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CPS 506 Comparative Programming Languages Sub-program and Parameter Passing.

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1 CPS 506 Comparative Programming Languages Sub-program and Parameter Passing

2 Topics Introduction Fundamentals of Subprograms Design Issues for Subprograms Local Referencing Environments Parameter-Passing Methods Parameters That Are Subprograms Overloaded Subprograms User-Defined Overloaded Operators Generic Subprograms Design Issues for Functions Co-routines 2

3 Fundamentals of Subprograms Three fundamentals – 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

4 Basic Definitions Two kinds of subprograms – Function – Procedure A subprogram definition describes – The interface – The actions A subprogram call is an explicit request that the subprogram be executed 4

5 Basic Definitions In Python, function definitions are executable; in all other languages, they are non-executable # def map( fun, list ): nlist = [] for item in list: nlist.append( fun( item ) ) return nlist # Make a sample test function def increment(x): return x+1 # Test them out! map( increment, [1,2,3,4,5] ) # should return [2,3,4,5,6] 5

6 Basic Definitions A subprogram header is the first part of the definition, including – Name – Kind of subprogram – Formal parameters The parameter profile (signature) of a subprogram is the – Number – Order – and Types of its parameters The protocol of subprogram is – Parameter profile – and its Return type (if it is a function) 6

7 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 7

8 Basic Definitions (continued) Functions as first-class entities – Can be stored in data structures – Pass as parameters – Returned from functions – Lua (anonymous functions) function cube(x) return x * x * x end cube = function (x) return x * x * x end 8

9 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 – All the languages support this method of parameter binding 9

10 Actual/Formal Parameter Correspondence Keyword – The name of the formal parameter to which an actual parameter is to be bound is specified with the actual parameter – Advantage: Parameters can appear in any order, thereby avoiding parameter correspondence errors – Disadvantage: User must know the formal parameter’s names – Python can use this type of parameter binding – Python, Ada, Fortran 95 can have both in one function call 10

11 Formal Parameter Default Values In certain languages (e.g., C++, Python, Ruby, Ada, PHP), formal parameters can have default values (if no actual parameter is passed) – In C++, default parameters must appear last because parameters are positionally associated – C++ float compute_pay(float income, float tax_rate, int exemptions = 1) pay = compute_pay( , 0.15); – Python Def compute_pay(income, exemptions = 1, tax_rate) pay = compute_pay( ,tax_rate = 0.15) 11

12 Formal Parameter Default Values (con’t) Variable number of parameters – C# methods can accept a variable number of parameters as long as they are of the same type—the corresponding formal parameter is an array preceded by params Public void DisplayList(params int[] list) { foreach (int next in list) { Console.WriteLine(“Next value {0}”, next); } Myclass myObject = new Myclass; int[] myList = new int[6] {2,4,6,8,10,12}; myObject.DisplayList(myList); myObject.DisplayList(2,4,3*x-1,17); 12

13 Formal Parameter Default Values (con’t) Variable number 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. (Details discussed in Ruby part) list = [2,4,6,8] def tester(p1,p2,p3,*p4) … end … tester(‘first’, mon =>72, tue =>68, wed =>59, *list) p1 is ‘first’ p2 is {mon =>72, tue =>68, wed =>59} p3 is 2 p4 is [4,6,8] 13

14 Formal Parameter Default Values (con’t) Variable number of parameters – In Python, the actual is a list of values and the corresponding formal parameter is a name with an asterisk (Details discussed in Python part) def fun1(p1,p2,*p3,**p4) … Fun1(2,4,6,8, mon =72, tue =68, wed =59) p1 is 2 P2 is 4 p3 is [6,8] p4 is {‘mon’:72, ‘tue’:68, ‘wed’:59} 14

15 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 15

16 Ruby Blocks # A method to compute and yield Fibonacci numbers up to # a limit def fibonacci(last) first, second = 1, 1 while first <= last yield first first, second = second, first + second end puts "Fibonacci numbers less than 100 are:" fibonacci(100) {|num| print num, " "} puts sum = 0 fibonacci(100) {|num| sum +=num} puts 16

17 Procedures and Functions There are two categories of subprograms – Procedures are collection of statements that define parameterized computations – Two ways of producing result Variables that are not formal parameters but are visible in both the procedure and the caller program unit Formal parameters that allow the transfer of data to the caller (call by reference parameters) 17

18 Procedures and Functions (con’t)... – Functions structurally resemble procedures but are semantically modeled on mathematical functions They are expected to produce no side effects – No modification on the parameters – No modification on variables defined outside Pure functions return only a value In practice, program functions have side effects Define user-defined operators (such as power) Overload operators by defining function in Ada, Python, Ruby, C++, and C# (discussed later) void functions in C-based languages work like procedures 18

19 Design Issues for Subprograms Are local variables static or dynamic? 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? 19

20 Local Referencing Environments Local variables can be stack-dynamic – Bound to storage when the subprogram begins execution – Unbounded from storage when that execution terminates – Advantages Support for recursion Storage for locals is shared with those of inactive subprograms (great advantage for old computers with small size of memory) – Disadvantages Allocation/de-allocation, initialization time Indirect addressing (access only during the execution) Subprograms cannot be history sensitive – Writing a subprogram for generating random numbers Local variables can be static – Advantages and disadvantages are the opposite of those for stack-dynamic local variables 20

21 Local Referencing Environments (con’t) In C and C++, local variables are stack-dynamic unless specifically declared to be static int adder(int list[], int listlen) { static int sum = 0; int count; for (count=0;count < listlen;count++) sum += list[count]; return sum; } Java, C# and Ada have only stack-dynamic local variables 21

22 Local Referencing Environments (con’t) In Fortran 95 a subprogram can be explicitly specified to be recursive. – So the local variables are stack-dynamic by default – Force variables to be static using Save keyword Recursive subroutine sub() Integer :: Count Save, Real :: Sum... End Subroutine sub In Python methods, all local variables are stack-dynamic 22

23 Semantic Models of Parameter Passing In mode – No change is returned. Some parameters are just passed to the subprogram Out mode – No parameter is passed. Just result is returned to the caller Inout mode 23

24 Models of Parameter Passing 24

25 Pass-by-Value (In Mode) The value of the actual parameter is used to initialize the corresponding formal parameter – Normally implemented by copying – Can be implemented by transmitting an access path but not recommended (enforcing write protection is not easy) – Disadvantages (if by physical move): additional storage is required (stored twice) and the actual move can be costly (for large parameters) – Disadvantages (if by access path method): must write-protect in the called subprogram and accesses cost more (indirect addressing) 25

26 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, by physical move – Require extra storage location and copy operation Potential problems – sub(p1, p1); whichever formal parameter is copied back will represent the current value of p1 – Evaluation time 26

27 Pass-by-Result (Out Mode) C# void Fixer(out int x,out int y) { x = 17; y =35; }... f.Fixer(out a, out a); void DoIt(out int x, int index) { x = 17; index = 42; }... sub = 21; f.DoIt(list[sub], sub); 27

28 Pass-by-Value-Result (in-out 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 28

29 Pass-by-Reference (In-out Mode) Pass an access path Also called pass-by-sharing Advantage: Passing process is efficient (no copying and no duplicated storage) Disadvantages – Slower accesses (compared to pass-by-value) to formal parameters Additional level of indirect addressing – Potentials for unwanted side effects (collisions) Unwanted aliases – Access to non-local variables (reduce readability and reliability) 29

30 Pass-by-Reference (In-out Mode) – C++ Collusion between actual parameters void fun(int &first, int &second) fun(total, total) first and second in fun will be aliases. Collusion between array elements fun(list[i], list[j]) fun1(list[i], list) 30

31 Pass-by-Reference (In-out Mode) – C++ Collusion between formal parameters and non-local variables that are visible int * global; void main(){... sub(global);... } void sub(int * param) {... } param and global are aliases. All these possible aliasing situations are eliminated if pass-by-value-result is used. 31

32 Pass-by-Name (In-out Mode) By textual substitution 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 Allows flexibility in late binding Algol procedure double(x); real x; begin x := x * 2 end; double(C[j]) is interpreted as C[j] := C[j] * 2. Usage – Compile time for macro – Generic subprograms in C++ and Ada 32

33 Implementing Parameter-Passing Methods In most language parameter communication takes place through the run-time stack Pass-by-reference are the simplest to implement; only an address is placed in the stack A subtle but fatal error can occur with pass-by-reference and pass-by-value-result: a formal parameter corresponding to a constant can mistakenly be changed 33

34 Function header: void sub(int a, int b, int c, int d) Function call in main: sub(w,x,y,z) (pass w by value, x by result, y by value-result, z by reference) 34

35 Parameter Passing Methods of Major Languages C – Pass-by-value – Pass-by-reference is achieved by using pointers as parameters Java – All parameters are passed by value – Object parameters are passed by reference Ada – Three semantics modes of parameter transmission in, out, in-out in is the default mode 35

36 Parameter Passing Methods of Major Languages (continued) Fortran 95 - Parameters can be declared to be in, out, or inout mode C# - Default method: pass-by-value – Pass-by-reference is specified by preceding both a formal parameter and its actual parameter with ref 36

37 Parameter Passing Methods of Major Languages Ada procedure Adder(A : in out Integer; B : in Integer; C : out Float) Fortran 95 Subroutine Adder(A, B, C) Integer,Intent(Inout) :: A Integer,Intent(In) :: B Integer,Intent(Out) :: C C# void sumer(ref int oldSum, int newOne, out nextOne) {...}... sumer(ref sum, newValue, nextValue); 37

38 Parameter Passing Methods of Major Languages (continued) PHP: very similar to C# – Pass-by-reference by preceding & Perl: all actual parameters are implicitly placed in a predefined array Python and Ruby use pass-by-assignment (all data values are objects), in effect is a pass-by-reference 38

39 Type Checking Parameters Considered very important for reliability FORTRAN 77 and original C: none C89 double sin(x) double x; {...} 0r double sin (double x) {...} 39

40 Type Checking Parameters Pascal, FORTRAN 90, Java, and Ada: it is always required ANSI C and C++: choice is made by the user – Prototypes double sin(double x) {...} C99 and C++: formal parameters in prototype form – Type checking could be avoided for some parameters by using “...” int printf(const char* format_string,...) At least one parameter 40

41 Type Checking Parameters (con’t) 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 41

42 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 42

43 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 Storage-mapping function address(mat[I,j]) = address(mat[0,0]) + I * number_of_columns + j void fun(int matrix[][10]) {...} void main() { int mat[5][10];... Fun(mat);... } Disallows writing flexible subprograms 43

44 Multidimensional Arrays as Parameters: Java and C# 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 44

45 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 45

46 Parameters that are Subprogram Names It is sometimes convenient to pass subprogram names as parameters Issues: – Are parameter types checked? 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 46

47 Parameters that are Subprogram Names: Parameter Type Checking Ada does not allow subprogram parameters; an alternative is provided via Ada’s generic facility (discussed later) Java does not allow method names to be passed as parameters 47

48 Parameters that are Subprogram Names Issues: (con’t) – Referencing environment for executing the passed subprogram In languages that allow nested subprograms Three choices 48

49 Parameters that are Subprogram Names: Referencing Environment Three choices – 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 49

50 Parameters that are Subprogram Names: Referencing Environment function sub1() { var x; function sub2() { alert(x); }; function sub3() { var x; x=3; sub4(sub2); }; function sub4(subx) { var x; x=4; subx(); }; x=1; sub3(); }; 50 – Shallow binding – Output from alert(x) is 4 – Deep binding – Output from alert(x) is 1 – Ad hoc binding – Output from alert(x) is 3

51 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 51

52 User-Defined Overloaded Operators Operators can be overloaded in Ada, C++, Python, and Ruby An Ada example (dot product) 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 C++ prototype for the same operator int operator * (const vector &a, const vector &b, int len); 52

53 Overloaded Subprograms 53 C++ (operator overloading) class complex{ float re, im; complex operator +(complex d){ complex ans; =; =; return ans; } complex c,d; c = c + d;

54 Overloaded Subprograms 54 C++ class C { int x; public static int sum(int v1, int v2) { return v1 + v2; } public int sum(int v3) { return x + v3; }

55 Overloaded Subprograms 55 C++ (ambiguous subprogram call) void fun(float b = 0.0); void fun();... fun();

56 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 56

57 Overloaded Subprograms 57 Java public class OverloadExample { public void print(int a) { System.out.println(a); } public void print(String a) { System.out.println(a); } void test() { int i = 1; String s = “Hi”; print(i); print(s); } public static void main(String[] args) { OverloadExample p = new OverloadExample(); p.test(); }

58 Generic Subprograms Scenario – Generic Sort subprogram to sort arrays of different element types A generic or polymorphic subprogram takes parameters of different types on different activations Overloaded subprograms provide ad hoc polymorphism 58

59 Generic Subprograms (continued) Ada – Versions of a generic subprogram are created by the compiler when explicitly instantiated by a declaration statement – Generic subprograms are preceded by a generic clause that lists the generic variables, which can be types on other subprograms 59

60 Generic Subprograms (continued) 60 Ada generic type Index_type is (<>); type Element_Type is private; type Vector is array (Integer range <>) of Element_Type; procedure Generic_Sort(List : in out Vector); procedure Generic_Sort(List : in out Vector) is Temp : Element_Type; begin for Top in List... for Bottom in Index_Type... if List(Top) > List(Bottom) then Temp := List(Top); List(Top) := List(Bottom); List(Bottom) := Temp; end if; end loop; end Generic_Sort;

61 Generic Subprograms (continued) 61 No code is generated by compiler unless it is instantiated for some type Array type: Int_Array Elements: Integer Subscripts: Integer procedure Integer_Sort is new Generic_Sort( Index_Type => Integer, Element_Type => Integer, Vector => Int_Array);

62 Generic Subprograms (continued) 62 Using generic subprogram in Ada to pass subprogram as parameter generic with function Fun(X: Float) return Float; procedure Integrate(Lowerbd : in Float; Upperbd : in Float; Result : in Float); procedure Integrate(Lowerbd : in Float; Upperbd : in Float; Result : in Float) is FunVal : Float; begin... FunVal := Fun(Lowerbd);... end Integrate procedure Integrate_Fun1 is new Integrate(Fun => Fun1);

63 Generic Subprograms (continued) C++ – Versions of a generic subprogram are created implicitly when the subprogram is named in a call or when its address is taken with the & operator – Generic subprograms are preceded by a template clause that lists the generic variables, which can be type names or class names 63

64 Generic Subprograms (continued) 64 C++ template Type max(Type first, Type second) { return first > second ? First : second; } int a, b, c; char d, e, f;... c = max(a, b); f = max(d, e); Why this is better than writing a macro? Like this: #define max(a,b) ((a) > (b)) ? (a) : (b)

65 Generic Subprograms (continued) 65 C++ generic sort subprogram template void generic_sort(Type list[], int len) { int top, bottom; Type temp; for (top = 0; top < len – 2; top++) for (bottom = top + 1; bottom < len – 1; bottom++) if (list[top] > list[bottom]) { temp = list[top]; list[top] = list[bottom]; list[bottom] = temp; } Float flt_list[100];... generic_sort(flt_list, 100);

66 Generic Subprograms (con’t) Java 5.0 – Differences between generics in Java 5.0 and those of C++ and Ada: Generic parameters in Java 5.0 must be classes Java 5.0 generic methods are instantiated just once as truly generic methods (by casting the return value) Restrictions can be specified on the range of classes that can be passed to the generic method as generic parameters (bounds) Wildcard types of generic parameters 66

67 Generic Subprograms (continued) 67 Java 5.0 generic method public static T doIt(T[] list) {... } doIt (myList); Java 5.0 wildcard type void printCollection(Collection c) { for (object e: c) { System.out.println(e); } Java 5.0 bounded wildcard type public void drawAll(ArrayList things) To draw any object whose type is a subclass of Shape

68 Generic Subprograms (con’t) C# 2005 – Supports generic methods that are similar to those of Java 5.0 – Differences No support for wildcard types Actual type parameters in call can be omitted if compiler can infer that 68

69 Generic Subprograms (continued) 69 C# 2005 class MyClass { public static T DoIt (T p1) {... } Which could be called int myInt = MyClass.DoIt(17); //calls DoIt String myStr =MyClass.DoIt(‘apples’); //calls DoIt

70 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 (handled by pointer type return values) – C++ is like C but also allows user-defined types or classes to be returned 70

71 Design Issues for Functions What types of return values are allowed? – 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 – Lua allows functions to return multiple values 71

72 Co-routines A co-routine is a subprogram that has multiple entries and controls them itself – supported directly in Lua Also called symmetric control: caller and called coroutines are on a more equal basis A co-routine call is named a resume 72

73 Co-routines (con’t) The first resume of a co-routine is to its beginning, but subsequent calls enter at the point just after the last executed statement in the coroutine Co-routines repeatedly resume each other, possibly forever Co-routines provide quasi-concurrent execution of program units (the co- routines); their execution is interleaved, but not overlapped 73

74 Co-routines Illustrated: Possible Execution Controls 74

75 Co-routines Illustrated: Possible Execution Controls 75

76 Co-routines Illustrated: Possible Execution Controls with Loops 76

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