UserGuide - Some Tips & Tricks

"Using a map to index a list"

lists provide a powerful way to build a structured storage system - however they have a disadvantage. If you aren't sure exactly where in the list a particular item is, you must examine each entry in the list to find the right one.

Maps also provide a similar function but are indexed by a key value instead - however they too have a disadvantage, they don't maintain the order elements are inserted into the list.

However by using a combination of the two, you can neatly avoid both of these issues...

This example loads a structured list of book data and builds an index of ISBN numbers using a map. It then demonstrates how to access the list using the index in the map.
  ; A book catalog structure.
  Structure BOOK
  ; Create a list to hold the catalog entries.
  NewList Catalog.BOOK()
  ; Create a map to hold the ISBN index.
  NewMap ISBN13.l()
  ; Working variables.
  Define.l Count, Index
  Define.s ISBN
  ; Add an empty element at the top of the list.
  ; The first element in a list is numbered zero, however the map will return zero if a requested entry isn't present.
  ; This empty element avoids a potential problem with an incorrect reference to the first catalog item being returned.
  For Count = 1 To 5
    ; Read the data from the table into the list.
    Read.s Catalog()\Title
    Read.s Catalog()\Author
    Read.s Catalog()\ISBN13
    Read.d Catalog()\Price
    ; Index the ISBN to the map.
    ISBN13(Catalog()\ISBN13) = ListIndex(Catalog())
  Next Count
  ; Find an entry.
  ISBN = "978-0340896983"
  Index = ISBN13(ISBN)
  If Index > 0
    Debug "Book with ISBN13 " + ISBN + " is at list index " + StrU(Index) + "."
    Debug ""
    ; We can now directly select the right list element without having to perform a search.
    SelectElement(Catalog(), Index)
    Debug "'" + Catalog()\Title + "' by " + Catalog()\Author
    Debug "ISBN: " + Catalog()\ISBN13
    Debug "Price: " + StrD(Catalog()\Price, 2)
    Debug "No book with that ISBN in the catalog."
  ; Some test data.
    Data.s "Carte Blanche", "Jeffery Deaver", "978-1444716474"
    Data.d 19.99
    Data.s "One Day", "David Nicholls", "978-0340896983"
    Data.d 7.99
    Data.s "Madeleine", "Kate McCann", "978-0593067918"
    Data.d 20.00
    Data.s "The Dukan Diet", "Dr Pierre Dukan", "978-1444710335"
    Data.d 8.99
    Data.s "A Game of Thrones", "George R. R. Martin", "978-0006479888"
    Data.d 9.99
    Data.s "The Help", "Kathryn Stockett", "978-0141039282"
    Data.d 8.99

UserGuide Navigation

< Previous: Advanced functions | Overview