CSC 3315
Programming Languages
Hamid Harroud
School of Science and Engineering, Akhawayn University
http://www.aui.ma/~H.Harroud/csc3315/
CSC3315 (Spring 2009)
1
Programming Languages
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A programming paradigm is a certain methodology for
programming a machine to solve problems.
A programming paradigm hence provides (and determines)
the view that the programmer has of the execution of a
program.
Major programming paradigms that exist today are:
– Imperative
– Functional
– Object-oriented
– Logic
Imperative Prgramming
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Imperative programming: the most widely
used programming paradigm
Emerged and developed alongside the first
computers, and remained popular
We will discuss the key features that
characterize the imperative programming
paradigm
Imperative languages are said to be Turing
Complete.
Imperative Prgramming
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Theorem: a programming language is Turing
Complete if it contains:
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integer variables, values, and operations
assignment statements
control constructs of statement sequencing
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Basically conditionals and branching statements
So this is all a programming language really
needs to be useful for writing programs,
But is this enough in practice?
Imperative Prgramming
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Modern imperative languages typically also
have a number of other features, such as:
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Variables
Basic data types (floats, strings, boolean)
Advanced control structures (loops, switch)
Complex data types (arrays, structures,
enumerations, pointers, etc.)
Subprograms
Input/output commands
Why? More friendly to programmers, and
hence better software maintainability
Variables
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A variable is an abstraction of a memory cell,
Use of variables relieves programmers from the burden of
memory addressing,
A variable exists as a name in a high-level program (written
by programmer). A variable exists as a memory location in
the executable program.
In high-level languages,
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a programmer uses variables to indirectly access and modify memory
memory management is left to the operating system; the detail of
where variables are actually stored during program execution is now
conveniently hidden from the programmer.
Variable Attributes
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Variables are characterized by six main
attributes:
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1) Name
2) Address (storage)
3) Value
4) Type
5) Lifetime
6) Scope
Static & Dynamic Binding
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A binding is some kind of association between
two entities.
Binding time is the time at which a binding takes
place
A binding is said to be static if it first occurs
before run time and remains unchanged
throughout the program execution
A binding is said to be dynamic if it first occurs
at runtime and/or can change during program
execution
Static & Dynamic Binding
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We are concerned with the binding of a
variable to its attributes
When are the attributes of a variable defined?
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Name: compile-time
Value: run-time
Address: load-time, run-time
Type: compile-ime, run-time
Scope: compile-time, run-time
Type Binding
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Possible binding times of the type of a
variable:
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1) Compile time: the binding is done during
compilation of the source program
=> Static type binding
2) Run time: the binding is done during the
execution of the program
=> Dynamic type binding
Static Type Binding
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Explicit type declaration: type of a variable is
specified via declaration statement in the program.
Implicit type declaration: a default mechanism for
specifying type of a variable based on its first
appearance in the program.
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Example: in Fortran the type of a variable is determined
based on first letter of variable name; I, J, K, L, M, N =>
integer, all other letters => float
Dynamic Type Binding
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Type is determined during runtime based on
RHS value of an assignment statement in which
variable appears as the LHS
 Example: JavaScript, Matlab
list = [2, 4.33, 6, 8]; => list is now an array of
floats
 list = ‘this’; => list is now a string
Flexible, but Inefficient (slow), because type checking
and storage allocation repeatedly done whenever an
assignment statement is executed
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Address Binding
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Address binding: when/how does the variable
become associated with a memory location
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=> Hence this involves allocating storage for the
variable (to store its value)
Obviously, this is related to the lifetime of the
variable
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Lifetime of a variable: the time interval during
which the variable is bound to a memory address
Address Binding
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Possible binding times of address of a variable:
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1) Load time: binding is done during linking/loading of
the executable program into memory (to prepare it for
execution)
=> Static address binding
2) Run time: binding is done during the execution of the
program
=> Dynamic address binding
(Compile time is never an address binding time!)
Type Binding
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Storage allocation: getting memory cell(s) from some pool of
available cells
Storage deallocation: putting a cell back into the pool
Who does storage allocation and deallocation?
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=> the operating system (OS)
Storage for variables comes from two places:
the runtime stack and the runtime heap
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These are two chunks of memory managed by the operating
system, and used to allocate memory for all executing programs
Address Binding
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There are four categories of variables, according to their address binding
properties:
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1) Static
static address binding (at load time)
2) Stack dynamic
3) Explicit heap dynamic
4) Implicit heap dynamic
1, 2, 3 dynamic address binding (at runtime)
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In C, C++, and Java, only the first three categories are used.
Implicit heap dynamic variables are typically used in conjunction with
dynamic type binding.
Address Binding & Lifetime
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Static variables:
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Stack-dynamic variables:
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execution of declaration stmt => execution of last stmt before
variable goes out of scope
Explicit heap-dynamic variables:
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load time => end of program execution
execution of alloc stmt => execution of dealloc stmt
Implicit heap-dynamic variables:
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execution of assignment stmt of some variable => execution
of next assignment stmt of same variable
Type Checking
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When discussing the type attribute of variables,
a related issue is that of type checking
Type checking basically has to do with making
sure the operands in any operation are of
compatible types
In general, these operands could be variables,
constants, or expressions containing both
Type Checking
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A compatible type: one that is either legal for
the operation, or is allowed to be implicitly
converted to a legal type, based on coercion
rules of the language
Coercion: implicit conversion of the type of a
variable or an expression
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Implicit means that the conversion is done
automatically by the compiler (by adding type
conversion instructions in the code) or interpreter
Type Checking
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Type checking is the activity of ensuring that
the operands of an operation are of compatible
types
A type error is the application of an operator to
an operand of an incompatible type
Type checking in a language is done based on
both its operator type compatibility rules and
coercion rules
Type Checking
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If all type bindings are static, then nearly all
type checking can be static, i.e. done at compile
time
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With some exceptions. Example: unions in C,
polymorphism in C++ (OOP)
If type bindings are dynamic, then type
checking must be dynamic (done at runtime);
the compiler cannot do type checking if the
type itself is unknown!
Scope
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The scope rules of a language specify how the
occurrence of a user-defined name in a program
statement is associated with the correct name
declaration
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The name could be that of a variable, data type,
subprogram
Example: if we have a program statement
i=1;
=> we need to know which variable declaration with
variable name i this occurrence of i corresponds to
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Scope
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Two common types of scope rules exist in
languages:
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Static scope: the scope of a variable is known at
compile-time
Dynamic scope: the scope of a variable can only be
determined at runtime
Static scope is the most commonly used in
languages today (C, C++, Java, Fortran, …).
Dynamic scope is used in Scheme (Lisp),
which is a functional language.
Static Scope
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Static scope rules are applied by the compiler
to associate a name reference with a declaration
as follows:
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When a reference to a name x is encountered,
compiler first searches declarations in the local
scope (i.e. block), then in the parent scope, then in
the grandparent scope, etc.; the search stops when a
declaration with the given name is found
If no declaration is found in any of the enclosing
scopes, then compiler flags a semantic error
Static Scope
Static Scope
Static Scope
Static Scope
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C++ and Java allow access to hidden
variables via explicit use of the scope
operator
Dynamic Scope
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Based on the subprogram call sequence (and
not the spatial layout of program units)
References to variables are connected to
declarations by searching back through the
chain of subprogram calls that led to this point
A called subprogram is granted access to local
variables of all other currently active
subprograms
Scope vs. Lifetime
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Scope and lifetime are sometimes closely related, but
are different concepts.
Consider a static variable in a C or C++ function:
void func() {
static int x = 0;
int y = 0;
...
}
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Scope of x and y is from point of declaration to end of
function block
Lifetime of x is entire execution of program
Lifetime of y is during execution of func()