OutlineNames and VariablesBindingBinding CategoriesScopeCS421 Lecture 16: Names and Variables1Mark [email protected] of Illinois at Urbana-ChampaignJuly 17, 20061Based on slides by Mattox Beckman, as updated by Vikram Adve, GulAgha, and Elsa GunterMark Hills CS421 Lecture 16: Names and VariablesOutlineNames and VariablesBindingBinding CategoriesScopeNames and VariablesBindingBinding CategoriesScopeMark Hills CS421 Lecture 16: Names and VariablesOutlineNames and VariablesBindingBinding CategoriesScopeObjectivesBy the end of this lecture, you should◮understand the attributes of variables in a programminglanguage, including concepts such as type and value◮understand the distinction between static and dynamicproperties of variables◮be familiar with the concept of scope, specifically differencesbetween static and dynamic scopeMark Hills CS421 Lecture 16: Names and VariablesOutlineNames and VariablesBindingBinding CategoriesScopeOverviewVariable AttributesNamesDef: A name is a string of characters used to identify some entityin a program2◮Names (or identifiers) give us a way to name programconcepts, such as types, values, classes, functions, etc.◮Different languages have different rules for names◮Case-sensitive (C, C++, Java) or not (Pascal, Lisp)?◮Are there special words in a language that are reserved words,or are there just keywords◮Naming requirements/conventions◮Name length and allowed characters2Sebesta,2nd edition,p.207Mark Hills CS421 Lecture 16: Names and VariablesOutlineNames and VariablesBindingBinding CategoriesScopeOverviewVariable AttributesVariablesA variable is just an abstraction of memory. We can define avariable as a sextuple of attributes:◮Name◮Address◮Value◮Type◮Lifetime◮ScopeMark Hills CS421 Lecture 16: Names and VariablesOutlineNames and VariablesBindingBinding CategoriesScopeOverviewVariable AttributesVariables: NamesRemember, names are defined with certain characters andconventions◮In Haskell and ML, variables start with lowercase letters◮In Haskell and Ml, type constructors start with uppercaseletters◮C-family languages have no similar restrictions, but generallyuse conventions (Java camel-case, etc)◮Older Fortran had a 6-character limit, all uppercase names,and “type inference” for variables based on the first letterMost variables have names, although some do not (heap-allocatedvariables may not, more on this later)Mark Hills CS421 Lecture 16: Names and VariablesOutlineNames and VariablesBindingBinding CategoriesScopeOverviewVariable AttributesVariables: AddressThe address is the machine memory address that is associated withthe variable. This can be trickier than it sounds:◮Multiple variables may refer to the same address – aliasing◮pointers and references◮Equivalence-type statements and unions◮function calls◮The same name may have multiple addresses at differenttimes◮OS-based – different ph ysical address, same virtual address◮language-based – variables in functions may have differentaddresses on each callSometimes called an l-value, since a variable’s address is used onthe left-hand side of an assignment.Mark Hills CS421 Lecture 16: Names and VariablesOutlineNames and VariablesBindingBinding CategoriesScopeOverviewVariable AttributesVariables: Typ e sThe type of a variable determines what kinds of values the variablecan refer to. We covered this in Lecture 6.Mark Hills CS421 Lecture 16: Names and VariablesOutlineNames and VariablesBindingBinding CategoriesScopeOverviewVariable AttributesVariables: ValuesThe value is what we typically think of when we talk aboutvariables. This is just the data associated with the variable.◮We will think in terms of abstract memory cells to hold thevalue – that way we don’t have to worry aboutrepresentational issues (word size, packing, bit formats, etc)◮The value is sometimes called the r-value, since it is used onthe right-hand side of an assignment; we must know thel-value to get the r-valueMark Hills CS421 Lecture 16: Names and VariablesOutlineNames and VariablesBindingBinding CategoriesScopeBindingBinding TimesStatic and DynamicDeclarations, Storage, and LifetimeBindingWhen we talk about associating attributes with variables, we aretalking about binding.◮Def: In the general sense, a binding is an associationbetween two items, such as varibles and attributes◮Def: The time this association occurs is the binding timeMark Hills CS421 Lecture 16: Names and VariablesOutlineNames and VariablesBindingBinding CategoriesScopeBindingBinding TimesStatic and DynamicDeclarations, Storage, and LifetimeBinding TimesBinding can take place at:◮Language design time (identifying ∗ as multiplication)◮Language implementation time (an int takes 4 bytes)◮Compile time (variable x is an int)◮Load time (variable x is in storage location 10)◮Link time (function f is in load module m)◮Run time (x has the value 10)Mark Hills CS421 Lecture 16: Names and VariablesOutlineNames and VariablesBindingBinding CategoriesScopeBindingBinding TimesStatic and DynamicDeclarations, Storage, and LifetimeExampleWhat are the bindings here, and when do they occur?count = count + 5;Mark Hills CS421 Lecture 16: Names and VariablesOutlineNames and VariablesBindingBinding CategoriesScopeBindingBinding TimesStatic and DynamicDeclarations, Storage, and LifetimeStatic vs. Dynamic BindingsBindings are either static or dynamic:◮Static bindings first occur before run time and remainunchanged during program execution◮Dynamic bindings either occur during run time or occur earlierbut can be changed during run timeWe’ve already seen one example of this in Lecture 6 – dynamictyping, where the type is either not computed prior to execution orcan change during execution.Mark Hills CS421 Lecture 16: Names and VariablesOutlineNames and VariablesBindingBinding CategoriesScopeBindingBinding TimesStatic and DynamicDeclarations, Storage, and LifetimeAn Aside: ConstantsConstants are similar to variables except the value, once assigned,cannot be changed.◮Usually they can be allocated statically◮Some languages allow constants that are defined once atruntime but then not changed (sometimes called readonly)◮Literals can be thought of as constants (i.e. 5, “hello”, etc.)Mark Hills CS421 Lecture 16: Names and VariablesOutlineNames and VariablesBindingBinding CategoriesScopeBindingBinding TimesStatic and DynamicDeclarations, Storage, and LifetimeDeclarationsWe can use a
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