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44 Cards in this Set
- Front
- Back
Imperative languages are |
abstractions of von Neumann architecture – Memory – Processor |
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Variables are characterized |
by attributes – To design a type, must consider scope, lifetime, type checking, initialization, and type compatibility |
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Names |
• Design issues for names: – Are names case sensitive? – Are special words reserved words or keywords? |
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Names (continued) • 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 |
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Names (continued) • Special characters |
– 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 |
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Names (continued) • 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) |
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Names (continued) • Special words |
– An aid to readability; used to delimit or separate statement clauses • A keyword is a word that is special only in certain contexts, e.g., in Fortran – Real VarName (Real is a data type followed with a name, therefore Real is a keyword) – Real = 3.4 (Real is a variable) – A reserved word is a special word that cannot be used as a user-defined name – Potential problem with reserved words: If there are too many, many collisions occur (e.g., COBOL has 300 reserved words!) |
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A variable is |
an abstraction of a memory cell • Variables can be characterized as a sextuple of attributes: – Name – Address – Value – Type – Lifetime – Scope |
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Variables Attributes • Name |
- not all variables have them |
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Variables Attributes Address - |
- the memory address with which it is associated – A variable may have different addresses at different times during execution – A variable may have different addresses at different places in a program – If two variable names can be used to access the same memory location, they are called aliases – Aliases are created via pointers, reference variables, C and C++ unions – Aliases are harmful to readability (program readers must remember all of them) |
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Variables Attributes (continued) • Type - |
- determines the range of values of variables and the set of operations that are defined for values of that type; in the case of floating point, type also determines the precision |
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Variables Attributes (continued)• Value - |
- the contents of the location with which the variable is associated - The l-value of a variable is its address - The r-value of a variable is its value |
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Variables Attributes (continued) • Abstract memory cell - |
- the physical cell or collection of cells associated with a variable |
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A binding is an |
association between an entity and an attribute, such as between a variable and its type or value, or between an operation and a symbol |
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• Binding time |
is the time at which a binding takes place. |
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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 |
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• A binding is static if |
it first occurs before run time and remains unchanged throughout program execution. v |
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• A binding is dynamic |
c if it first occurs during execution or can change during execution of the program |
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Type Binding |
• How is a type specified? • When does the binding take place? • If static, the type may be specified by either an explicit or an implicit declaration |
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• An explicit declaration is |
a program statement used for declaring the types of variables |
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• An implicit declaration is |
a default mechanism for specifying types of variables through default conventions, rather than declaration statements • Fortran, BASIC, Perl, Ruby, JavaScript, and PHP provide implicit declarations (Fortran has both explicit and implicit) – Advantage: writability (a minor convenience) – Disadvantage: reliability (less trouble with Perl) |
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Explicit/Implicit Declaration (continued) • Some languages use |
type inferencing to determine types of variables (context) – C# - a variable can be declared with var and an initial value. The initial value sets the type – Visual BASIC 9.0+, ML, Haskell, F#, and Go use type inferencing. The context of the appearance of a variable determines its type |
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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 |
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Variable Attributes (continued) • Storage Bindings & Lifetime |
– Allocation - getting a cell from some pool of available cells – Deallocation - putting a cell back into the pool • The lifetime of a variable is the time during which it is bound to a particular memory cell |
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Categories of Variables by Lifetimes • Static |
--bound to memory cells before execution begins and remains bound to the same memory cell throughout execution, e.g., C and C++ static variables in functions – Advantages: efficiency (direct addressing), history-sensitive subprogram support – Disadvantage: lack of flexibility (no recursion) |
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Categories of Variables by Lifetimes • Stack-dynamic |
--Storage bindings are created for variables when their declaration statements are elaborated. (A declaration is elaborated when the executable code associated with it is executed) • If scalar, all attributes except address are statically bound – local variables in C subprograms (not declared static) and Java methods • Advantage: allows recursion; conserves storage • Disadvantages: – Overhead of allocation and deallocation – Subprograms cannot be history sensitive – Inefficient references (indirect addressing) |
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Categories of Variables by Lifetimes • Explicit heap-dynamic |
-- Allocated and deallocated by explicit directives, specified by the programmer, which take effect during execution • Referenced only through pointers or references, e.g. dynamic objects in C++ (via new and delete), all objects in Java • Advantage: provides for dynamic storage management • Disadvantage: inefficient and unreliable |
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Categories of Variables by Lifetimes • Implicit heap-dynamic- |
c--Allocation and deallocation caused by assignment statements – all variables in APL; all strings and arrays in Perl, JavaScript, and PHP • Advantage: flexibility (generic code) • Disadvantages: – Inefficient, because all attributes are dynamic – Loss of error detection |
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Variable Attributes: 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 |
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Static Scope |
• Based on program text • To connect a name reference to a variable, you (or the compiler) must find the declaration • Search process: search declarations, first locally, then in increasingly larger enclosing scopes, until one is found for the given name • Enclosing static scopes (to a specific scope) are called its static ancestors; the nearest static ancestor is called a static parent • Some languages allow nested subprogram definitions, which create nested static scopes (e.g., Ada, JavaScript, Common LISP, Scheme, Fortran 2003+, F#, and Python)
• 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 |
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Blocks |
– A method of creating static scopes inside program units--from ALGOL 60- Note: legal in C and C++, but not in Java and C# - too error-prone |
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Declaration Order |
• C99, C++, Java, and C# allow variable declarations to appear anywhere a statement can appear – In C99, C++, and Java, the scope of all local variables is from the declaration to the end of the block – In C#, the scope of any variable declared in a block is the whole block, regardless of the position of the declaration in the block • However, a variable still must be declared before it can be used |
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The LET Construct |
• Most functional languages include some form of let construct • A let construct has two parts – The first part binds names to values – The second part uses the names defined in the first part |
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• In C++, Java, and C#, variables can be declared in |
for statements – The scope of such variables is restricted to the for construct |
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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 |
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Global Scope (continued) • 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 |
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Global Scope (continued) • Python |
– A global variable can be referenced in functions, but can be assigned in a function only if it has been declared to be global in the function |
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Evaluation of Static Scoping |
• Works well in many situations • Problems: – In most cases, too much access is possible – As a program evolves, the initial structure is destroyed and local variables often become global; subprograms also gravitate toward become global, rather than nested |
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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 |
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Scope Example • Evaluation of Dynamic Scoping: |
– Advantage: convenience – Disadvantages: 1. While a subprogram is executing, its variables are visible to all subprograms it calls 2. Impossible to statically type check 3. Poor readability- it is not possible to statically determine the type of a variable |
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Scope and Lifetime |
• Scope and lifetime are sometimes closely related, but are different concepts • Consider a static variable in a C or C++ function |
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• The referencing environment of a statement is the collection of |
all names that are visible in the statement • In a static-scoped language, it is the local variables plus all of the visible variables in all of the enclosing scopes • A subprogram is active if its execution has begun but has not yet terminated • In a dynamic-scoped language, the referencing environment is the local variables plus all visible variables in all active subprograms |
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Named Constants |
• A named constant is a variable that is bound to a value only when it is bound to storage • Advantages: readability and modifiability • Used to parameterize programs • The binding of values to named constants can be either static (called manifest constants) or dynamic • Languages: – Ada, C++, and Java: expressions of any kind, dynamically bound – C# has two kinds, readonly and const - the values of const named constants are bound at compile time - The values of readonly named constants are dynamically bound |
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Summary |
• Case sensitivity and the relationship of names to special words represent design issues of names • Variables are characterized by the sextuples: name, address, value, type, lifetime, scope • Binding is the association of attributes with program entities • Scalar variables are categorized as: static, stack dynamic, explicit heap dynamic, implicit heap dynamic • Strong typing means detecting all type errors |