Chapter 9

Subprograms

2

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

3

Basic Definitions

A subprogram definition describes the interface to and the actions of the subprogram abstraction

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

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

4

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

5

Actual/Formal Parameter Correspondence Two approaches: 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

Parameters can appear in any order

6

Parameter Correspondence: Example Example on Keyword: Ada call statement can be as

Sumer(Length => My_Length,

List => My_Array,

Sum => My_Sum); Where the definition of Sumer has the formal parameters

Length, List, and Sum. Ada and Fortran 95 allow bothe and they can be mixed in a call,

as in

Sumer( My_Length,

List => My_Array,

Sum => My_Sum);

7

Formal Parameter Default Values

In C++, Fortran 95, Ada, and PHP, formal parameters can have default values. A default value is used if no actual parameter is passed to the formal parameter in the subprogram header.

Example: AdaFunction Compute_Pay(Income : Float;

Exemptions : Integer := 1;Tax_Rate : Float) return Float;

Consider the following callPay := Compute_Pay(20000.0, Tax_Rate => 0.15);

No need to include comma for the absent actual parameter, because all actual parameter after an absent one must be keyword parameters.

8

Formal Parameter Default Values

In C++, default parameters must appear last because parameters are positionally associated.

Once a default parameter is omitted in a call, all remaining formal parameters must have defualt values.

Example: C++

float compute_pay (float income, float tax_rate,

integer exemptions = 1); Notice that the parameters are rearanged so that the one with

default value is last. An example call is

pay := compute_pay(20000.0, 0.15);

9

Procedures and Functions

There are two categories of subprograms

1. Procedures are collection of statements that define parameterized computations.

Procedures can produce results in two ways: Change global variables. Through parameters that allow the transfer of data.

2. Functions are modeled on mathematical functions They returns a value. They can be used in any expression. They are expected to produce no side effects In practice, program functions have side effects

10

Design Issues for Subprograms

What parameter passing methods are provided?

Are parameter types checked? Are local variables static or dynamic? Can subprogram definitions appear in other

subprogram definitions? Can subprograms be overloaded? Can subprogram be generic?

11

Local Variables Environments

Local variables can be stack-dynamic (bound to storage) Advantages

Support for recursion Storage for locals is shared among some subprograms

Disadvantages Allocation/de-allocation, initialization time Indirect addressing Subprograms cannot be history sensitive

Local variables can be static More efficient (no indirection) No run-time overhead Cannot support recursion

12

Parameter Passing Methods

Ways in which parameters are transmitted to and/or from called subprograms Pass-by-value Pass-by-result Pass-by-value-result Pass-by-reference Pass-by-name

13

Models of Parameter Passing

14

Pass-by-Value (In Mode)

The value of the actual parameter is used to initialize the corresponding formal parameter. Normally implemented by copying When copies are used, additional storage is

required Storage and copy operations can be costly

15

Pass-by-Result (Out Mode)

When a parameter is passed by result, no value is transmitted to the subprogram; the corresponding formal parameter acts as a local variable; its value is transmitted to caller’s actual parameter when control is returned to the caller Require extra storage location and copy operation

Potential problem: sub(p1, p1); whichever formal parameter is copied back last will represent the current value of p1.

16

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

17

Pass-by-Reference (Inout Mode)

Pass an access path Also called pass-by-sharing Passing process is efficient (no copying

and no duplicated storage) Disadvantages

Potentials for un-wanted side effects Potentials unwanted aliases

18

Pass-by-Name (Inout Mode)

By textual substitution. The actual parameters is textually substituted for the corresponding formal parameter in all its occurrences in the subprogram.

Formals are bound to an access method at the time of the call, but actual binding to a value or address takes place at the time of a reference or assignment

Not used by widely used languages.

19

Implementing Parameter-Passing Methods In most language parameter communication takes place

thru the run-time stack. Pass-by-value: parameters have their values copied into

stack location. The stack then serve as storage for the formal parameters.

Pass-by-result: the values assigned to the actual parameters are places in the stack, where they can be retrieved by the calling program upon termination of the subprogram.

Pass-by-value-result: The stack location for the parameters is initialized by the call and is then used like a local variable in the called subprogram.

Pass-by-reference are the simplest to implement; only an address is placed in the stack.

20

Parameter Passing Methods of Major Languages

Fortran Always used the inout model (pass-by-reference or pass-by-value-

result). Before Fortran 77: pass-by-reference. Fortran 77 and later: simple variables are often passed by value-

result. C

Pass-by-value. Pass-by-reference is achieved by using pointers as parameters.

C++ Pass-by-value. A special pointer type called reference type for pass-by-reference.

Java All parameters are passed by value. Object parameters are passed by reference.

21

Parameter Passing Methods of Major Languages

Ada Three modes of parameter transmission: in, out, in out. in is the default mode. Formal parameters declared out can be assigned but not

referenced; those declared in can be referenced but not assigned; in out parameters can be referenced and assigned.

Procedure Adder( A : in out Integer;

B : in Integer;

C : out Float)

22

Parameter Passing Methods of Major Languages

C# Default method: pass-by-value Pass-by-reference is specified by preceding both a formal

parameter and its actual parameter with ref

Void sumer( ref int oldsum, int newone) { … }

…

sumer( ref sum, newvalue);

The first parameter to sumer is pass-by-reference and the second is pass-by-value.

23

Type Checking Parameters

Considered very important for reliability

FORTRAN 77 and original C: No type checking

Pascal, Fortran 90, Java, and Ada: Type checking

Relatively new languages Perl, JavaScript, and PHP do not require type checking

24

Multidimensional Arrays as Parameters: C, C++

Multidimensional Arrays are arrays of arrays, and they are stored in row major order. The mapping function for mat[i, j] is

Address(mat[i, j]) = address(mat[0, 0]) + i * number_of_columns + j

This doesn’t need the number of rows. When an array is passed as a parameter, the formal

parameter must include the number of columns but not the number of rows. Consider the prototype

void fun (int mat [ ][10]);

25

Multidimensional Arrays as Parameters: C, C++

Disallows writing flexible subprograms (arrays of different number of rows)

Solution: Pass a pointer to the array and the sizes of the dimensions as other parameters. Consider the prototype

void fun (float *mat_ptr, int num_rows, int num_cols);

26

Multidimensional Arrays as Parameters: Pascal and Ada

Pascal Type checking. Declared size is part of the array’s type

Ada Constrained arrays - Type checking Unconstrained arrays

The index ranges are not given in the array type definition.

Definition of variables of unconstrained array types must include index ranges. Example

typ Mat_Type is array (Integer range <>, Integer range <>) of Float;

Mat_1 : Mat_Type(1 .. 100, 1 .. 20);

27

Multidimensional Arrays as Parameters: 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.

28

Parameters that are Subprogram Names: Parameter Type Checking

C and C++: Functions cannot be passed as parameters Pointers to functions can be passed The type of a pointer to a function is the function’s protocol Because the protocol includes all parameter types, such

parameters can be type checked FORTRAN 95

It has a mechanism for subprograms to be passed as parameters

Type checked Ada

Does not allow subprogram to be passed as parameters The functionality of passing subprogram as parameters is

provided by Ada’s generic facility.

29

Parameters that are Subprogram Names: Referencing Environment

In languages that allow nested subprograms, such as JavaScript, the question is what referencing environment for executing the passed subprogram should be used.

There are three choices:1. Shallow binding: The environment of the call statement

that enacts the passed subprogram.

2. Deep binding: The environment of the definition of the passed subprogram.

3. Ad hoc binding: The environment of the call statement that passed the subprogram.

30

Parameters that are Subprogram Names: Referencing Environment – JavaScript Example

function sub1 {

var x;

function sub2 () {

alert(x); // prints x

};

function sub3 () {

var x;

x = 3;

sub4(sub2);

};

function sub4 (subx) {

var x;

x =4;

subx();

};

x = 1;

sub3();

};

31

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 Different in number, order, and type of its parameters,

and the type of its return type.

C++, Java, C#, and Ada include predefined overloaded subprograms many classes have overloaded constructors.

32

Overloaded Subprograms

In Ada, the return type of an overloaded function can be used to disambiguate calls. two overloaded functions can have the same

parameters and differ only in their return types.

Ada, Java, C++, and C# allow users to write multiple versions of subprograms with the same name

33

Generic Subprograms

A generic or polymorphic subprogram takes parameters of different types on different activations.

Programmer should not need to write four different sort sub-programs to sort four arrays that differ only in element type.

Java 5.0, Ada, and C++ support generic subprogram.

34

User-Defined Overloaded Operators

Operators can be overloaded in Ada and C++ 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

35

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

36

Coroutines Illustrated: Possible Execution Controls

37

Coroutines Illustrated: Possible Execution Controls

38

Coroutines Illustrated: Possible Execution Controls with Loops