barcode generator dll c# MULTIPLE ASSIGNMENT in Font

Maker Quick Response Code in Font MULTIPLE ASSIGNMENT

CHAPTER 11 MULTIPLE ASSIGNMENT
Drawing QR-Code In None
Using Barcode encoder for Font Control to generate, create QR Code image in Font applications.
www.OnBarcode.com
Barcode Encoder In None
Using Barcode generator for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
6 6 6 6 6 6
Paint Data Matrix In None
Using Barcode generator for Font Control to generate, create ECC200 image in Font applications.
www.OnBarcode.com
Making GTIN - 13 In None
Using Barcode creator for Font Control to generate, create EAN13 image in Font applications.
www.OnBarcode.com
61$0( 6PLWK -RQHV %ODNH &ODUN $GDPV
QR Maker In None
Using Barcode generation for Font Control to generate, create Quick Response Code image in Font applications.
www.OnBarcode.com
PDF 417 Generation In None
Using Barcode generator for Font Control to generate, create PDF 417 image in Font applications.
www.OnBarcode.com
67$786     
Make UPC Symbol In None
Using Barcode generation for Font Control to generate, create Universal Product Code version A image in Font applications.
www.OnBarcode.com
USS Code 93, USS 93 Maker In None
Using Barcode maker for Font Control to generate, create Code 93 Full ASCII image in Font applications.
www.OnBarcode.com
&,7< /RQGRQ 3DULV 3DULV /RQGRQ $WKHQV
Encode Denso QR Bar Code In None
Using Barcode creation for Online Control to generate, create Denso QR Bar Code image in Online applications.
www.OnBarcode.com
QR Code 2d Barcode Drawer In None
Using Barcode generation for Microsoft Excel Control to generate, create QR Code JIS X 0510 image in Office Excel applications.
www.OnBarcode.com
6 6 6 6 6 6 6 6 6 6 6 6 6
1D Barcode Printer In .NET
Using Barcode generation for ASP.NET Control to generate, create Linear 1D Barcode image in ASP.NET applications.
www.OnBarcode.com
Generating GTIN - 13 In Objective-C
Using Barcode encoder for iPad Control to generate, create EAN / UCC - 13 image in iPad applications.
www.OnBarcode.com
3 3 3 3 3 3 3 3 3 3 3 3 3
Generating Code 128A In Java
Using Barcode encoder for Java Control to generate, create USS Code 128 image in Java applications.
www.OnBarcode.com
Print Quick Response Code In None
Using Barcode drawer for Software Control to generate, create QR Code image in Software applications.
www.OnBarcode.com
47<            
Decode EAN / UCC - 13 In VB.NET
Using Barcode scanner for .NET framework Control to read, scan read, scan image in .NET applications.
www.OnBarcode.com
Barcode Printer In None
Using Barcode generation for Microsoft Word Control to generate, create Barcode image in Word applications.
www.OnBarcode.com
3 3 3 3 3 3 3
Barcode Creator In None
Using Barcode printer for Software Control to generate, create Barcode image in Software applications.
www.OnBarcode.com
Encoding UPC Code In None
Using Barcode printer for Online Control to generate, create UCC - 12 image in Online applications.
www.OnBarcode.com
31$0( 1XW %ROW 6FUHZ 6FUHZ &DP &RJ
Data Matrix ECC200 Creation In Visual C#.NET
Using Barcode generator for .NET Control to generate, create Data Matrix 2d barcode image in Visual Studio .NET applications.
www.OnBarcode.com
2D Barcode Drawer In C#.NET
Using Barcode creation for Visual Studio .NET Control to generate, create Matrix Barcode image in .NET applications.
www.OnBarcode.com
&2/25 5HG *UHHQ %OXH 5HG %OXH 5HG
:(,*+7      
&,7< /RQGRQ 3DULV 2VOR /RQGRQ 3DULV /RQGRQ
CHAPTER 11 MULTIPLE ASSIGNMENT
(the one that attempts to move supplier S1 to Paris) is shorthand for the following: S := WITH ( ( ( ( ( S WHERE S# = S# ('S1') ) AS T1 , EXTEND T1 ADD ( 'Paris' AS NEWCITY ) ) AS T2 , T2 { ALL BUT CITY } ) AS T3 , T3 RENAME ( NEWCITY AS CITY ) ) AS T4 : S MINUS T1 ) UNION T4 ;
It follows that the successful UPDATE in the example is indeed a multiple assignment; in fact, it involves precisely two target variables, relvar S and relvar P, and it assigns one value to one of these variables and another to the other. By contrast, the UPDATEs that failed were both single assignments. And so we have here our first example of a multiple assignment that can t be simulated by a sequence of single assignments; as we claimed in the introduction, there are some things that can t be done at all without multiple assignment, and multiple assignment thus truly is a new primitive operator. Aside: In case you re not familiar with the WITH construct (see the expanded version of the first UPDATE in the foregoing discussion), we digress for a moment to explain it. Basically, WITH allows us to introduce names for subexpressions and thereby to break a large, complicated expression into smaller, more digestible pieces. The introduced names can be regarded as denoting compiler-generated temporary variables. In the example, therefore, the overall assignment to relvar S can be thought of as equivalent to the following sequence of assignments T1 T2 T3 T4 S := := := := := S WHERE S# = S# ('S1') ; EXTEND T1 ADD ( 'Paris' AS NEWCITY ) ; T2 { ALL BUT CITY } ; T3 RENAME ( NEWCITY AS CITY ) ; ( S MINUS T1 ) UNION T4 ;
with the important distinction, of course, that the overall assignment is indeed a single statement, not a sequence of five separate statements. Alternatively, and more accurately, we could say that the original assignment to relvar S is equivalent to the following single assignment: S := ( S MINUS ( S WHERE S# = S# ('S1') ) ) UNION ( ( ( EXTEND ( S WHERE S# = S# ('S1') ) ADD ( 'Paris' AS NEWCITY ) ) { ALL BUT CITY } ) RENAME ( NEWCITY AS CITY ) ) ; End of aside. Let s get back to the example. As that example correctly suggests, it s the fact that certain constraints are in effect that makes it impossible for certain multiple assignments to be replaced by a sequence of single assignments. But you might also have realized that we re making a tacit assumption here: To be specific, we re assuming that all constraint checking is immediate, meaning it s done at end-of-statement or, loosely, at semicolons. You might therefore be thinking that if we had deferred the checking instead (to end-of-transaction), there would have been no need for the multiple assignment. And of course you d be right in thinking that way. But we have good reasons for insisting that checking never be deferred in this sense reasons that we ll be discussing in detail in the next section and thus we believe the multiple assignment was necessary in the example after all. Incidentally, the foregoing discussion serves to highlight a significant difference in emphasis (not the only one) between our multiple form of assignment and Dijkstra s concurrent form.
CHAPTER 11 MULTIPLE ASSIGNMENT
Our major reason for wanting to be able to perform several separate though presumably interrelated assignments as a single operation is to ensure that no integrity checking is done until all of the assignments in question have been executed. This emphasis on our part on the importance of integrity checking is understandable, given our database background. By contrast, Dijkstra s language presumably didn t support integrity constraints (in the usual database sense) at all; hence, the problems solved by our multiple assignment proposal presumably didn t arise. To say it again, therefore: Given our assumption regarding immediate checking, there s no way the multiple assignment in the example we ve been discussing can be simulated by any sequence of single assignments, and so multiple assignment really is a new primitive operator. Note: We first proposed such an operator in The Third Manifesto, but we didn t get it quite right in fact, we got it wrong twice, in two different ways! We ll elaborate on this point in the section on semantics, later.
Copyright © OnBarcode.com . All rights reserved.