Difference between revisions of "Objective-C Operators and Expressions"

Revision as of 14:35, 8 October 2009

In the previous chapters we looked at using variables and constants in Objective-C 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 Objective-C code. The primary method for working with data is in the form of expressions. In this chapter we will look in detail at Objective-C expressions and operators.

What is an Expression?

The most basic expression consists of an operator, two operands and an assignment. The following is an example of an expression:

int 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 an integer 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 various types of operators available in Objective-C.

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 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. The following examples are all valid uses of the assignment operator:

int x; // declare the variable

x = 10;  // Assigns the value 10 to a variable named x

x = y + z; // Assigns the result of variable y added to variable z to variable x

x = y;   // Assigns the value of variable y to variable x

Assignment operators may also be chained to assign the same value to multiple variables. For example, the following code example assigns the value 20 to the x, y and z variables:

int x, y, z;

x = y = z = 20;

Objective-C Arithmetic Operators

Objective-C 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:

int x = -10; // Unary - operator used to assign -10 to a variable named x

x = y - z; // Subtraction operator. Subtracts z from y

<google>ADSDAQBOX_FLOW</google> The following table lists the primary Objective-C arithmetic operators:

Note that multiple operators may be used in a single expression.

For example:

x = y * 10 + z - 5 / 4;

Whilst the above code is perfectly valid it is important to be aware that Objective-C does not evaluate the expression from left to right or right to left, but rather in an order specified by the precedence of the various operators. Operator precedence is an important topic to understand since it impacts the result of a calculation and will be covered in detail the chapter entitled Objective-C 2.0 Operator Precedence.

Compound Assignment Operators

In an earlier section we looked at the basic assignment operator (=). Objective-C 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

The above expression performs exactly the same task as x = x + y but saves the programmer some typing.

Numerous compound assignment operators are available in Objective-C. The most frequently used are outlined in the following table:

Increment and Decrement Operators

Another useful shortcut can be achieved using the Objective-C increment and decrement operators (also referred to as unary operators because they operate on a single operand). As with the compound assignment operators described in the previous section, consider the following Objective-C code fragment:

x = x + 1; // Increase value of variable x by 1

x = x - 1; // Decrease value of variable y by 1

These expressions increment and decrement the value of x by 1. Instead of using this approach 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 is performed before any other operations are performed on the variable. For example, in the following example, x is incremented before it is assigned to y, leaving y with a value of 10:

int x = 9;
int y;

y = ++x;

In the following example, 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.

int x = 9;
int y;

y = x--;

Comparison Operators

In addition to mathematical and assignment operators, C# also includes set of logical operators useful for performing comparisons. These operators all return a Boolean (bool) true or false result depending on the result of the comparison. These operators are binary in that they work with two operands.

Comparison operators are most frequently used in constructing program flow control. For example an if statement may be constructed based on whether one value matches another:

if (x == y)
      System.Console.WriteLine ("x is equal to y");

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:

bool result;
int x = 10;
int y = 20;

result = x < y;

Clearly 10 is less than 20, resulting in a true evaluation of the x < y expression. The following table lists the full set of C# comparison operators:

Boolean Logical Operators

Objective-C also provides a set of so called logical operators designed to return boolean true and false. In practice true equates to 1 and false equates to 0. These operators both return boolean results and take boolean values as operands. The key operators are NOT (!), AND (&&), OR (||) and XOR (^).

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 1 (true), prefixing the variable with a '!' character will invert the value to 0 (false):

bool flag = true; //variable is true
bool secondFlag;

secondFlag = !flag; // secondFlag set to false

The OR (||) operator returns 1 if one of its two operands evaluates to true, otherwise it returns 0. For example, the following example evaluates to true because at least one of the expressions either side of the OR operator is true:

if ((10 < 20) || (20 < 10))
        NSLog (@"Expression is true");

The AND (&&) operator returns 1 only if both operands evaluate to be true. The following example will return 0 because only one of the two operand expressions evaluates to true:

if ((10 < 20) && (20 < 10))
      NSLog (@"Expression is true");

The XOR (^) operator returns 1 if one and only one of the two operands evaluates to true. For example, the following example will return 1 since only one operator evaluates to be true:

if ((10 < 20) ^ (20 < 10))
      System.Console.WriteLine("Expression is true");

If both operands evaluated to be true, or both were false the expression with return false.

The Ternary Operator

Objective-C uses something called a ternary operator to provide a shortcut way of making decisions. The syntax of the ternary operator (also known as the conditional operator) is as follows:

[condition] ? [true expression] : [false expression]

The way this works is that [condition] is replaced with an expression that will return either true (1) or false (0). 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:

int x = 10;
int y = 20;

NSLog(@"Largest number is %i", 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 NSLog call for display to the user:

2009-10-07 11:14:06.756 t[5724] Largest number is 20

Bitwise Operators

In the chapter entitled Objective-C 2.0 Data Types we talked about the fact that computers 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, Objective-C provides a range of bit operators. Those familiar with bitwise operators in other languages such as C, C++, C# and Java will find nothing new in the this area of the Objective-C 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 Objective-C's bitwise operators:

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 Objective-C code, therefore, we should find that the result is 3 (00000011):

        int x = 171;
        int y = 3;
        int z;

        z = x & y; // Perform a bitwise AND on the values held by variables x and y

        NSLog(@"Result is %i", z);

2009-10-07 15:38:09.176 t[12919] Result is 3

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 the our example numbers, the result will be as follows:

10101011 OR
00000011
========
10101011

If we perform this operation in an Objective-C example we see the following:

        int x = 171;
        int y = 3;
        int z;

        z = x | y;

        NSLog(@"Result is %i", z);


2009-10-07 15:41:39.647 t[13153] Result is 171

Bitwise XOR

The bitwise XOR (commonly referred to as exclusive OR and represent 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 Objective-C code:

int x = 171;
int y = 3;
int z;

z = x ^ y;

NSLog(@"Result is %i", z);

When executed, we get the following output from NSLog:

2009-10-07 16:09:40.097 t[13790] 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. As the bits are shifted to the left, zeros are placed in the vacated right most (low order) positions. Note also that the once the left most (high order) bits are shifted beyond the size of the variable containing the value, those high order are discarded:

10101011 Left Shift one bit
========
101010110

In Objective-C 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:

int x = 171;
int z;

z = x << 1;

NSLog(@"Result is %i", 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.

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. 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 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 and, unfortunately, on the particular system and Objective-C implementation in use.

10101011 Right Shift one bit
========
01010101

The bitwise right shift is represented by the '>> character sequence followed by the shift count:

int x = 171;
int z;

z = x >> 1;

NSLog(@"Result is %i", 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: