MELJUN CORTES Programming Languages Subprograms

8/8/2019 MELJUN CORTES Programming Languages Subprograms

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Subprograms

Programming Languages

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Subprograms

These are the fundamental building blocksof programs and are therefore among the

most important concepts in programminglanguage design.

Two forms:

Procedure

Function

Components:

Subprogram header

Subprogram implementation

Entry point

Exit point


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Subprograms


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Subprograms

Classifications:

Internal subprogram

External subprogram

Advantages:

Extensibility

Modularity

Reusability and maintainability

Abstraction


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Subprograms


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CALL-RETURN

Mechanisms

Necessary mechanism in connection withthe subprogram call:

A mechanism for saving the status ofexecution of the program unit which called it(or the caller)

A mechanism for passing parameters

A mechanism for allocating and bindingstorage to local variables declared in thesubprogram

A mechanism for bringing the control to thecalled subprogram (or the callee)

Necessary mechanism in connection to asubprogram return:

A mechanism for moving the values of the

formal parameters outmode A mechanism for deallocating the storage

used by the local variables

A mechanism for restoring the status ofexecution of the caller

A mechanism for transferring control backto the caller which invoked the call


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Recursive Subprogram

A recursive subprogram is one that callsitself, either directly or by invoking some

other subprogram that calls it.

Example:

class Factorial {

int fact(int n) {

int result;if ( n ==1) return 1;

result = fact (n-1) * n;

return result;

}

}

class Recursion { public static void main (String args[]) {

Factorial f =new Factorial();

System.out.println(“Factorial of 3 is ”

+ f.fact(3));

System.out.println(“Factorial of 3 is ”+ f.fact(4));

System.out.println(“Factorial of 3 is ”

+ f.fact(5));}

}


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Categories of

Subprograms

rocedures perform a specific action orproduces results.

These provide user-defined parameterizedcomputation statements.

• These computations are enacted by single callstatements, in effect the procedures definenew statements

Produces results via two methods:• If there are variable that are not formal

parameters but are still visible in both theprocedure and the calling program, theprocedure can change them

• If the subprogram has formal parameter hatallow the transfer of data to the caller, thoseparameters can be changed.

Composed of two parts:• Procedure specification

• Procedure body


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Categories of

Subprograms

You write procedure using the syntax PROCEDURE name [(parameter[, parameter,

...])] IS[local declarations]

BEGIN

executable statements

[EXCEPTION

exception handlers]

END [name]; where parameter stands for the followingsyntax:

parameter_name [IN | OUT | IN OUT]

datatype [{:= | DEFAULT} expr]

You cannot impose the NOT NULL

constraint on a parameter. Also, you cannot specify a constraint on

the datatype. For example, the followingdeclaration of emp_id is illegal:

PROCEDURE ... (emp_id NUMBER(4)) IS --illegal; should be NUMBER

BEGIN ... END;


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Categories of

Subprograms

unctions

compute a value.

These provide user-defined operators that

are semantically modeled on mathematicalfunctions.

• If a function is a faithful model, it produces noside effects.

• It modifies neither its parameters nor any variables defined outside the function.

Composed of two parts:

• Function specification • Function body

– Declarative part

– Executable part

– Exception-handling part


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Categories of

Subprograms

You write functions using the syntax

FUNCTION name [(parameter[, parameter,

...])] RETURN datatype IS[local declarations]

BEGIN

executable statements

[EXCEPTION

exception handlers]

END [name];

where parameter stands for the followingsyntax:

parameter_name [IN | OUT | IN OUT]datatype [{:= | DEFAULT} expr]

Remember, you cannot impose the NOTNULL constraint on a parameter, and youcannot specify a constraint on thedatatype.


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Parameters

Define information that is passed to afunction

Types:

Input parameters

Output parameters

Input/output parameters

Categories:

Formal parameters – variables are declaredin the procedure or function definition

Actual parameters – appear in the procedurecall statement or the function reference

Positional parameters – or plain arguments,are parameters without keyword

Keyword parameters – which either actual orformal, are expressions or variable namespreceded by a keyword and an equal sign “=”that identifies which parameter is beingpassed


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Formal Parameters

These are dummy variables found in thesubprogram header and used in the

subprogram.

Consider the following example:

public int mult(int x, int y)

{return x * y;

}

The formal parameters for subprogram mult are the integers x and y.


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Actual Parameters

These represent a value or address used inthe subprogram call statement.

Consider the following example:

public int mult(int x, int y) {

return x * y;

}

// Where the method mult is used

int length = 10;

int width = 5;

int area = mult(length, width);

The actual parameters are the integerslength and width.


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Positional Parameters

The binding of actual parameters to formalparameters is by position: the first actual

parameter is bound to the first formalparameter and so forth.

These parameters are safe and effective.


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Keyword Parameters

The name of the formal parameter to whichan actual parameter is to be bound is

specified with the actual parameter.

Advantage:

They can appear in any order in the actualparameter list.

Disadvantage:

The user of the subprogram must know thenames of formal parameters.

The only restriction with this is that after a

keyword parameter appears in the list, allremaining parameters must be keyworded.


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Subprograms


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Parameter-Passing

Methods

Ways in which parameters are transmittedto and/or from called subprograms

Pass-by-value Pass-by-result

Pass-by-value-result

Pass-by-reference

Pass-by-name

Semantics Models for Formal Parameters

In Mode

Out mode

Inout Mode


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Pass-by-Value Method

The value of the actual parameter is usedto initialize the corresponding formal

parameter.

It uses In mode semantic.

It can be implemented using actual data

transfer or through the use of an accesspath.

Example in Java:

int num = 29;

newNum(num);

System.out.println(num);

public void newNum(int num) {

num = 16;

}


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Pass-by-Result Method

In this method, no value is transferred toor given to the formal parameters of the

subprogram.

Thus, the formal parameters become thelocal variables.

It uses Out mode semantics.


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Pass-by-Value-Result

Method

It is a combination of pass-by value andpass-by-result.

It uses Inout mode semantics.

It is sometimes called pass by copy because the actual parameter is copied to

the formal parameter at subprogram entryand then copied back at subprogramtermination.


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Pass-by-Reference

Method

In this method, an access path or anaddress to the called subprogram is

transmitted instead of copying data values before and after the called subprogram’sexecution.


Advantage:

The passing process is efficient in terms oftime and space.

Disadvantages:

Access to the formal parameters will beslower than pass-by-value, because ofadditional level of indirect addressing that isrequired.

Inadvertent and erroneous changes may bemade to the actual parameter.


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Pass-by-Name Method

When parameters are passed by name, theactual parameter is, in effect, textually

substituted for the corresponding formalparameter in all its occurrences in thesubprogram.

A formal parameter is bound to an accessmethod at the time of the subprogram call,

but the actual binding to a value or anaddress is delayed until the formalparameter is assigned or referenced.



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Type-Checking

Parameters

The type-checking feature of programminglanguages specifically checks whether the

types of actual parameters are consistentor compatible with the types of thecorresponding formal parameters.

Consider the following function call:

ans – EX_FUNC(9.26);

The actual parameter, or argument, is 9.26 which is floating point number. What if thereceiving formal parameter of EX_FUNC isof integer type, obviously without a featurelike parameter type-checking, no error isdetected by the compiler, an incorrectresult is given by the function andassigned to the variable ans.


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Subprograms


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Overloaded

Subprograms

An overloaded subprogram works with adifferent set and/or numbers of

parameters.

An overloaded operator is one that hasmultiple meanings and/or operates ondifferent data types.

The meaning of a particular instance of anoverloaded operator is determined by thetype of its operand.

An overloaded subprogram is a

subprogram that has a same name asanother subprogram in the samereferencing environment.


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Generic Subprograms

Generic Units

Ada generics are used to provide thefunctionality of parameters that are

subprograms; generic part is a subprogram.

C++ Template Functions

The terms generic formal object (or simplyformal object), generic formal type (or simplyformal type), and generic formal subprogram

(or simply subprogram) are used to refer tocorresponding generic formal parameters.

The only form of subtype indication allowed within a generic formal part is a type mark(that is, the subtype indication must notinclude an explicit constraint).

The designator of a generic subprogram

must be an identifier.

Default expressions for generic formalobjects and default names for formalsubprograms are only evaluated for genericinstantiations that use such defaults.

Default expressions for parameters of formal

subprograms are only evaluated for calls ofthe formal subprograms that use suchdefaults.


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S b


* P t f STI

Accessing NONLOCAL

Environments

COMMON Block (FORTRAN)

External Declarations and Modules (Ada and Modula-2)

Extern Statement (C)

MELJUN CORTES Programming Languages Subprograms

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