Transcript Chapter 1

Chapter 9
Subprograms
ISBN 0-321-49362-1
Chapter 9 Topics
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Introduction
Fundamentals of Subprograms
Design Issues for Subprograms
Local Referencing Environments
Parameter-Passing Methods
Parameters That Are Subprograms
Overloaded Subprograms
Generic Subprograms
Design Issues for Functions
User-Defined Overloaded Operators
Coroutines
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Introduction
• Two fundamental abstraction facilities
– Process abstraction
• Emphasized from early days
– Data abstraction
• Emphasized in the1980s
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Fundamentals of Subprograms
• Each subprogram has a single entry point
• The calling program is suspended during
execution of the called subprogram
• Control always returns to the caller when
the called subprogram’s execution
terminates
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Basic Definitions
•
A subprogram definition describes the interface to and the actions of the subprogram
abstraction.
–
In Python, function definitions are executable; in all other languages, they are non-executable. For
example:
if …
•
•
def fun(…):
else
def fun(…):
…
A subprogram call is an explicit request that the subprogram be executed.
A subprogram header is the first part of the definition, including the name, the kind of
subprogram, and the formal parameters. For example:
–
–
–
–
Subroutine Adder ( parameters )
Procedure Adder (parameters)
def adder ( parameters ):
void adder ( parameters)
Fortran
Ada
Python
C
•
The parameter profile (aka signature) of a subprogram is the number, order, and types of
its parameters.
•
The protocol is a subprogram’s parameter profile and, if it is a function, its return type.
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Basic Definitions (continued)
• Function declarations in C and C++ are often
called prototypes
• A subprogram declaration provides the protocol,
but not the body, of the subprogram
• A formal parameter is a dummy variable listed in
the subprogram header and used in the
subprogram
• An actual parameter represents a value or address
used in the subprogram call statement
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Actual/Formal Parameter
Correspondence
•
Positional
–
–
•
The binding of actual parameters to formal parameters is by position: the first
actual parameter is bound to the first formal parameter and so forth
Safe and effective
Keyword
–
–
The name of the formal parameter to which an actual parameter is to be bound is
specified with the actual parameter
Example call to a Python function:
Sumer( length = my_length, list = my_array, sum = my_sum )
–
–
Advantage: Parameters can appear in any order, thereby avoiding parameter
correspondence errors
Disadvantage: User must know the formal parameter’s names
• Both Positional and Keyword
–
–
Ada, Fortran95, and Python
Example: Sumer( my_length, sum = my_sum, list = my_array )
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Formal Parameter Default Values
• In some languages, C++, Python, Ruby, Ada, and
PHP, formal parameters can be assigned a default
values, if the actual parameter is not passed.
Python function example:
def compute_pay ( income, exemptions = 1, tax_rate )
Function call example:
pay = compute_pay(20000.0, tax_rate = 0.15)
– In C++, default parameters must appear last because
parameters are positionally associated. For example:
Float compute_pay ( float income, float tax_rate, int
exemptions = 1)
– To call : pay = compute_pay(20000.0, 0.15)
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Formal Parameter Default Values
• Variable numbers of parameters
–
C, C++, Perl and JavaScript allow the number of actual parameters to be fewer than
the number of formal parameters.
• For example: printf function of C
–
C# allows a variable number of parameters as long as they are of the same type. For
example:
public void DisplayList ( params int [] list ) {
foreach (int next in list ) {
Console.WriteLine(“Next value {0}”, next);
}
}
Call examples:
myObject.DisplayList(myList);// myList is an int array.
myObject.DisplayList(2, 4, 3 * x – 1, 17);
- Ruby and Python also support variable numbers of parameters. In Ruby, the actual
parameters are sent as elements of a hash literal and the corresponding formal
parameter is preceded by an asterisk.
- In Python, the actual is a list of values and the corresponding formal parameter is a
name with an asterisk
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Procedures and Functions
• There are two categories of subprograms
– Procedures are collection of statements that
define parameterized computations
– Functions structurally resemble procedures but
are semantically modeled on mathematical
functions
• They are expected to produce no side effects
• In practice, program functions have side effects
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Design Issues for Subprograms
• Are local variables static or dynamic?
• Can subprogram definitions appear in other
subprogram definitions?
• What parameter passing methods are provided?
• Are parameter types checked?
• If subprograms can be passed as parameters and
subprograms can be nested, what is the
referencing environment of a passed subprogram?
• Can subprograms be overloaded?
• Can subprogram be generic?
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Local Variables
Are local variables static or dynamic?
• Local variables can be stack-dynamic
- Advantages
• Support for recursion
• Storage for locals is shared among some subprograms
– Disadvantages
• Allocation/de-allocation, initialization time
• Uses indirect addressing
• Subprograms cannot be history sensitive
• Local variables can be static
– Advantages and disadvantages are the opposite of those
for stack-dynamic local variables
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Local Variables
• Most languages implement local variables as stack-dynamic
variables. Ada and methods of C++, C# and Java.
• Fortran95 allows the user to choose via the Save statement.
• In this PL/I example, Count is stack-dynamic due to the fact that the
subroutine is marked as Recursive, whereas, Sum is so due to the
keyword Save.
Recursive Subroutine Sub()
Integer :: Count
Save, Real :: Sum
…
End Subroutine Sub
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Local Variables
• The default for local variables in C/C++ functions is stackdynamic, however, by using the keyword static in the
declaration, they can be implemented as static variables.
– Example:
int adder(int list[], int listlen) {
static int sum = 0;
int count;
for (count = 0; count < listlen; count++)
sum += list[count];
return sum;
}
Non-static version:
int adder(int list[], int listlen) {
int sum = 0;
int count;
for (count = 0; count < listlen; count++)
sum += list[count];
return sum;
}
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Nested Subprograms
• This idea originated with Algol 60.
• Motivation was to create a hierarchy of both logic
and scope.
• Combined with static scoping, this provides a
highly structured way to grant access to nonlocal
variables in enclosing subprograms.
• Allowed by Algol60, Algol 68, Pascal and Ada.
• Not allowed by C, C++ and Java.
• Allowed by JavaScript, Python and Ruby.
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Parameter Passing
• Semantic Models of Parameter Passing
– In mode
– Out mode
– Inout mode
• Conceptual Models of Transfer
– An actual value is copied to the caller, to the
callee, or both ways (see above)
– An access path is transmitted, such as a simple
pointer or reference.
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Three Semantic Models of Parameter Passing When Values
are Copied.
Models of Parameter Passing
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Implementation Model of Parameter
Passing
• Implementation Models of Parameter
Passing
–
–
–
–
–
Pass-by-value
Pass-by-result
Pass-by-value–result
Pass-by-reference
Pass-by-name
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Pass-by-Value (In Mode)
• The value of the actual parameter is used to
initialize the corresponding formal parameter,
which then acts as a local.
– Normally implemented by copying.
• Disadvantages: (if by physical move): additional storage is
required (stored twice) and the actual move can be costly
(for large parameters).
– Can be implemented by transmitting an access path.
• Disadvantages: must write-protect the variable in the
called subprogram and accesses cost more (indirect
addressing).
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Pass-by-Result (Out Mode)
• No value is transmitted to the subprogram.
• The corresponding formal parameter acts as a local variable, with
its value transmitted to caller’s actual parameter when control is
returned to the caller, by a physical move.
• Same advantages/disadvantages of pass-by-value.
• If values are returned by access path, then there is the problem of
ensuring that the initial value of the actual paramete ris not used in
the called subprogram. The difficulties of this lead to values
returned by copy method.
• If values are returned by copy, then disadvantages are:
– Extra storage and overhead of the copy operation.
– Actual parameter collision.
– Time at which the return address is evaluated.
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Pass-by-Result (Out Mode)
• Actual parameter collision.
• Consider the call: sub(p1, p1); whichever formal
parameter is copied to its corresponding actual
parameter last will represent the current value of p1 in
the caller.
• Example in C#.
void Fixer (out int x, out int y) {
s = 17;
y = 35;
}
…
f.Fixer(out a, out a);
What is the value of a upon return from the function call.
• Depends upon the implementation. Which of the copies from formal
to actual occurs last.
• Since this order may vary by different implementations, the
results of the same program may also vary.
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Pass-by-Result (Out Mode)
• Problem of the implementation’s choice of time at which the return
address is evaluated.
– Time of the call
– Time of the return.
– Example in C#.
void DoIt( out int x, int index) {
x = 17;
index = 42;
}
..
sub = 21;
f.DoIt(list[sub], sub);
– Result if address of list[sub] is evaluated at the time of the call:
• Value of 17 will be returned to list[21]
– Result if address of list[sub] is evaluated at the time of the return:
• Value of 17 will be returned to list[42];
– Again, since implementations may vary, the results of the same
program may also vary.
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Pass-by-Value-Result (inout Mode)
• A combination of pass-by-value and
pass-by-result.
• Sometimes called pass-by-copy.
• Formal parameters have local storage
• Disadvantages:
– Those of pass-by-result
– Those of pass-by-value
• Advantages are relative to pass-byreference.
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Pass-by-Reference (Inout Mode)
• Passes an access path.
• Also called pass-by-sharing
• Advantage:
– Passing process is efficient
– No duplicated storage
• Disadvantages
– Slower accesses (compared to pass-by-value) to
formal parameters
– Potentials for unwanted changes to the actual
parameter.
– Unwanted aliases (access broadened)
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Pass-by-Reference (Inout Mode)
• Unwanted aliases examples
– Collisions between actual parameters. Example in C++
void fun (int &first, int &second)
Now consider call of: fun(total, total);
first and second are now aliases.
– Collisions between array elements.
Consider call of: fun( list[i], list[j] );
If i and j are equal, then first and second are now
aliases.
– Collisions between formal parameters and nonlocal variables that are
visible. Example in C
int * global;
void main() {
…
sub(global);
}
void sub(int * param) {….}
Param and global are now aliases.
Parameter Passing Methods of Major
Languages
•
Pascal provides for two:
–
–
X: integer is a pass-by-value
Var X: integer is a pass-by-reference
• C (from Algol 68)
– Pass-by-value only, but pass-by-reference is achieved by using pointers
as parameters
• C++
• Actual parameter is &i in the call -- subA(&i);
• Formal parameter is *x in the function heading -- subA(int *x);
– Has a special pointer type called reference type for pass-by-reference,
which are implicitly dereferenced.
• Java
• void fun (const int & p1, int p2, int &p3) {…} where,
p1 and p3 are both implicitly dereferenced and p1 cannot be
changed.
– All parameters are passed are passed by value. However, since you are
passing a reference in the case of Object parameters, you are giving
write-access to the object.
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Parameter Passing Methods of Major
Languages (continued)
•
Ada
–
–
–
Three semantics modes of parameter transmission: in, out, in out; in is the default mode
Formal parameters
• declared out can be assigned but not referenced
• declared in can be referenced but not assigned
• declared in out parameters can be referenced and assigned
Example: Procedure Adder (A: in out Integer; B : in Integer; C : out Float)
•
Fortran 95
•
C#
- Default method: pass-by-value
- Parameters can be declared to be in, out, or inout mode
– Example: Subroutine Adder(A, B, C)
Integer, Intent(Inout) :: A
Integer, Intent(In) :: B
Integer, Intent(out) :: C
–
–
•
•
•
Pass-by-reference is specified by preceding both a formal parameter and its actual parameter
with ref
Example:
void sumer (ref int oldSum, int newOne) {..}
call is sumer(ref sum, newValue);
PHP: very similar to C#
Perl: all actual parameters are implicitly placed in a predefined array named @_
Python and Ruby use pass-by-assignment (all data values are objects)
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Implementing Parameter-Passing
Methods
• In most languages parameter transmissions takes place
through the run-time stack.
– Pass-by-value actual parameters have their values copied into
those stack locations which serve as storage for their
corresponding formal parameters.
– Pass-by-result actual parameters are assigned the final value of
their corresponding formal parameters when the subprogram
terminates.
– Pass-by-value-result have their values copied into those stack
locations which serve as storage for their corresponding formal
parameters, where The formal parameters are used as local
variables. When the subprogram terminates, the final values of
these locals are copied back into the actual parameters as with
pass-by-result.
– Pass-by-reference is the simplest to implement; only an address
is placed in the stack.
• Subtle but fatal errors can occur with pass-by-reference and
pass-by-value-result.
– A formal parameter corresponding to a constant can mistakenly
be changed.
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Main calls sub with actual
parameters: w, x, y and z
Which correspond to the actual
parameters a, b, c and d in
function sub.
void sub(int a, int b, int c, int d)
with the following parameter
passing mechanisms:
(by-value, by-result,
by-value-result, by-reference)
Implementing Parameter-Passing
Methods
• Subtle but fatal errors can occur with pass-by-reference and passby-value-result if implementations do not prevent their occurrence.
•
– A formal parameter corresponding to a constant can mistakenly be
changed.
– Example:
• Hypothetical Subprogram uses pass-by-reference
Sub1( Integer: P1, Integer: P2)
begin
P1 = 8
P2 = 5
end.
• Calling program (assume that b is 10
Sub1(a, 10);
result = b * 10; // result will be 50 vs. 100
– Actually happened in Fortran IV
–
C and C++ provide for the typing of formal parameters as pointers to constants to
allow for the efficiency of pass-by-reference with the security of pass by value.
Even if the actual parameter is not a constant it is coerced to a constant.
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Type Checking Parameters
• Considered very important for reliability.
• No type checking required in FORTRAN 77 and original C.
• C89 the user can choose.
– Prototypes are used for this purpose, where
• formal parameters can also be defined as:
double sin(double x)
{…}
–
Vs.
double sin(x)
double x;
{…}
• C99 and C++ all functions must be in prototype form.
• Always required in Pascal, FORTRAN 90, Java, and Ada.
• Relatively new languages Perl, JavaScript, and PHP do not
require type checking.
• In Python and Ruby, variables do not have types (objects do),
so parameter type checking is not possible.
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Multidimensional Arrays as Parameters
• If a multidimensional array is passed to a
subprogram and the subprogram is
separately compiled, the compiler needs to
know the declared size of that array to
build the storage mapping function
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Multidimensional Arrays as Parameters:
C and C++
• Programmer is required to include the
declared sizes of all but the first subscript
in the actual parameter
• Disallows writing flexible subprograms
• Solution: pass a pointer to the array and the
sizes of the dimensions as other
parameters; the user must include the
storage mapping function in terms of the
size parameters
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Pass-by-Name (Inout Mode)
• The actual parameter is, in effect, textually substituted for the
corresponding formal parameter in all of its occurrences in the
subprogram.
• Not part of any widely used language.
– Used at compile time by the macros in assembly languages
– Used for generic parameters of the generic programs of C++ and Ada.
• Allows flexibility in late binding
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Multidimensional Arrays as Parameters:
Ada, Java and C#
• Ada – not a problem
– Constrained arrays – size is part of the array’s type
– Unconstrained arrays - declared size is part of the object
declaration
• Java and C#
– Similar to Ada
– Arrays are objects; they are all single-dimensioned, but
the elements can be arrays
– Each array inherits a named constant (length in Java,
Length in C#) that is set to the length of the array when
the array object is created
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Multidimensional Arrays as Parameters:
Fortran
• Formal parameter that are arrays have
a declaration after the header
– For single-dimension arrays, the
subscript is irrelevant
– For multidimensional arrays, the sizes
are sent as parameters and used in the
declaration of the formal parameter, so
those variables are used in the storage
mapping function
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Design Considerations for Parameter
Passing
• Two important considerations
– Efficiency
– One-way or two-way data transfer
• But the above considerations are in conflict
– Good programming suggest limited access to
variables, which means one-way whenever
possible
– But pass-by-reference is more efficient to pass
structures of significant size
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Parameters that are Subprogram
Names
• It is sometimes convenient to pass
subprogram names as parameters
• Issues:
1. Are parameter types checked?
2. What is the correct referencing environment for
a subprogram that was sent as a parameter?
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Parameters that are Subprogram
Names: Parameter Type Checking
•
•
•
•
C and C++: functions cannot be passed as
parameters but pointers to functions can be
passed and their types include the types of the
parameters, so parameters can be type checked
FORTRAN 95 type checks
Ada does not allow subprogram parameters; an
alternative is provided via Ada’s generic facility
Java does not allow method names to be passed
as parameters
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Parameters that are Subprogram
Names: Referencing Environment
• Shallow binding: The environment of the
call statement that enacts the passed
subprogram
- Most natural for dynamic-scoped
languages
• Deep binding: The environment of the
definition of the passed subprogram
- Most natural for static-scoped languages
• Ad hoc binding: The environment of the call
statement that passed the subprogram
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Overloaded Subprograms
• An overloaded subprogram is one that has the
same name as another subprogram in the same
referencing environment
– Every version of an overloaded subprogram has a unique
protocol
• C++, Java, C#, and Ada include predefined
overloaded subprograms
• In Ada, the return type of an overloaded function
can be used to disambiguate calls (thus two
overloaded functions can have the same
parameters)
• Ada, Java, C++, and C# allow users to write
multiple versions of subprograms with the same
name
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Generic Subprograms
• A generic or polymorphic subprogram takes parameters of
different types on different activations
• Overloaded subprograms provide ad hoc polymorphism
• A subprogram that takes a generic parameter that is used in
a type expression that describes the type of the parameters
of the subprogram provides parametric polymorphism
- A cheap compile-time substitute for dynamic binding
• In Ada, generic subprograms are instantiated explicitly
• In C++, they are instantiated implicitly, when the
subprogram is named in a call or when its address is taken
with the & operator
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Generic Subprograms (continued)
• Java 5.0
- Differences between generics in Java 5.0 and
those of C++ and Ada:
1. Generic parameters in Java 5.0 must be classes
2. Java 5.0 generic methods are instantiated just
once as truly generic methods
3. Restrictions can be specified on the range of
classes that can be passed to the generic method
as generic parameters
4. Wildcard types of generic parameters
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Generic Subprograms (continued)
• C# 2005
- Supports generic methods that are similar
to those of Java 5.0
- One difference: actual type parameters in
a call can be omitted if the compiler can
infer the unspecified type
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Examples of parametric
polymorphism: C++
template <class Type>
Type max(Type first, Type second) {
return first > second ? first : second;
}
• The above template can be instantiated for any
type for which operator > is defined
int max (int first, int second) {
return first > second? first : second;
}
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Design Issues for Functions
•
Are side effects allowed?
–
•
Parameters should always be in-mode to reduce side
effect (like Ada)
What types of return values are allowed?
–
–
–
–
–
–
Most imperative languages restrict the return types
C allows any type except arrays and functions
C++ is like C but also allows user-defined types
Ada subprograms can return any type (but Ada
subprograms are not types, so they cannot be returned)
Java and C# methods can return any type (but because
methods are not types, they cannot be returned)
Python and Ruby treat methods as first-class objects, so
they can be returned, as well as any other class
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User-Defined Overloaded
Operators
• Operators can be overloaded in Ada, C++,
Python, and Ruby
• An Ada example
function "*" (A,B: in Vec_Type): return Integer
is
Sum: Integer := 0;
begin
for Index in A'range loop
Sum := Sum + A(Index) * B(Index)
end loop
return sum;
end "*";
…
c = a * b; -- a, b, and c are of type Vec_Type
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Coroutines
• A coroutine is a subprogram that has multiple
entries and controls them itself
• Also called symmetric control: caller and called
coroutines are on a more equal basis
• A coroutine call is named a resume
• The first resume of a coroutine is to its beginning,
but subsequent calls enter at the point just after
the last executed statement in the coroutine
• Coroutines repeatedly resume each other, possibly
forever
• Coroutines provide quasi-concurrent execution of
program units (the coroutines); their execution is
interleaved, but not overlapped
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Coroutines Illustrated: Possible
Execution Controls
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Coroutines Illustrated: Possible
Execution Controls
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Coroutines Illustrated: Possible
Execution Controls with Loops
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Summary
• A subprogram definition describes the actions
represented by the subprogram
• Subprograms can be either functions or
procedures
• Local variables in subprograms can be stackdynamic or static
• Three models of parameter passing: in mode, out
mode, and inout mode
• Some languages allow operator overloading
• Subprograms can be generic
• A coroutine is a special subprogram with multiple
entries
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Ruby Blocks
•
•
•
Ruby includes a number of iterator functions, which are often used to
process the elements of arrays
Iterators are implemented with blocks, which can also be defined by
applications
Blocks are attached methods calls; they can have parameters (in vertical
bars); they are executed when the method executes a yield statement
def fibonacci(last)
first, second = 1, 1
while first <= last
yield first
first, second = second, first + second
end
end
puts "Fibonacci numbers less than 100 are:"
fibonacci(100) {|num| print num, " "}
puts
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