Variable definitions were introduced in the section called “The Basics of Variable Definitions”.
We will now examine them in greater detail.
A variable definition allocates memory to store a value, and assigns
the memory location a name and a data type. The compiler uses the data
type to determine which machine instructions to use to process the
data. For example, adding two integers is done with a different
machine instruction than adding two floating point values. The
integer add instruction is faster, so defining a variable as
int rather than
double will lead to a
A C variable definition consists of a data type followed by one or more variable names separated by commas, and ultimately a semicolon:
type name [, name ...];
double height, width, area;
A Fortran 90 variable definition consists of a type, followed by two colons, followed by a list of variable names separated by commas:
type :: name [, name ...]
real(8) :: height, width, area
Variable names must begin with a letter or an underscore, and may contain letters, underscores, and digits after that.
The words “definition” and “declaration”
are often used interchangeably in programming. In the context of
languages such as C and C++, definition and declaration have
different meanings. A definition in C allocates memory, whereas
a declaration merely alludes to a variable or function defined
elsewhere. For example, all C programs contain a global variable
errno, which contains the error code from
the most recent standard library function. We can
declare it in a given function by using the
extern modifier to tell the compiler that this variable
is defined somewhere else and we want to access it from here:
extern int errno; // Declare (allude to), not define, errno
extern modifier, we would be defining
a new local variable called
rather than alluding to the one already defined globally.
The local variable would take precedence over the preexisting
global variable under the variable scope rules of C, which
means it would be impossible to access the global variable
where the local variable is defined.
Note that while we can declare
an allusion as shown above, the modern method is to include
errno.h, which contains this allusion:
Both C and Fortran support assigning an initial value to a variable in the definition.
double sum = 0.0;
real(8) :: sum = 0.0d0
Using this feature reduces the length of a program slightly by eliminating an assignment statement further down. Some would argue that it is less cohesive, however. It is generally a good practice to group related statements together in one cohesive block. In this context, it would mean initializing variables immediately before the code that depends on that initial value. This way, we can see that the variable is initialized properly without having to scroll up or search through the source code. This will save time and distractions while debugging.
double sum; // Suppose there are 100 lines of additional code here // A cohesive block of code sum = 0.0; for (c = 0; c < list_size; ++c) sum += value;
double sum = 0.0; // Suppose there are 100 lines of additional code here // This is not cohesive since the initialization of sum is far away from // the loop that requires it for (c = 0; c < list_size; ++c) sum += value;
Memory locations for variables are generally allocated together in a block. Suppose we have the following variable definitions:
double height, width, area; int list_size, age;
real(8) :: height, width, area integer :: list_size, age
The program will have 8 bytes (64 bits) of memory allocated for each double or real(8) variable, and 4 bytes (32 bits) for each integer variable. A possible map of the memory space, assuming the block begins at memory address 4000, is shown in Table 16.3, “Memory Map”. Recall that each memory location contains 1 byte (8 bits), so each of these variables will occupy multiple memory addresses.
Table 16.3. Memory Map
|4000 to 4007||height|
|4008 to 4015||width|
|4016 to 4023||area|
|4024 to 4027||list_size|
|4028 to 4031||age|
What does a variable definition do?
What are the rules for naming variables?
What does a variable declaration do?
int age, year; float gpa;