data structures howeverit should be possible to indicate the desirability of packing at least in those cases in which more than one component would fit into single wordi when gain of storage economy by factor of and more could be achieved we propose the convention to indicate the desirability of packing by prefixing the symbol array (or recordin the declaration by the symbol packed representation of records records are mapped onto computer store by simply juxtaposing their components the address of component (fieldri relative to the origin address of the record is called the field' offset ki it is computed as ki si- where sj is the size (in wordsof the -th component we now realize that the fact that all components of an array are of equal type has the welcome consequence that ki the generality of the record structure does unfortunately not allow such simplelinear function for offset address computationand it is therefore the very reason for the requirement that record components be selectable only by fixed identifiers this restriction has the desirable benefit that the respective offsets are known at compile time the resulting greater efficiency of record field access is well-known the technique of packing may be beneficialif several record components can be fitted into single storage word (see fig since offsets are computable by the compilerthe offset of field packed within word may also be determined by the compiler this means that on many computers packing of records causes deterioration in access efficiency considerably smaller than that caused by the packing of arrays padded fig representation of packed record representation of sets set is conveniently represented in computer store by its characteristic function (sthis is an array of logical values whose ith component has the meaning " is present in sas an examplethe set of small integers { is represented by the sequence of bitsby bitstringc( the representation of sets by their characteristic function has the advantage that the operations of computing the unionintersectionand difference of two sets may be implemented as elementary logical operations the following equivalenceswhich hold for all elements of the base type of the sets and yrelate logical operations with operations on setsi in ( + ( in xor ( in yi in ( * ( in ( in yi in ( - ( in ~( in