subprograms - virginia tech
TRANSCRIPT
![Page 1: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/1.jpg)
Subprograms
In Text: Chapter 9
![Page 2: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/2.jpg)
Outline
• Definitions• Design issues for subroutines• Parameter passing modes and
mechanisms• Advanced subroutine issues
N. Meng, S. Arthur 2
![Page 3: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/3.jpg)
Subroutine
• A sequence of program instructions that perform a specific task, packaged as a unit
• The unit can be used in programs whenever the particular task should be performed
N. Meng, S. Arthur 3
![Page 4: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/4.jpg)
Subroutine
• Subroutines are the fundamental building blocks of programs
• They may be defined within programs, or separately in libraries that can be used by multiple programs
• In different programming languages, a subroutine may be called a procedure, a routine, a method, or a subprogram
N. Meng, S. Arthur 4
![Page 5: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/5.jpg)
Characteristics of Subroutines/Subprograms
• Each subroutine has a single entry point• The caller is suspended during the
execution of the callee subroutine• Control always returns to the caller
when callee subroutine’s execution terminates
N. Meng, S. Arthur 5
![Page 6: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/6.jpg)
Parameters
• A subroutine may be written to expect one or more data values from the calling program
• The expected values are called parameters or formal parameters
• The actual values provided by the calling program are called arguments or actual parameters
N. Meng, S. Arthur 6
![Page 7: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/7.jpg)
Actual/Formal Parameter Correspondence
• Two options– Positional parameters• In nearly all programming languages, the binding
is done by position• E.g., the first actual parameter is bound to the
first formal parameter– Keyword parameters• Each formal parameter and the corresponding
actual parameter are specified together• E.g., Sort (List => A, Length => N)
N. Meng, S. Arthur 7
![Page 8: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/8.jpg)
Keyword Parameters
• Advantages– Order is irrelevant–When a parameter list is long, developers
won’t make the mistake of wrongly ordered parameters
• Disadvantages– Users must know and specify the names of
formal parameters
N. Meng, S. Arthur 8
![Page 9: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/9.jpg)
Default Parameter
• A parameter that has a default value provided to it
• If the user does not supply a value for this parameter, the default value will be used
• If the user does supply a value for the default parameter, the user-specified value is used
N. Meng, S. Arthur 9
![Page 10: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/10.jpg)
An Example in Ada
procedure sort (list : List_Type;
length : Integer := 100);
...
sort (list => A);
N. Meng, S. Arthur 10
![Page 11: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/11.jpg)
Design issues for subroutines• What parameter passing methods are
provided?• Are parameter types checked?• What is the referencing environment of
a passed subroutine?• Can subroutine definitions be nested?• Can subroutines be overloaded?• Are subroutines allowed to be generic?• Is separate/independent compilation
supported?N. Meng, S. Arthur 11
![Page 12: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/12.jpg)
Parameter-Passing Methods
• Ways in which parameters are transmitted to and/or from calleesubroutines– Semantic models– Implementation models
N. Meng, S. Arthur 12
![Page 13: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/13.jpg)
Semantic Models
• Formal parameters are characterized by one of three distinct semantic models– In mode: They can receive data from the
corresponding actual parameters– Out mode: they can transmit data to the
actual parameters– Inout mode: they can do both
N. Meng, S. Arthur 13
![Page 14: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/14.jpg)
Models of Parameter Passing
N. Meng, S. Arthur 14
![Page 15: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/15.jpg)
An Examplepublic int[] merge(int[] arr1, int[] arr2) {
int[] arr = new int[arr1.length + arr2.length];for (int i = 0; i < arr2.length; i++) {
arr[i] = arr1[i];arr2[i] = arr1[i] + arr2[i];arr[i + arr1.length] = arr2[i];
}return arr;
}
N. Meng, S. Arthur
Which parameter is in mode, out mode, or inout mode?
15
![Page 16: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/16.jpg)
Implementation Models
• A variety of models have been developed by language designers to guide the implementation of the three basic parameter transmission modes– Pass-by-value– Pass-by-result– Pass-by-value-result– Pass-by-reference– Pass-by-name
N. Meng, S. Arthur 16
![Page 17: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/17.jpg)
Pass-by-Value
• The value of the actual parameter is used to initialize the corresponding formal parameter, which then acts as a local variable in the subprogram
• Implement in-mode semantics• Implemented by copy
N. Meng, S. Arthur 17
![Page 18: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/18.jpg)
Pros and Cons
• Pros– Fast for scalars, in both linkage cost and
access time– No side effects to the parameters
• Cons– Require extra storage for copying data– The storage and copy operations can be
costly if the parameter is large, such as an array with many elements
N. Meng, S. Arthur 18
![Page 19: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/19.jpg)
Pass-by-Result
• No value is transmitted to a subroutine• The corresponding formal parameter acts
as a local variable, whose value is transmitted back to the caller’s actual parameter– E.g., void Fixer(out int x, out int y) {
x = 17;y = 35;
}• Implement out-mode parameters
N. Meng, S. Arthur 19
![Page 20: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/20.jpg)
Pros and Cons
• Pros– Same as pass-by-value
• Cons– The same cons of pass-by-value– Parameter collision• E.g., Fixer(x, x), what will happen?• If the assignment statements inside Fixer() can
be reordered, what will happen?
N. Meng, S. Arthur 20
![Page 21: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/21.jpg)
Pass-by-Value-Result
• A combination of pass-by-value and pass-by-result, also called pass-by-copy
• Implement inout-mode parameters• Two steps– The value of the actual parameter is used to
initialize the corresponding formal parameter– The formal parameter acts as a local variable,
and at subroutine termination, its value is transmitted back to the actual parameter
N. Meng, S. Arthur 21
![Page 22: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/22.jpg)
Pros and Cons
• Pros– Same as pass-by-reference, which is to be
discussed next• Cons– Same as pass-by-result
N. Meng, S. Arthur 22
![Page 23: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/23.jpg)
Pass-by-Reference
• A second implementation model for inout-mode parameters
• Rather than copying data values back and forth, it shares an access path, usually an address, with the caller – E.g., void fun(int &first, int &second)
N. Meng, S. Arthur 23
![Page 24: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/24.jpg)
Pros and Cons
• Pros– Passing process is efficient in terms of
time and space• Cons– Access to the formal parameters is slower
than pass-by-value parameters due to indirect access via reference
– Side effects to parameters– Aliases can be created
N. Meng, S. Arthur 24
![Page 25: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/25.jpg)
An Example: pass-by-value-result vs. pass-by-reference
program foo;var x: int;
procedure p(y: int);begin
y := y + 1;y := y * x;
endbegin
x := 2;p(x);print(x);
end
N. Meng, S. Arthur 25
pass-by-value-result pass-by-reference
x y x y
(entry to p)
(after y:= y + 1)
(at p’s return)
2 2 2 2
2 3 3 3
6 6 9 9
![Page 26: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/26.jpg)
Aliases can be created due to pass-by-reference
• Given void fun(int &first, int &second),– Actual parameter collisions• E.g., fun(total, total) makes first and second to be
aliases– Array element collisions• E.g., fun(list[i], list[j]) can cause first and second
to be aliases if i == j– Collisions between formals and globals• E.g., int* global;
void main() { … sub(global); … }void sub(int* param) { … }
• Inside sub, param and global are aliasesN. Meng, S. Arthur 26
![Page 27: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/27.jpg)
Pass-by-Name
• Implement an inout-mode parameter transition method
• The body of a function is interpreted at call time after textually substituting the actual parameters into the function body
• The evaluation method is similar to C preprocessor macros
N. Meng, S. Arthur 27
![Page 28: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/28.jpg)
An Example in Algol
procedure double(x);real x;
beginx := x * 2;
end;Therefore, double(C[j]) is interpreted as C[j] = C[j] * 2
N. Meng, S. Arthur 28
![Page 29: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/29.jpg)
Another Example
• Assume k is a global variable,procedure sub2(x: int; y: int; z: int);begin
k := 1;y := x;k := 5;z := x;
end;• How is the function call sub2(k+1, j, i)
interpreted?
N. Meng, S. Arthur 29
![Page 30: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/30.jpg)
Disadvantages of Pass-by-Name
• Very inefficient references• Too tricky; hard to read and understand
N. Meng, S. Arthur 30
![Page 31: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/31.jpg)
Implementing Parameter-Passing Methods
• Most languages use the runtime stack to pass parameters– Pass-by-value• Values are copied into stack locations
– Pass-by-result• Values assigned to the actual parameters are placed
in the stack– Pass-by-value-result• A combination of pass-by-value and pass-by-result
– Pass-by-reference• Parameter addresses are put in the stack
N. Meng, S. Arthur 31
![Page 32: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/32.jpg)
An Example
• Function header: void sub (int a, int b, int c, int d)– a: pass by value– b: pass by result– c: pass by value result– d: pass by reference
• Function call: main() calls sub(w, x, y, z)
N. Meng, S. Arthur 32
![Page 33: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/33.jpg)
N. Meng, S. Arthur 33
![Page 34: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/34.jpg)
Design Considerations for Parameter Passing
• Efficiency• Whether one-way or two-way data
transfer is needed
N. Meng, S. Arthur 34
![Page 35: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/35.jpg)
One Software Engineering Principle
• Access by subroutine code to data outside the subroutine should be minimized– In-mode parameters are used whenever no
data is returned to the caller– Out-mode parameters are used when no data
is transferred to the callee but the subroutine must transmit data back to the caller
– Inout-mode parameters are used only when data must move in both directions between the caller and callee
N. Meng, S. Arthur 35
![Page 36: Subprograms - Virginia Tech](https://reader034.vdocuments.mx/reader034/viewer/2022042715/6267247e4a68446e67181309/html5/thumbnails/36.jpg)
A practical consideration in conflict with the principle
• Pass-by-reference is the fastest way to pass structures of significant size
N. Meng, S. Arthur 36