Variables

Like most programming languages, C is able to use and process named variables and their contents. Variables are simply names used to refer to some location in memory - a location that holds a value with which we are working.

It may help to think of variables as a placeholder for a value. You can think of a variable as being equivalent to its assigned value. So, if you have a variable i that is initialized (set equal) to 4, then it follows that i+1 will equal 5.

Since C is a relatively low-level programming language, before a C program can utilize memory to store a variable it must claim the memory needed to store the values for a variable . This is done by declaring variables. Declaring variables is the way in which a C program shows the number of variables it needs, what they are going to be named, and how much memory they will need.

Within the C programming language, when managing and working with variables, it is important to know the type of variables and the size of these types. Since C is a fairly low-level programming language, these aspects of its working can be hardware specific - that is, how the language is made to work on one type of machine can be different from how it is made to work on another.

All variables in C are "typed". That is, every variable declared must be assigned as a certain type of variable.

Declaring, Initializing, and Assigning Variables

Here is an example of declaring an integer, which we've called some_number. (Note the semicolon at the end of the line; that is how your compiler separates one program statement from another.)

int some_number;

This statement means we're declaring some space for a variable called some_number, which will be used to store integer data. Note that we must specify the type of data that a variable will store. There are specific keywords to do this - we'll look at them in the next section.

Multiple variables can be declared with one statement, like this:

int anumber, anothernumber, yetanothernumber;

We can also declare and assign some content to a variable at the same time. This is called initialization because it is the "initial" time a value has been assigned to the variable:

int some_number=3;

In C, all variable declarations (except for globals) must be done at the beginning of a block. You cannot declare your variables, insert some other statements, and then declare more variables. Variable declarations (if there are any) are always the first part of any block.

After declaring variables, you can assign a value to a variable later on using a statement like this:

some_number=3;

You can also assign a variable the value of another variable, like so:

anumber = anothernumber;

Or assign multiple variables the same value with one statement:

anumber = anothernumber = yetanothernumber = 3;

This is because the assignment ( x = y) returns the value of the assignment. x = y = z is really shorthand for x = (y = z).

Naming Variables

Variable names in C are made up of letters (upper and lower case) and digits. The underscore character ("_") is also permitted. Names must not begin with a digit. Unlike some languages (such as Perl and some BASIC dialects), C does not use any special prefix characters on variable names.

Some examples of valid (but not very descriptive) C variable names:

foo
Bar
BAZ
foo_bar
_foo42
_
QuUx

Some examples of invalid C variable names:

2foo (must not begin with a digit)
my foo (spaces not allowed in names)
$foo ($ not allowed -- only letters, digits, and _)
while (language keywords cannot be used as names)

As the last example suggests, certain words are reserved as keywords in the language, and these cannot be used as variable names.

In addition there are certain sets of names that, while not language keywords, are reserved for one reason or another. For example, a C compiler might use certain names "behind the scenes", and this might cause problems for a program that attempts to use them. Also, some names are reserved for possible future use in the C standard library. The rules for determining exactly what names are reserved (and in what contexts they are reserved) are too complicated to describe here, and as a beginner you don't need to worry about them much anyway. For now, just avoid using names that begin with an underscore character.

The naming rules for C variables also apply to other language constructs such as function names, struct tags, and macros, all of which will be covered later.

Literals

Anytime within a program in which you specify a value explicitly instead of referring to a variable or some other form of data, that value is referred to as a literal. In the initialization example above, 3 is a literal. Literals can either take a form defined by their type (more on that soon), or one can use hexadecimal (hex) notation to directly insert data into a variable regardless of its type. Hex numbers are always preceded with 0x. For now, though, you probably shouldn't be too concerned with hex.

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