Searching Damian Gordon. Google PageRank Damian Gordon.

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Presentation transcript:

Searching Damian Gordon

Google PageRank Damian Gordon

Google Search Algorithm First Draft: PROGRAM GoogleCollect: NextLink <- random website; WHILE (NextLink != NULL) DO IF (No copy of this page in google collection) THEN copy this page into google collection; ENDIF; NextLink <- Next link on this page; ENDWHILE; END.

Google Search Algorithm First Draft: PROGRAM GoogleSearch: READ SearchString; Get First Webpage from collection; WHILE (Webpages Left to Search) DO IF (SearchString IN Current-Web-Page) THEN Put this page on the list; ENDIF; Get Next Webpage; ENDWHILE; Order the list according to PageRank; END.

Database Searching Damian Gordon

Searching Oracle DB2 MySQL SQL Server PostgreSQL

Searching

Array Searching Damian Gordon

Searching Let’s remember our integer array from before:

Searching ……..…

Searching Let’s say we want to find everyone who is aged 18:

Searching: Sequential Search

This is a SEQUENTIAL SEARCH. If the array is 40 characters long, it will take 40 checks to complete. If the array is 1000 characters long, it will take 1000 checks to complete.

Here’s how we could do it: PROGRAM SequentialSearch: integer SearchValue <- 18; integer ArraySize <- 40; FOR N IN 0 TO ArraySize-1 DO IF Age[N] = SearchValue THEN PRINT “User “ N “is 18”; ENDIF; ENDFOR; END. Searching: Sequential Search

Searching: Binary Search If the data is sorted, we can do a BINARY SEARCH

Searching: Binary Search If the data is sorted, we can do a BINARY SEARCH ……..…

Searching: Binary Search If the data is sorted, we can do a BINARY SEARCH

Searching: Binary Search If the data is sorted, we can do a BINARY SEARCH This means we jump to the middle of the array, if the value being searched for is less than the middle value, all we have to do is search the first half of that array.

Searching: Binary Search If the data is sorted, we can do a BINARY SEARCH This means we jump to the middle of the array, if the value being searched for is less than the middle value, all we have to do is search the first half of that array. We search the first half of the array in the same way, jumping to the middle of it, and repeat this.

Searching: Binary Search

The BINARY SEARCH just takes five checks to find the right value in an array of 40 elements. For an array of 1000 elements it will take 11 checks. This is much faster than if we searched through all the values.

If the data is sorted, we can do a BINARY SEARCH PROGRAM BinarySearch: integer First <- 0; integer Last <- 40; boolean IsFound <- FALSE; WHILE First <= Last AND IsFound = FALSE DO Index = (First + Last)/2; IF Age[Index] = SearchValue THEN IsFound <- TRUE; ELSE IF Age[Index] > SearchValue THEN Last <- Index-1; ELSE First <- Index+1; ENDIF; ENDWHILE; END. Searching: Binary Search

etc.