Does the integer metadata for a field count towards its allocated length?
Integer metadata values like field lengths and offsets are completely separate from the actual data. They don’t use up the bytes reserved for the field itself, but they do contribute to the length of the record.
For example, in lecture we looked at the way that the record
('1234567', 'comp sci', 200) would be represented using
variable-length records with a header of offsets. In that example,
we used 2-byte offsets but 4-byte integer values, so the length
of the record ended up being 27 bytes:
When do we need to use a # delimiter in a fixed-length record?
We use a # as a delimiter at the end of a string value in a
fixed-length record if:
the string is in a variable-length field (e.g., a VARCHAR) and
the string is shorter than the maximum length of the variable-length field.
We don’t need those delimiter characters in any other circumstance.
When we do include a # character, it uses up 1 byte of the field’s allocated number of bytes.
For example, let’s say that we have a VARCHAR(10) field as part
of a fixed-length record.
If you needed to store the string 'abc' in that field, you would
use the string 'abc#------' (abc, followed by #, followed by
6 hyphens), and the overall length of that field would be 10.
If you needed to store the string '0123456789' in that field,
you would use the string '0123456789' with no # and no hyphens,
since the value itself already uses up all 10 of the available
bytes.
In 2.3, when showing the before-and-after pictures for each increase in the number of buckets, should we include the key that causes the increase?
Yes. The problem states that a split should occur whenever the total number of items is greater than (3*number of buckets). This means that the split occurs after we go over the threshold, not before.
In the “before” table, you should include the key of the item that takes you over the threshold, and then the “after” table will also include that key.
For example, in the lecture notes, when we used the slightly different rule that a split occurs when the total number of items is greater than (2*number of buckets), you’ll see for example that we have a “before” picture with 3 buckets and 7 items, and then an “after” picture with 4 buckets and the same 7 items.
I’m getting an error when I try to compile/run my code. It says something like “Exception in thread “main” java.sql.SQLException: No suitable driver found for jdbc:sqlite:movie.sqlite”. What can I do?
In VS Code, make sure that you have opened the correct folder –
the one that actually contains the Java files. If you have done it
correctly, when you look at the Explorer pane, the top-level
folder should be your problem3 folder, and the Java files should
all be in that folder. If necessary, try using File->Close Folder
and then try reopening the correct folder.
If this doesn’t work, you can use VS Code on the virtual desktop.
I’m failing one of the Autograder tests, but my return value looks like it matches the expected return value. What should I do?
Double-check that you:
When I test oscarsForYear() or one of the other methods that
calls it, I seem to be missing one of the awards that should be in
the results. Why could that be?
When you use JDBC to process a SQL query that can have
an arbitrary number of results, you need to use a while loop. As
mentioned in our overview, the structure of
these loops looks something like this:
while (results.next()) { // code to process each row goes here }
If your method is missing one row of the results, it’s possible
that you are calling next() before the loop in order to
determine whether the query has any results. You shouldn’t do
that! Here’s why: If the query has one or more rows of results,
the initial call to next() will position the ResultSet object
on the first row of results, and then the call to next() in the
header of while loop will advance the ResultSet to the second
row of results. And because the code to process each row is inside
the loop, you won’t actually process the first row!
In order to handle the case in which the query has no results, we recommend that you do the following:
Before beginning the loop, initialize your result variable to the empty string. (Optionally, you could also initialize a counter variable to 0.)
Inside the loop, concatenate the strings needed for the current row of results. (If you’re using a counter variable, increment it as well.)
If there are no results, the while loop condition will be
false the very first time that it is evaluated, and thus the
loop will perform 0 repetitions.
To check for cases with no results, you can simply check to see if your result variable is still the empty string or if your counter variable is still 0.
In Java, when should I use == and when should I use the
equals() method?
When you use the == operator with a variable, it bases its
comparison on the value stored in that variable. Thus, it should
typically only be used in two cases: (1) with variables and values of
primitive data types (int, double, char, boolean); or (2)
when checking to see if the value stored in a reference variable is
null.
If you are comparing two objects (e.g., two strings) to see if
they are equivalent to each other, you should use the equals
method, since it bases its result on the internals of the
objects. If you use == with objects, it compares the memory
addresses of the objects, which isn’t typically what you want.
Note: In addition to the questions above, we strongly encourage you to consult the solutions to Lab 5, which includes a number of query problems that are similar to the ones in this assignment. Those solutions will be posted on the Labs page after the final lab is held.
My query has a syntax error, but I can’t find it. Do you have any suggestions?
Here are some things to double-check:
Variable names should begin with a dollar sign ($).
You should have commas between different components of a
for clause, let clause and return clause.
Each condition in a where clause should a boolean
expression – something that would give you true or false –
and multiple conditions should be separated by either
and or or.
If you are defining new start and end tags in your
return clause, make sure that your end tag includes
a backslash. For example, the following is incorrect:
<location>{ string($b) }<location>
Instead, it should be:
<location>{ string($b) }</location>
My query doesn’t have any syntax errors (BaseX says it is OK), but I’m not getting any results or I’m missing some of the results that I should be seeing. Do you know why that would be?
One thing to double-check is that all of your element and attribute names are correct. To remind yourself of what the elements and attributes look like in our database, you can consult our description of its schema.
For example, if you wanted to obtain all movie elements, the correct XPath expression is:
//movie
If you accidentally included an s at the end of the word
(//movies), you wouldn’t produce an error, but you wouldn’t
get any results!
In addition:
Make sure that all attribute names are preceded by an @
symbol.
If you use a predicate in an XPath expression, make sure it is in the correct position within the expression. For example, in Lab 5 we considered the following XPath expression:
//account[branch = $b]/balance
The following incorrect version of this expression would not produce any results:
//account/balance[branch = $b]
If you are testing for the presence of a single ID value
within the value of an IDREFS attribute, make sure that you
use the contains function and not the = operator.
When using contains, make sure that the first input
is the larger string and the second input is the substring
that you are looking for.
See the next question for some other suggestions for debugging your query.
My query produces some results, but they’re not fully correct. Do you have any suggestions for how to debug my query?
One thing that can help is to try executing simpler versions of your query to see if the various components of the query are giving you what you expect them to.
For example, consider the following incorrect query, which is similar to one from Lab 5:
for $b in distinct-values(//branch) let $balances := //account/balance[branch = $b] return <branch> { <location>{ $b }</location>, <total_balance>{ sum($balances) }</total_balance> } </branch>
You could start by running just the XPath expression used in
the for clause:
distinct-values(//branch)
If you did so, you would see that it gives you the expected results: the names of all of the branches in the bank database from Lab 5.
Next, you could simplify the query as follows to see if the
let clause is producing the correct results:
for $b in distinct-values(//branch) let $balances := //account/balance[branch = $b] return ($b, $balances)
If you ran this simpler query, you would only see the branch
names, which indicates that $balances is always being assigned
an empty set. This would help you to realize that you need to
revise the XPath expression in the let clause.
Next, you could try a revised version of that XPath expression:
for $b in distinct-values(//branch) let $balances := //account[branch = $b]/balance return ($b, $balances)
This time, you would see the correct balance elements for each
branch, so you would know that the let clause is functioning
correctly.
Finally, you could restore the original return clause and
confirm that it works as well.
I know we need to use curly braces in some of our return
clauses, but I’m not sure when we need them.
You only need them if you are creating new types of elements. For example, consider the following query from Lab 5:
for $c in //customer let $c_accounts := //account[contains(@owners, $c/@customer_num)] return <customer> { $c/name, for $a in $c_accounts return <account> { string($a/branch), "-", string($a/balance) } </account> } </customer>
We’re creating new elements of type customer, so we need to
specify the start and end tags of those elements, and we also
need curly braces inside of those tags so that XQuery will
evaluate the expressions needed to produce the contents of the new
elements.
In addition, we are creating new elements of type account, so
we need to specify their start and end tags and have curly braces
for their contents as well.
However, we are using the existing name child element of the
customer element assigned to $c – without changing its
existing tags – so we can simply say $c/name, without
specifying start and end tags and without an additional set
of curly braces.
On query 5, I’m not sure what components I need to include in my FLWOR expression. Can you point me to a similar problem that I can use as a model?
Yes. In Lab 5, query problem 4 is somewhat similar to query 5 in the assignment. In the original version of that lab problem, we wanted to produce new elements for every customer in the database. In the solutions, we also show how we could modify the query so that it only produces a new element for customers that have at least one account. That modified query is a good starting point for what you need to do in query 5 of the assignment.
Last updated on October 9, 2024.