Variables
Variables are locations in a computer's memory.
A typical desktop or laptop computer will have 4-16GB of memory,
or four-sixteen billion addressable memory locations,
A typical C program will use 4 bytes to hold a single integer value,
or 8 bytes to hold a single floating-point value.
Any variable can only hold a single value at any time - they do not
maintain a history of past values they once had.
Naming our variables
To make programs more readable,
we provide variables with simple names.
We should carefully choose names to reflect the role
of the variable in our programs.
- While variable names can be almost anything
(but not the same as the keywords in C)
there's a simple restriction on the permitted characters in a name -
- they must commence with an alphabetic or the underscore character
(_ A-Z a-z), and
- be followed by zero or more alphabetic,
underscore or digit characters (_ A-Z a-z 0-9).
- C variable names are case sensitive, thus:
MYLIMIT, mylimit, Mylimit and MyLimit
are four different variable names.
- While not required,
it's preferred that variable names do not consist entirely
of uppercase characters.
We'll consistently use uppercase-only
names for constants provided by the C preprocessor,
or user-defined type names:
MAXLENGTH, AVATAR, BUFSIZ, and ROT
- Older C compilers limited variable names to, say, 8 unique characters.
Thus, for them,
turn_nuclear_reactor_coolant_on and turn_nuclear_reactor_coolant_off
are the same variable!
Keep this in mind if ever developing portable code for old environments.
CITS2002 Systems Programming, Lecture 2, p4, 25th July 2023.
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CITS2002