- Variables in a program store information needed by the
program to perform its task
- Each variable in modern programming languages has:
- a name:
symbolic name used to
identify (refer to) the variable
- an address:
the location in memory where variable is stored
- the computer uses the address to
find the variable in
memory
- a type:
tells the compiler
the size of the variable and
how to use the value in the variable -
see discussion about data type
( click here )
- a scope:
the range
where the variable is accessible/visible
More on scoping much later... - a lifetime:
the duration when the variable exists
(variables can be "created" and "destroyed" while the program is running)
More on lifetime much later...See the following note on "creating" and "destroying" variables: click here
- a name:
symbolic name used to
identify (refer to) the variable
- Variable name consists of:
- letters
- digits
- _ (underscore)
- Variable name must begin with a letter
- Defining uninitialized variables:
Uninitialized variables do not have a usable value when the variable is created (i.e., you need to initialize it before you can use its value)
DataType VariableName1, VariableName2, ... Examples: int a, b, c; // 3 uninitialzied integer variables double x, y; // 2 uninitialzied double precision floats (NOTE: semi-colon is used to terminate a variable definition) - Defining initialized variables:
DataType VarName1=Value1, VarName2=Value2, ... Examples: int a=5, b=9, c=7; // 3 initialzied integer variables double x=7.0, y; // 1 initialzied and 1 uninitialzied // double precision floats
- Variables are used by the computer program to store values
To do so, the computer needs to use some memory (see click here)
- When a computer program is started to run inside the computer,
a
portion
of the computer memory will be taken up by the
computer program , but
another portion
remains
unused.
- Variable definitions will cause the computer to
obtain (i.e., reserve)
some UNUSED memory space for the variable.
- This is easier said that done
and
a number of things must happen before all will go well:
- The computer must make sure that the
memory space
must
NOT be used (to store another variable or program instruction)...
- The computer must make sure that it can find the variable back when it is used...
- The computer must make sure that the
memory space
must
NOT be used (to store another variable or program instruction)...
- The compiler must remember the following
2 pieces of information
to find a variable in memory:
- location (or address)
where the variable is stored in memory
- type (implies the size) (computer need to know the number of bytes to retrieve and what operations it can perform to the value)
- location (or address)
where the variable is stored in memory
- The following figure summarized what happens inside the computer
when a variable is defined:
- Compiler encounters a variable definition
(" int a; ")
- Compiler find and allocate space for the new variable
- Compiler store Name, Address, Type (and size) information about the new variable in its " symbol table"
- Compiler encounters a variable definition
(" int a; ")
- NOTE:
I will use the following picture to represent the fact
that variables have:
- a name
- an address
- a value
- Note: the variable also has a type; it is not represented in the figure, but it is assumed that the computer also know that (we don't need to use the data type of the variable so much)
