A variable is a seqeuence of program code with a name (also called its identifier ). A name or identifier in C can be anything from a single letter to a word. The name of a variable must begin with an alphabetic letter or the underscore ‘_’ character but the other characters in the name can be chosen from the following groups:

a .. z (any letter from a to z)
A .. Z (any letter from A to Z)
0 .. 9 (any digit from 0 to 9)
_ (the underscore character)

Some examples of valid variable names are:

a total Out_of_Memory VAR integer etc... In C variables do not only have names: they also have types. The type of a variable conveys to the the compiler what sort of data will be stored
in it. In BASIC and in some older, largely obsolete languages, like PL/1, a special naming convention is used to determine the sort of data which can be held in particular variables. e.g. the dollar symbol ‘$’ is commonly used in BASIC to mean that a variable is a string and the percentage ‘%’ symbol is used to indicate an integer. No such convention exists in C. Instead we specify the types of variables in their declarations.

This serves two purposes:

• It gives a compiler precise information about the amount of memory
that will  have to be given over to a variable when a program is finally
run and what sort of arithmetic will have to be used on it (e.g. integer
only or floating point or none).

• It provides the compiler with a list of the variables in a convenient place
so that it can cross check names and types for any errors.
There is a lot of different possible types in C. In fact it is possible for
us to define our own, but there is no need to do this right away: there are
some basic types which are provided by C ready for use. The names of these
types are all reserved words in C and they are summarized as follows:

char A single ASCII character
short A short integer (usually 16-bits)
short int A short integer
int A standard integer (usually 32-bits)

long A long integer
long int A long integer (usually 32-bits, but increasingly 64 bits)
float A floating point or real number (short)
long float
a long floating point number
double A long floating point number

There is some repetition in these words. In addition to the above, the word
unsigned can also be placed in front of any of these types. Unsigned means
that only positive or zero values can be used. (i.e. there is no minus sign).
The advantage of using this kind of variable is that storing a minus sign
takes up some memory, so that if no minus sign is present, larger numbers
can be stored in the same kind of variable. The ANSI standard also allows
the word signed to be placed in front of any of these types, so indicate
the opposite of unsigned. On some systems variables are signed by default,
whereas on others they are not.

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