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subprograms

summary

    This subchapter looks at subprograms and subroutines.

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stub section

    This subchapter is a stub section. It will be filled in with instructional material later. For now it serves the purpose of a place holder for the order of instruction.

    Professors are invited to give feedback on both the proposed contents and the propsed order of this text book. Send commentary to Milo, PO Box 1361, Tustin, California, 92781, USA.

subprograms and subroutines

    This subchapter looks at subprograms, subroutines, procedures, and functions.

    Alan Turing’s famous 1936 paper “On Computable Numbers, with an Application to the Entscheidungsproblem”, in which he described the Turing Machine, first introduced the idea of m-functions (also called m-m configuration functions or skeleton tables).

    Alan Turing’s m-functions are equivalent to and the predecesors of modern functions, procedures, suboutines, and methods.

valid function identifiers

    Quick summary of the rules for building valid function identifiers in several major languages, using regular expressions:

Ada	[a-zA-Z](_?[a-zA-Z0-9])*
ALGOL-68	[a-z][a-z0-9 ]*
Awk	[_a-zA-Z][_a-zA-Z0-9]*
B	[_a-zA-Z][_a-zA-Z0-9]*
BourneShell	[_a-zA-Z0-9]+
C	[_a-zA-Z][_a-zA-Z0-9]*
C#	[_a-zA-Z][_a-zA-Z0-9]*
C++	[_a-zA-Z][_a-zA-Z0-9]*
COBOL	[a-zA-Z][a-zA-Z0-9-]* 30 character maximum
Classic REXX	[a-zA-Z!?@#][a-zA-Z0-9!?@#]*
Common Lisp	anything without a space and is not a number
E	[_a-zA-Z][_a-zA-Z0-9]*
Eiffel	[a-zA-Z][_a-zA-Z0-9]*
F#	[_a-zA-Z][_a-zA-Z0-9']*
FORTRAN	[A-Z][A-Z0-9]* maximum of six characters
Forth	anything without a space and is not a number
GNU-bc	[a-z][a-z0-9_]*
Haskell	[_a-z][_a-zA-Z0-9']*
Java	[_a-zA-Z$][_a-zA-Z0-9$]*
JavaScript	[_a-zA-Z$][_a-zA-Z0-9$]*
Lisp	anything without a space and is not a number
Maple	[_a-zA-Z][_a-zA-Z0-9]*
Mathematica	[a-zA-Z][a-zA-Z0-9]*
Matlab	[a-zA-Z][_a-zA-Z0-9]*
Mercury	[_a-z][_a-zA-Z0-9]*
merd	[_a-z][_a-zA-Z0-9]*[!?']*
Modula-3	[a-zA-Z][_a-zA-Z0-9]*
MUMPS	[a-zA-Z%][a-zA-Z0-9]*
OCaml	[_a-z][_a-zA-Z0-9']*
Pascal	[a-zA-Z][a-zA-Z0-9]*
Perl	[_a-zA-Z0-9]+
Perl6	[_a-zA-Z0-9]+
PHP	[_a-zA-Z][_a-zA-Z0-9]*
PL/I	[a-zA-Z][a-zA-Z0-9]*
Pliant	[_a-zA-Z][_a-zA-Z0-9]* or '[^']*'
Prolog	[_a-z][_a-zA-Z0-9]*
Python	[_a-zA-Z][_a-zA-Z0-9]*
Rebol	[_a-zA-Z?!.'+*&|=~-][_a-zA-Z0-9?!.'+*&|=~-]* or [^0-9[](){}":;/][^ \n\t[](){}":;/]*
Ruby	[_a-zA-Z][_a-zA-Z0-9]*[!?]?
Scheme	[_a-zA-Z!0&*/:<=>?^][_a-zA-Z!0&*/:<=>?^0-9.+-]*
SmallTalk	[a-zA-Z][a-zA-Z0-9]*
SML	[_a-z][_a-zA-Z0-9']*
Tcl	[^ \t\n\r\f]+

   “Procedure names should reflect what they do; function names should reflect what they return.ÊFunctions are used in expressions, often in things like ifs, so they need to read appropriately.
        if(checksize(x))
   “is unhelpful because we can’t deduce whether checksize returns true on error or non-error; instead
        if(validsize(x))
   “makes the point clear and makes a future mistake in using the routine less likely.” —Rob Pike, Notes on Programming in C, February 21, 1989

mismatched types

    According to Brian Kernighan, co-creator of the AWK and AMPL programming languages and co-author of K&R C, one of the three most common errors for new programmers is mismatched types in a fucntion call.

Ada

BASIC

assembly language instructions

subroutine return pointer

    Some RISC processors include a special subroutine return pointer rather than using a stack in memory. The return address for subroutine calls is stored in this register rather than in memory. More than one level of subroutine calls requires storing and saving the contents of this register to and from memory.

• Motorola 88100: r1 is a 32 bit register containing the return pointer generated by bsr and jsr instructions; the register can be read or overwritten by software and can even be used as a temporary general purpose data register

See also Registers

other

   “Documentation that is structured and contained within the program is able to immediately satisfy the changing information demands of the maintainer. Those needs are determined in part by the subtask on which he is currently working. For solving nontrivial error correction or modification problems, the maintainer must have a detailed understanding of the program. To locate a section of code, knowledge of the program’s structure is required. Knowing how an instruction sequence relates to other parts of the program is important for altering and testing software. The documentor can inform the unknowledgeable programmer in each subtask demand by varying the message content and effectively using the visual space.
   “Information may be conveyed to the maintainer in several ways. One is an abstract summary of the module at the beginning of the routine. Another is through the titles and headings of processing sections positioned in the instruction sequence. The third is in phrases and short sentences to the right of the code. They describe the processing steps and relate them to other parts of the program. The descriptions are organized into an outline that reflects the processing divisions of the routine.
   “The size and complexity of the module determine whether the information will be used. Small routines may need only comments to the right of the code. A more complete description is required for large programs.
   “The type of documentation that has just been read has a bearing on the processing of code. Documentation formats act as advance organizers of thought. Each type primes the maintainer for a different response to the instructions encountered. Messages that are consistent with the structure of the program aid recognition and recall. … Programs are documented to enhance the maintainer’s performance.” —Dennis Smith. Designing Maintainable Software. Springer-Verlag, 1999, pg. 103

   “A second recommendation has to do with procedures and other major units of the program—Introductory module comments are also in order. Comments following a procedure header explaining the general nature of the procedure are not only in order but may be necessary. Keeping in mind the bridge aspect, we need not describe the calling environment. The professional assumes that the reader has read the program to the degree that the procedure calls are understood—but maybe not the procedure itself. As such, the procedure header comments should be short and help the reader understand the next level of detail in the program.” —Henry Ledgard. Professional Software Volume II: Programming Practice, Addison-Wesley, 1987, pg 65

   “6. Symmetry is a complexity reducing concept (co-routines include sub-routines); seek it everywhere.” —Alan Perlis, Epigrams on Programming, ACM’s SIGPLAN Notices Volume 17, No. 9, September 1982, pages 7-13

chapter contents

• procedures
• functions
• paramters
• coroutines

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