Objective-C 2.0 Data Types

When we look at the different types of software that run on computer systems, from financial applications to graphics intensive games, it is easy to forget that computers are really just binary machines. Binary systems work in terms of 0 and 1, true or false, set and unset. All the data sitting in RAM, stored on disk drives and flowing through circuit boards and buses are nothing more than sequences of 1s and 0s. Each 1 or 0 is referred to as a bit and bits are grouped together in blocks of 8, each group being referred to a byte. When people talk about 32-bit and 64-bit computer systems they are talking about the number of bits that can be handled simultaneously by the CPU bus. A 64-bit CPU, for example, is able to handle data in 64-bit blocks, resulting in faster performance than a 32-bit based system.

Humans, of course, don't think in binary. We work with decimal numbers, letters and words. In order for a human to easily (easily being a relative term in this context) program a computer, some middle ground between human and computer thinking is needed. This is where programming languages such as Objective-C come into play. Programming languages allow us humans to express instructions to a computer in terms and structures we understand, and then compile it down to a format that can be executed by a computer.

One of the fundamentals of any program involves data, and programming languages such as Objective-C define a set of data types that allow us to work with data in a format we understand when writing a computer program. For example, if we want to store a number in an Objective-C program we could do so with the following syntax:

int mynumber = 10;

In the above example, we have created a variable named mynumber of data type integer by using the keyword int. We then assigned the value of 10 to this variable. Once we know that int means we are specifying a variable of integer data type we have an understanding of what is happening in this particular line of an Objective-C program. When we compile the source code down to the machine code used by the CPU, the the number 10 is seen by the computer in binary as:

1010

Similarly, we can express a letter, the visual representation of a digit ('0' through to '9') or punctuation mark (referred to in computer terminology as characters) using the following syntax:

char myletter = 'c';

Once again, this is understandable by a human programmer, but gets compiled down to a binary sequence for the CPU to understand. In this case, the letter 'c' is represented by the decimal number 99 using the ASCII table (an internationally recognized standard that assigns numeric values to human readable characters). When converted to binary, it is stored as:

10101100011

Now that we have a basic understanding of the concept of data types and why they are necessary we can take a closer look at the different data types and qualifiers supported by Objective-C.

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int Data Type

The Objective-C int data type can store a positive or negative whole number (in other words a number with no decimal places). The actual size, or range of integer that can be handle by the int data type is machine dependent. Typically the amount of storage allocated to int values in either 32-bit or 64-bit depending on the implementation of Objective-C on that platform or the CPU on which the compiler is running. it is important to note, however, that the operating system also plays a role in whether ints are 32 or 64-bit. For example the CPU in a computer may be 64-bit but the operating system running on it may only be 32-bit.

For example, on a 32-bit implementation, the maximum range of an unsigned int is 0 to 4294967295. On a 64-bit system this range would be 0 to 18,446,744,073,709,551,615. When dealing with signed int values, the ranges are −2,147,483,648 to +2,147,483,647 and −9,223,372,036,854,775,808 to +9,223,372,036,854,775,807 for 32-bit and 64-bit implementations respectively.

When writing an Objective-C program, the only guarantee you have is that an int will be at least 32-bits. To avoid future problems when compiling the code on other platforms it is safer to limit int values to the 32-bit range, rather than assume that 64-bit will be available.

By default, int values are decimal (i.e. based on number base 10). To express an int in Octal (number base 8) simply precede the number with a zero (0). For example:

int myoctal = 024;

Similarly, an int may be expressed in number base 14 (hexadecimal) by preceding the number with 0x, for example:

int myhex = 0xFFA2;

char Data Type

The Objective-C char data type is used to store a single character such as a letter, numerical digit or punctuation mark or space character. For example, the following lines assign a variety of different characters to char type variables:

char myChar = 'w';
char myChar = '2';
char myChar = ':';

Special Characters/Escape Sequences

In addition to the standard set of characters outlined above, there are also a range of special characters (also referred to as escape sequences) available for specifying items such as a new line or tab. These special characters are identified by prefixing the character with a backslash a concept referred to as escaping. For example, the following assigns a new line to the variable named newline:

char newline = '\n';

In essence, any character that is preceded by a backslash is considered to be a special character and is treating accordingly. This raises the question as to what to do if you actually want a backslash character. This is achieved by escaping the backslash itself:

char myslash = '\\'; Assign a backslash to a variable

Commonly used special characters supported by Objective-C are as follows:

\a - Sound alert \b - Backspace \f - Form feed \n - New line \r - Carriage return \t - Horizontal tab \v - Vertical tab \\ - Backslash \" - Double quote (when using when placing a double quote into a string declaration) \' - Single quote (when using when placing a double quote into a string declaration)

float Data Type

The Objective-C float data type is used to store floating point values, in other words values containing decimal places. For example, 456.12 would be stored in a float data type. In practice, all floating point values are stored as a different data type (called double) by default. We will be covering the double data type next, but if you specifically want to use a float data type, you must append an f into the end of the value. For example:

float myfloat = 123.432f

For convenience when working with exceptionally large number, both floating point and double data type values can be specified using scientific notation (also known as standard form or exponential notation). For example, we can express 67.7 x 10⁴ in Objective-C as:

float myfloat = 67.7e4

double Data Type

The Objective-C double data type is used to store larger values than can be handled by the float data type. The term double comes from the fact that a double can handle values twice the size of a float. As previously mentioned, all floating point values are stored as double data types unless the value is followed by an 'f' to specifically specify a float.