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<== Date ==> | <== Thread ==> |
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Subject: |
Long expressions using MAX and MIN in CALC fields |

From: |
"Redman, Russell O." <Russell.Redman@nrc-cnrc.gc.ca> |

To: |
"Tech-Talk (E-mail)" <tech-talk@aps.anl.gov> |

Date: |
Tue, 25 Feb 2003 15:57:11 -0800 |

I would like to suggest a small enhancement for the calcRecord, calcoutRecord and waitRecord code that uses the postfix routine. This enhancement will allow the MAX and MIN operators to be used on many more input fields than is currently possible due to the limited number of characters permitted in the CALC fields of these records. My motivation for this change is that I would like to use a SINGLE calcout record to evaluate MIN(2,MAX(A,B,C,D,E,F,G,H,I)) i.e. the MAX of nine input fields with an upper limit of 2. The calcout record has trouble with this because the MAX(X,Y) function has only two arguments. The shortest expression I can come up with to evaluate this is MIN(2,MAX(MAX(MAX(MAX(A,B),MAX(C,D)),MAX(MAX(E,F),MAX(G,H))),I)) which is difficult to read and contains 64 characters, far more than the 36 characters permitted. I know I can do the calculation with a select record and a calcout, but these records are used in a time sensitive part of my database and the calculation is nested quite deeply. If at all possible, I would like to complete the operation using a single calcout record. I would like to suggest a change in the expression syntax for the MIN and MAX functions to allow these kinds of long expressions. Currently, the expression MAX(A,B) generates RPCL FETCH_A FETCH_B MAX In an ideal world, I would like the MIN and MAX operators to accept a variable number of arguments, so that an expression like MAX(A,B,C,D) might generate RPCL of the form FETCH_A FETCH_B FETCH_C FETCH_D MAX MAX MAX However, I do not understand the postfix subroutine well enough to see how to do this. A much easier approach to implement is to treat MAX and MIN as binary operators. In the existing postfix.c code MAX and MIN are handled as UNARY operators, but mathematically they can equally well be considered to be binary operators that are commutative and associative, similar in many ways to the '+' and '*' operators. With an infix notation, we could write my expression as 2 min (A max B max C max D max E max F max G max H max I). This is fairly easy to understand but does not save enough characters and might be difficult to distinguish from the MAX and MIN functions in existing databases . Unfortunately, most of the obvious punctuation characters have already been used for other operations, but I suggest that "|<" and ">|" might be suitable as binary MIN and MAX operators, respectively. I do not believe that these combinations would ever occur in a valid expression in an existing database. My expression would then become 2|<(A>|B>|C>|D>|E>|F>|G>|H>|I). This is more cryptic, but contains only 30 characters and is simple to implement. In the postfix.c code, insert the two lines {">|", 7, 8, BINARY_OPERATOR, MAX}, /* maximum of 2 args */ {"|<", 7, 8, BINARY_OPERATOR, MIN}, /* minimum of 2 args */ in the table of operators immediately after the MAX and MIN entries. This is the only required code change. It is then necessary to rebuild libCom and iocCore, to touch rec/calcRecord.c, rec/calcoutRecord.c and waitRecord.c and rebuild rec, and (for me at least) to rebuild my record support. With this simple change, the assignment -> dbpf "simb:b.CALC","(A>|B>|C)|<D" gives RPCL: 0x010203434304447f094343434343434343484000 which I interpret as 0x01 = FETCH_A 0x02 = FETCH_B 0x03 = FETCH_C 0x43 = MAX 0x43 = MAX 0x04 = FETCH_D 0x44 = MIN 0x7f = END_STACK (Ignore the garbage after the END_STACK token.) With this simple change it is possible to take the MAX or MIN of every input field in a calcRecord or a calcoutRecord, as well as more complex expressions such as the one I was after. In sCalcPostfix.c there is code for an alternative implementation of the postfix operation. This generates different tokens in the RPCL field and is not used for the calcRecord or calcoutRecord in R3.13.5. I have made the corresponding changes in sCalcPostfix.c to the table of operators, but have not been able to test them. I have attached both postfix.c and sCalcPostfix.c to this message. Dr. Russell O. Redman Tel: (250) 363-6917 | Fax: (250) 363-0045 <mailto:Russell.Redman@nrc-cnrc.gc.ca> National Research Council Canada | Conseil national de recherches Canada 5071 West Saanich Road | 5071 West Saanich Road Victoria, B. C. V9E 2E7 | Victoria, C.-B. V9E 2E7 Government of Canada | Gouvernement du Canada

**Attachment:
postfix.c**

**Attachment:
sCalcPostfix.c**

- Replies:
**Re: Long expressions using MAX and MIN in CALC fields***Andrew Johnson***Re: Long expressions using MAX and MIN in CALC fields***Tim Mooney***Re: Long expressions using MAX and MIN in CALC fields***Tim Mooney*

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