TIP #351: ADD STRIDING SUPPORT TO LSEARCH =========================================== Version: $Revision: 1.19 $ Author: Peter da Silva Donal K. Fellows Harald Oehlmann Andreas Leitgeb State: Draft Type: Project Tcl-Version: 8.7 Vote: Pending Created: Thursday, 09 July 2009 URL: https://tip.tcl-lang.org351.html Post-History: ------------------------------------------------------------------------- ABSTRACT ========== This TIP allows the searching of lists that are grouped into collections of several elements. RATIONALE =========== When operating on strided lists (for example key-value lists) it's normal to convert them between lists and arrays and back again. If it was possible to efficiently perform a strided search of the list it would be possible to (for example) search just the keys and ignore the values. Indeed, Tcl has a long tradition of working with lists which are structured into groups through *foreach* and *array get*, and this is strengthened further with dictionaries [TIP #111] and striding sorts [TIP #326]. However, there is currently no facility for searching such lists; this TIP proposes fixing this. PROPOSED CHANGE ================= We propose adding a *-stride* option to *lsearch*, by exact analogy with the option added to *lsort* in [TIP #326], whose semantics it should closely match. If *-stride* is supplied, the list will be treated as consisting of groups of grpSize elements. The search will be operated within this group as it is a first level of nested lists (see /Conceptual Backround/ below). The first element of *-index* is used to seach for an item of the group. The option *-start* always points to the beginning of the group, even if a position within the group is given. Returned indices are the first element of the striding group(s) that is/are being indicated. The list length must be a multiple of *grpSize*, which in turn must be at least 2. CONCEPTUAL BACKROUND ====================== STRIDING EQUIVALENT TO FIRST LEVEL OF NESTED LISTS ---------------------------------------------------- The striding within the list is seen as the first level of list nesting. E.g. *Nested list*: set deep {{1 a A} {2 b B} {3 c C}} *Flat strided list*: set flat {1 a A 2 b B 3 c C} Functions should operate the same way on both representation, with the only difference, that *-stride 3* must be specified in the second case. Unfortunately, the current implementation of *lsort* is not doing this. It interpretes *-index ""* as *-index 0*: % lsort -stride 2 {A 1 A 2 A 0} A 1 A 2 A 0 % lsort -stride 2 -index "" {B 2 B 1 A 3} A 3 B 2 B 1 NUMERIC POSITION INDICES -------------------------- Numerical positional indices (-start parameter, return value) follow the flattened list and not the grouped list. This is different to the nested list view. Furthermore, if option *-subindices* is given and a non-empty argument for *-index*, then the group-start and index-into-group are added up. This gives compatibility with lindex, as in the no-stride case. EXAMPLES ========== In these examples, the variable /kvlist/ holds the key-value list: set kvlist {K1 V1 K2 V1 K1 K1} Example 1: find keys even if they exist multiple times: % lsearch -all -stride 2 -index 0 -exact $kvlist K1 0 4 Example 2: find existance of a value: % lsearch -all -stride 2 -index 1 -exact $kvlist V1 0 2 Remark that the indexes of the first group elements are returned. The real values are at "result+index" eq *1 3*. Example 3: extract a sub-kv-list starting from key K2: % lrange $kvlist [lsearch -stride 2 -index 0 -exact $kvlist K2] end K2 V1 K1 K1 Example 4: find a group within a list: % lsearch -stride 2 -exact $kvlist {K2 V1} 2 Example 5: find in combined strided and nested list % lsearch -stride 2 -index {1 1} -exact\ {K0 {V0.0 V0.1} K1 {V1.0 V1.1}}\ V1.1 2 Example 6: subindices with strided list: % lsearch -stride 2 -index {1 1} -subindices {1 {a A} 2 {b B}} B 3 1 (that is: 2 for the group-start plus 1 for the intra-group index, and separately 1 for the further nested index. % lindex {1 {a A} 2 {b B}} 3 1 B to be consisten with: % lsearch -index {1 1} -subindices {{1 {a A}} {2 {b B}}} B 1 1 1 % lindex {{1 {a A}} {2 {b B}}} 1 1 1 B COPYRIGHT =========== This document has been placed in the public domain. ------------------------------------------------------------------------- TIP AutoGenerator - written by Donal K. Fellows