not2.gif - 1916 Bytes

d. Logical NOT

The logical NOT is the class exclusion operator. The result of a logical NOT between two classes is the class containing all  the elements of the first class which do not also belong to the second class. 

Thus, if A and B are two terms representing the two classes of documents indexed with terms A and B respectively, the logical  NOT between A and B is the class of documents indexed with term A but not  with the term B. The symbol used  to indicate the logical NOT operation is the not sign (^). The logical NOT should be used with extreme caution, since it may  easily result in the inadvertent loss of relevant material. For example, a search request might concern documents about food  additives but excluding references to flavour enhancement. One might be tempted to formulate the query as follows: 


It is quite possible, however, for a document on the subject to contain a section on food addittives. This document would then  be missed if one formulated the query as above. Note further that unlike the logical OR and logical AND, the logical NOT  operation does not give the same results if the two operands are inverted. Thus, in general A ^ B is not the same as B ^ A.

Note:  Do not use the negation sign as the first operator in your query. The query:


is illegal

CDS-ISIS2.gif - 2872 Bytes