1. Chapter 9 Subprograms

2. 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. 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. 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. 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. 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. 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: Ada Function Compute_Pay(Income : Float; Exemptions : Integer := 1; Tax_Rate : Float) return Float; Consider the following call Pay := 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. 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. 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. 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. 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. 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. 13 Models of Parameter Passing

14. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 36 Coroutines Illustrated: Possible Execution Controls

37. 37 Coroutines Illustrated: Possible Execution Controls

38. 38 Coroutines Illustrated: Possible Execution Controls with Loops