of x (9) is assigned to variable y before the decrement is performed. After the expression is evaluated the value of y will be 9 and the value of x will be 8.
var x = 9 var y = x--
Comparison Operators
Swift also includes a set of logical operators useful for performing comparisons. These operators all return a Boolean result depending on the result of the comparison. These operators are binary operators in that they work with two operands.
Comparison operators are most frequently used in constructing program flow control logic. For example an if statement may be constructed based on whether one value matches another:
if x == y {
// Perform task
}
The result of a comparison may also be stored in a Bool variable. For example, the following code will result in a true value being stored in the variable result:
var result: Bool? var x = 10 var y = 20 result = x < y
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| Operator | Description |
|---|---|
| x == y | Returns true if x is equal to y |
| x > y | Returns true if x is greater than y |
| x >= y | Returns true if x is greater than or equal to y |
| x < y | Returns true if x is less than y |
| x <= y | Returns true if x is less than or equal to y |
| x != y | Returns true if x is not equal to y |
Boolean Logical Operators
Swift also provides a set of so called logical operators designed to return Boolean true or false values. These operators both return Boolean results and take Boolean values as operands. The key operators are NOT (!), AND (&&) and OR (||).
The NOT (!) operator simply inverts the current value of a Boolean variable, or the result of an expression. For example, if a variable named flag is currently true, prefixing the variable with a '!' character will invert the value to false:
var flag = true // variable is true var secondFlag = !flag // secondFlag set to false
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if (10 < 20) || (20 < 10) {
print("Expression is true")
}
The AND (&&) operator returns true only if both operands evaluate to be true. The following example will return false because only one of the two operand expressions evaluates to true:
if (10 < 20) && (20 < 10) {
print("Expression is true")
}
Range Operators
Swift includes two useful operators that allow ranges of values to be declared. As will be seen in later chapters, these operators are invaluable when working with looping in program logic.
The syntax for the closed range operator is as follows:
x…y
This operator represents the range of numbers starting at x and ending at y where both x and y are included within the range. The range operator 5…8, for example, specifies the numbers 5, 6, 7 and 8.
The half-closed range operator, on the other hand uses the following syntax:
x..<y
In this instance, the operator encompasses all the numbers from x up to, but not including, y. A half closed range operator 5..<8, therefore, specifies the numbers 5, 6 and 7.
The Ternary Operator
Swift supports the ternary operator to provide a shortcut way of making decisions within code. The syntax of the ternary operator (also known as the conditional operator) is as follows:
condition ? true expression : false expression
The way the ternary operator works is that condition is replaced with an expression that will return either true or false. If the result is true then the expression that replaces the true expression is evaluated. Conversely, if the result was false then the false expression is evaluated. Let's see this in action:
let x = 10
let y = 20
print("Largest number is \(x > y ? x : y)")
The above code example will evaluate whether x is greater than y. Clearly this will evaluate to false resulting in y being returned to the print call for display to the user:
Largest number is 20
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Bitwise Operators
As previously discussed, computer processors work in binary. These are essentially streams of ones and zeros, each one referred to as a bit. Bits are formed into groups of 8 to form bytes. As such, it is not surprising that we, as programmers, will occasionally end up working at this level in our code. To facilitate this requirement, Swift provides a range of bit operators.
Those familiar with bitwise operators in other languages such as C, C++, C#, Objective-C and Java will find nothing new in this area of the Swift language syntax. For those unfamiliar with binary numbers, now may be a good time to seek out reference materials on the subject in order to understand how ones and zeros are formed into bytes to form numbers. Other authors have done a much better job of describing the subject than we can do within the scope of this book.
For the purposes of this exercise we will be working with the binary representation of two numbers. Firstly, the decimal number 171 is represented in binary as:
10101011
Secondly, the number 3 is represented by the following binary sequence:
00000011
Now that we have two binary numbers with which to work, we can begin to look at the Swift bitwise operators:
Bitwise NOT
The Bitwise NOT is represented by the tilde character and has the effect of inverting all of the bits in a number. In other words, all the zeros become ones and all the ones become zeros. Taking our example 3 number, a Bitwise NOT operation has the following result:
00000011 NOT ======== 11111100
The following Swift code, therefore, results in a value of -4:
let y = 3
let z = ~y
print("Result is \(z)")
Bitwise AND
The Bitwise AND is represented by a single ampersand (&). It makes a bit by bit comparison of two numbers. Any corresponding position in the binary sequence of each number where both bits are 1 results in a 1 appearing in the same position of the resulting number. If either bit position contains a 0 then a zero appears in the result. Taking our two example numbers, this would appear as follows:
10101011 AND 00000011 ======== 00000011
As we can see, the only locations where both numbers have 1s are the last two positions. If we perform this in Swift code, therefore, we should find that the result is 3 (00000011):
let x = 171
let y = 3
let z = x & y
print("Result is \(z)")
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Bitwise OR
The bitwise OR also performs a bit by bit comparison of two binary sequences. Unlike the AND operation, the OR places a 1 in the result if there is a 1 in the first or second operand. The operator is represented by a single vertical bar character (|). Using our example numbers, the result will be as follows:
10101011 OR 00000011 ======== 10101011
If we perform this operation in a Swift example the result will be 171:
let x = 171
let y = 3
let z = x | y
print("Result is \(z)")
Bitwise XOR
The bitwise XOR (commonly referred to as exclusive OR and represented by the caret '^' character) performs a similar task to the OR operation except that a 1 is placed in the result if one or other corresponding bit positions in the two numbers is 1. If both positions are a 1 or a 0 then the corresponding bit in the result is set to a 0. For example:
10101011 XOR 00000011 ======== 10101000
The result in this case is 10101000 which converts to 168 in decimal. To verify this we can, once again, try some Swift code:
let x = 171
let y = 3
let z = x ^ y
print("Result is \(z)")
When executed, we get the following output from print:
Result is 168
Bitwise Left Shift
The bitwise left shift moves each bit in a binary number a specified number of positions to the left. Shifting an integer one position to the left has the effect of doubling the value.
As the bits are shifted to the left, zeros are placed in the vacated right most (low order) positions. Note also that once the left most (high order) bits are shifted beyond the size of the variable containing the value, those high order bits are discarded:
10101011 Left Shift one bit ======== 101010110
In Swift the bitwise left shift operator is represented by the '<<' sequence, followed by the number of bit positions to be shifted. For example, to shift left by 1 bit:
let x = 171
let z = x << 1
print("Result is \(z)")
When compiled and executed, the above code will display a message stating that the result is 342 which, when converted to binary, equates to 101010110.
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Bitwise Right Shift
A bitwise right shift is, as you might expect, the same as a left except that the shift takes place in the opposite direction. Shifting an integer one position to the right has the effect of halving the value.
Note that since we are shifting to the right there is no opportunity to retain the lower most bits regardless of the data type used to contain the result. As a result the low order bits are discarded. Whether or not the vacated high order bit positions are replaced with zeros or ones depends on whether the sign bit used to indicate positive and negative numbers is set or not.
10101011 Right Shift one bit ======== 01010101
The bitwise right shift is represented by the '>>' character sequence followed by the shift count:
let x = 171
let z = x >> 1
print("Result is \(z)")
When executed, the above code will report the result of the shift as being 85, which equates to binary 01010101.
Compound Bitwise Operators
As with the arithmetic operators, each bitwise operator has a corresponding compound operator that allows the operation and assignment to be performed using a single operator:
| Operator | Description |
|---|---|
| x &= y | Perform a bitwise AND of x and y and assign result to x |
| x |= y | Perform a bitwise OR of x and y and assign result to x |
| x ^= y | Perform a bitwise XOR of x and y and assign result to x |
| x <<= n | Shift x left by n places and assign result to x |
| x >>= n | Shift x right by n places and assign result to x |
Summary
Operators and expressions provide the underlying mechanism by which variables and constants are manipulated and evaluated within Swift code. This can take the simplest of forms whereby two numbers are added using the addition operator in an expression and the result stored in a variable using the assignment operator. Operators fall into a range of categories, details of which have been covered in this chapter.
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| Swift Data Types, Constants and Variables | Swift Flow Control |
