Searching+ordered+lists

//by Richard Russell, December 2006//

The fastest way to search an //ordered// list is the **binary search**; you can find a description of how it works in the [|main BB4W documentation]. The code below is a slightly simplified version, which searches an alphabetically-ordered string array for an exact match to a supplied string:

code format="bb4w" DEF FNwhere(A$, S$) LOCAL B%, H%, T%     T% = DIM(A$,1) H% = 2 WHILE H%=A$(B%+H%) B% += H%       H% /= 2 UNTIL H%=0 IF S$=A$(B%) THEN = B% ELSE = -1 code If the supplied string matches one of the elements in the array the function returns the index of the matching element. If there is no match the function returns -1.

Note that the entire array must be pre-sorted into alphabetical sequence.


 * Important note:**

It is important that the method you use to **sort** the array is compatible with the method you use to **search** the array, i.e. that they use the same //collation order//. Specifically, if you use the above **FNwhere** function you can sort the array using version 1.3 (or later) of the **SORTLIB** library and set the second parameter of **FN_sortinit** to **2**.

If you use an earlier version of SORTLIB, or set the parameter to a different value, then the comparisons you make when searching the string should also use the CompareString function:

code format="bb4w" DEF FNwhere(A$, S$) LOCAL B%, H%, R%, T%     T% = DIM(A$,1) H% = 2 WHILE H%1 B% += H%        ENDIF H% /= 2 UNTIL H%=0 IF S$=A$(B%) THEN = B% ELSE = -1 code

By changing the second parameter of **CompareString** from 0 to 1 a //case insensitive// comparison is made. That would be appropriate if you specified the **ignore case** option when using SORTLIB.