Re: Example of misusage of exceptions.
From: Skybuck Flying (nospam_at_hotmail.com)
Date: 11/01/04
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Date: Mon, 1 Nov 2004 19:36:26 +0100
"Nicolai Hansen" <nic@aub.dk> wrote in message
news:d96764ff.0411010304.36146331@posting.google.com...
> "Skybuck Flying" <nospam@hotmail.com> wrote in message
news:<cm38jr$6mu$1@news6.zwoll1.ov.home.nl>...
> > Hello,
> >
> > First I like to say I appreciate that this person has made a website
about
> > adaptive huffman compression and even supplied some code.
> >
> > However the code is pretty shocking ;) and a good example of misusage of
> > exceptions... probably because out of lazyness ;)
> >
> > http://www.cs.sfu.ca/cs/CC/365/li/squeeze/AdaptiveHuff.html
> >
> > This person/code is using/throwing exceptions instead of returning a
boolean
> > value as I think that he should have done in the first place.
> >
> > The first function tries to find a character if it can't find it it
simply
> > (?!) throws an exception.
> >
> > The second function tries to find a tree node with a certain frequency
if it
> > doesn't find it it also throws an exception.
> >
> > The update tree method has a try catch block... this whole construction
> > obfuscates the logic path quite considerably...
> >
> > For example at first glance it looks like the 'catch' block can be
'entered'
> > from both functions... in reality however the second function will
always
> > find the frequency... so the programmer did not clearly express that in
the
> > code.
> >
> >
> > // two functions from other class.
> >
> > /**
> > * findChar
> > *
> > * Returns the index of the character
> > *
> > */
> >
> > public int findChar (char letter) throws NoSuchElementException
> > {
> >
> > for (int i=0; i < 2*ALPH_SIZE; i++) {
> > if (tree[i].letter == letter) return i;
> > }
> >
> > throw new NoSuchElementException(" in findChar");
> > }
> >
> > /*
> > * HighestInBlock
> > *
> > * Returns the value of the node that is the highest in the block
> > * i.e. of all nodes with the same count
> > *
> > */
> >
> > public int highestInBlock (int count)
> > {
> >
> > int highest = -1;
> >
> > for (int i=0; i < 2*ALPH_SIZE; i++)
> > {
> > if (tree[i].count == count) highest=i;
> > }
> >
> > if (highest == -1) throw new NoSuchElementException("No such
node
> > with count of " + count);
> >
> > return highest;
> > }
> >
> >
> > public void updateTree(char newchar)
> > {
> > int current;
> > int max;
> >
> >
> > try { // first appearance
for
> > symbol
> > current = findChar(newchar); // Go to symbol
external
> > node
> >
> > // could enter catch block ?! yes
> >
> > max = highestInBlock(tree[current].count); // Node number max
in
> > block?
> >
> > // could enter catch block ?! probably not !
> >
> > if (current != max && tree[current].parent != max) {
> > addMessage(" Swapping nodes " +current+ " and " +max);
> > swap(current, max); // Switch node with
> > highest node in block
> > pause();
> > current = max;
> > }
> > addMessage(" Increasing count for '" +newchar+ "'");
> > tree[current].count++; // Increment node
weight
> > pause();
> >
> >
> > } catch (NoSuchElementException e) { // Yes
> > addMessage(" Spawning new node for '" +newchar+ "'");
> > current = spawn(newchar); // NYT gives birth
to
> > new NYT and external node
> > current = tree[current].parent; // Go to old NYT
node
> > tree[current].count++; // Increment count
of
> > old NYT node
> > pause();
> >
> > }
> >
> > while (current != root) // Is this the root
> > node?
> > {
> > current = tree[current].parent; // Go to parent node
> >
> > // no exception handling at all ?
> >
> > max = highestInBlock(tree[current].count); // Node number max
in
> > block?
> >
> > // no exception handling at all ?
> >
> > if (current != max && tree[current].parent != max) {
> > addMessage(" Swapping nodes " +current+ " and " +max);
> > swap(current, max); // Switch node with highest
node
> > in block
> > pause();
> > current = max;
> > }
> >
> > tree[current].count++; // Increment node
weight
> >
> > }
> >
> > drawTrie(dispRect, null);
> >
> > }
> >
> > Pretty lazy isn't it ?!?
> >
> > Bye,
> > Skybuck.
>
> 1. This is Java (ie wrong NG)
No because I was translating it to Delphi :D
> 2. No, it's pretty smart code. Notice that many of the methods has a
> return value. When they have a return value they can not also return a
> boolean. Instead they throw an exception, telling the caller that they
> have finished without a result. The exception tells the caller what
> the problem was, and this is handled by the caller (in the catch
> block).
This is a non-issue.
This is the way it should have been written ;)
( Besides from that I believe the java code actually has many potential
bugs... but let's not go into that... the code is probably ment only as an
example of the update tree algorithm... )
function FindCharacter( const Letter : Talphabet_letter; var Position :
integer ) : boolean;
var
i : integer;
begin
result := false;
for i:=0 to (2 * const_alphabet_size)-1 do
begin
if (tree[i].letter = Letter) then
begin
Position := i;
result := true;
break;
end;
end;
end;
Bye,
Skybuck.
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