Dictionary lookup - getting the keys

When you have an array you can simply loop through the contents to examine what's there, but when you have a dictionary you need to have the list of keys so that you can look up each value in the dictionary.  For that, we need a getKeys$() function.  The following function returns a single string with the keys from our global dictionary$ variable, each separated by a delimiter that we can specify.

function getKeys$(delimiter$)
  pointer = 1
  while pointer > 0
    'get the next key
    pointer = instr(dictionary$, "~key~", pointer)
    if pointer then
      keyPointer = pointer + 5
      pointer = instr(dictionary$, "~value~", pointer)
      key$ = mid$(dictionary$, keyPointer, pointer - keyPointer)
      if instr(keyList$, "~key~" + key$) = 0 then
        getKeys$ = getKeys$ + key$ + delimiter$
        keyList$ = keyList$ + "~key~" + key$
      end if
    end if
  wend
end function

Once have this string we can tease out each key.  Here is an quick example that shows how to do this.  The variable allKeys$ will hold all the keys, each separated by "~".  Then we use the word$() function to get each key.

global dictionary$

call setValueByName "first", "Tom"
call setValueByName "last", "Thumb"
call setValueByName "phone", "555-555-1234"

allKeys$ = getKeys$("~")
print allKeys$

key = 1
while word$(allKeys$, key, "~") <> ""
  key$ = word$(allKeys$, key, "~")
  print "Key number "; key; " is "; key$
  print "    value = "; getValue$(key$)
  key = key + 1
wend


Here is what the resulting output looks.

phone~last~first~
Key number 1 is phone
    value = 555-555-1234
Key number 2 is last
    value = Thumb
Key number 3 is first
    value = Tom

Notice that the keys do not come out in the order that we put them in.  This is typical in dictionary style lookup mechanisms.  The ordering of keys is not guaranteed.

Dictionary lookup - Garbage collection

If we need to use our keyed dictionary lookup functions for a purpose where we will change the values for any or all keys the string we save in dictionary$ will get larger each time we set a key and value.  This is because the setValueByName subroutine sets a key and value by adding onto the front of the dictionary$ variable but it does not remove any preexisting value for that key.  So if for example I set a key of "storeFolder" and a value of "c:\myStoreFolder" and then later I change the value to "c:\myOtherFolder" I will have two different entries for the key "storeFolder".  Only the latest value will be returned by the getValue$() function.

So, how do we fix this?  We implement a garbage collector.  We can create a subroutine that makes a copy of dictionary$ that only has the latest value for each key.

Here is a first stab at a garbage collector subroutine.

sub collectGarbage
  pointer = 1
  while pointer > 0
    'get the next key
    pointer = instr(dictionary$, "~key~", pointer)
    if pointer then
      keyPointer = pointer + 5
      pointer = instr(dictionary$, "~value~", pointer)
      key$ = mid$(dictionary$, keyPointer, pointer - keyPointer)
      if instr(keyList$, key$) = 0 then
        value$ = getValue$(key$)
        newDictionary$ = "~key~" + key$ + "~value~" + value$ + newDictionary$
        keyList$ = keyList$ + key$
      end if
    end if
  wend
  dictionary$ = newDictionary$
end sub

Keyed dictionary lookup in Liberty BASIC

Liberty BASIC has a way to manage collections of data by using arrays and you look up the information by numeric position.  You can do a lot with this but it doesn't let you look up information by name.

We can provide an easy to use way to do this in Liberty BASIC by using the string functions of Liberty BASIC.  By using a single string we can have easy lookup of values by name and also have the ability to store the collection of values in a file and retrieve it simply.  In some versions of BASIC this is only useful for small lists of information because of string size limitations of 255.  Liberty BASIC permits strings of millions of characters so this is not a problem.

Here is a very simple demo of the concept just to get us started.  In future postings we will explain and enhance the way this works.

global dictionary$

call setValueByName "first", "Tom"
call setValueByName "last", "Thumb"
call setValueByName "phone", "555-555-1234"

print getValue$("last")
print getValue$("blah")
print getValue$("phone")
print getValue$("first")

sub setValueByName key$, value$
  dictionary$ = "~key~"+key$+"~value~"+value$+dictionary$
end sub

function getValue$(key$)
  getValue$ = chr$(0)
  keyPosition = instr(dictionary$, "~key~"+key$)
  if keyPosition > 0 then
    keyPosition = keyPosition + 5  'skip over key tag
    valuePosition = instr(dictionary$, "~value~",  keyPosition)
    if valuePosition > 0 then
      valuePosition = valuePosition + 7   'skip over value tag
      endPosition = instr(dictionary$, "~key~", valuePosition)
      if endPosition > 0 then
        getValue$ = mid$(dictionary$, valuePosition, endPosition - valuePosition)
      else
        getValue$ = mid$(dictionary$, valuePosition)
      end if
    end if
  end if
end function