Names
•Length
–If too short, they cannot be connotative
–Language examples:
•FORTRAN 95: maximum of 31
•C99: no limit but only the first 63 are significant; also, external names are limited to a maximum of 31
•C#, Ada, and Java: no limit, and all are significant
•C++: no limit, but implementers often impose one
•Special character
–PHP: all variable names must begin with dollar signs
–Perl: all variable names begin with special characters, which specify the variable’s type
–Ruby: variable names that begin with @ are instance variables; those that begin with @@ are class variables
•Case sensitivity
–Disadvantage: readability (names that look alike are different)
•Names in the C-based languages are case sensitive
•Names in others are not
•Worse in C++, Java, and C# because predefined names are mixed case (e.g. IndexOutOfBoundsException)
Possible Binding Times
•Language design time -- bind operator symbols to operations
•Language implementation time-- bind floating point type to a representation
•Compile time -- bind a variable to a type in C or Java
•Load time -- bind a C or C++ static variable to a memory cell)
•Runtime -- bind a nonstatic local variable to a memory cell
Dynamic Type Binding
•Dynamic Type Binding (JavaScript, Python, Ruby, PHP, and C# (limited))•Specified through an assignment statement e.g., JavaScript
list = [2, 4.33, 6, 8];
list = 17.3;
–Advantage: flexibility (generic program units)
–Disadvantages:
•High cost (dynamic type checking and interpretation)
•Type error detection by the compiler is difficult Scope
•The scope of a variable is the range of statements
over which it is visible
•The local
variables of a
program unit are those that are declared in that unit
•The nonlocal
variables of a
program unit are those that are visible in the unit but not declared there
•Global variables are a special category of nonlocal
variables
•The scope rules of a language determine
how references to names are associated with variables
•Variables can be hidden from a unit by
having a "closer" variable with the same name
•Ada allows access to these
"hidden" variables
–E.g., unit.name
Global Scope
•C, C++, PHP, and Python support a program
structure that consists of a sequence of function definitions in a file
–These
languages allow variable declarations to appear outside function definitions
•C and C++have both declarations (just
attributes) and definitions (attributes and storage)
–A declaration outside a function definition specifies that it is defined in another file
•PHP
–Programs
are embedded in HTML markup documents, in any number of fragments, some
statements and some function definitions
–The
scope of a variable (implicitly) declared in a function is local to the
function
–The
scope of a variable implicitly declared outside functions is from the
declaration to the end of the program, but skips over any intervening functions
•Global
variables can be accessed in a function through the $GLOBALS
array or by declaring it global
Dynamic Scope
•Based on calling sequences of program units, not their textual layout (temporal versus spatial)
•References to variables are connected to declarations by searching back through the chain of subprogram calls that forced execution to this point
Scope Example
function
big() {
function
sub1()
var
x = 7;
function
sub2() {
var
y = x;
}
var
x = 3;
}
–Static
scoping
•Reference
to x in sub2 is
to big's x
–Dynamic
scoping
•Reference
to x in sub2 is
to sub1's x
Reference
•Robert W. Sebesta - Concept of Programming Languages (Tenth Edition), Chapter
5
