If I run this code
CALL pagerank.get()
YIELD node, rank;
everything works like a charm. But If I expand the query just a little with sorting it gets stuck.
CALL pagerank.get()
YIELD node, rank
ORDER BY rank DESC;
I don't get any results or errors. What could be the reason for this? Should I add something to my code?
Whenever you have a CALL procedure which yields something, you must have a RETURN clause. Hence, the correct usage would be:
CALL pagerank.get()
YIELD node, rank
RETURN node, rank;
Similarly, when trying to sort the results by some value, first add the RETURN clause:
CALL pagerank.get()
YIELD node, rank
RETURN node, rank
ORDER BY rank DESC;
Related
I am Trying to solve a problem in hacker rank, Which title is breaking best and worst. but the function I write didn't give proper output for any of the test case but in my visual code ide it is giving the correct result.
The problem is find out the sequence of increasing number and decreasing. finally return the result in two space separated integers. I've write the function to the point and my IDE is giving the accurate result but in hacker rank it's giving error
here is the link of problem
my solution approach
function breakingRecords(scores) {
let increase = 0;
let decrease =0;
{code that properly increase the value of
increase and decrease.}
}
const result = (increase,decrase);
return result;
}
What I made mistake in my code is I return my value normally in two variable but those value I have to return in array like
const result = [increase,decrase];
return result;
That's it!!
I understand recursion and what the advantages it brings to writing code efficiently. While I can code recursive functions, I cannot seem to wrap my head around how they work. I would like someone to explain me recursion instinctively.
For example, this code:
int fact(int n)
{ if n<0:
return -1
elif n==0:
return 1
else
return n*fact(n-1)
}
These are some of my questions:
Let's say n=5. On entering the function,the control goes to the last return statement since none of the previous conditions are satisfied.
Now, roughly, the computer 'writes' something like this: 5*(fact(4))
Again, the fact() function is called and the same process gets repeated except now we have n=4.
So, how exactly does the compiler multiply 5*4 and so on until 2 since its not exactly 5*4 but 5*fact(4). How does it 'remember' that it has to multiply two integers and where does it store the temporary value since we haven't provided any explicit data structure?
Again let's say n=5. The same process goes on and eventually n gets decremented to 0. My question is why/how doesn't the function simply return 1 as stated in the return statement. Similar to my previous question, how does the compiler 'remember' that it also has 180 stored for displaying?
I'd be really thankful if someone explains this to me completely so that can understand recursion better and intuitively.
Yeah, for beginners recursion can be quite confusing. But, you are already on the right track with your explanation under "1.".
The function will be called recursively until a break condition is satisfied. In this case, the break condition is satisfied when n equals 0. At this point, no recursive calls will be made anymore. The result of each recursive call is returned to the caller. The callers always "wait" until they get a result. That's how the algorithm "knows" the receiver of the results. The flow of this procedure is handled by the so called stack.
Hence, in your informal notation (in this example n equals 3):
3*(fact(2)) = 3*(2*fact(1)) = 3*(2*(1*fact(0))).
Now, n equals 0. The inner fact(0) therefore returns 1:
3*(2*(1*(1)))) = 3*(2*(1)) = 3*(2) = 6
You can see a bit like this
The function fact(int n) is like a class and every time you call fact(int n) you create an instance of that class. By creating them (calling them) from the same function, you are creating a chain of instances. Once you reach break condition, those functions start returning one by one and the value they returned to calculate a new value in the return statement return n*fact(n-1) e.g. return 3*fact(2);
I have the following pseudo-code:
function X(data, limit, level = 0)
{
result = [];
foreach (Y(data, level) as entity) {
if (level < limit) {
result = result + X(entity, limit, level + 1);
} else {
//trivial recursion case:
result = result + Z(entity);
}
}
return result;
}
which I need to turn into a plain (e.g. without recursive calls). So far I'm out of ideas regarding how to do that elegantly. Following this answer I see that I must construct the entire stack frames which are basically the code repetitions (i.e. I will place same code again and again with different return addresses).
Or I tried stuff like these suggestions - where there is a phrase
Find a recursive call that’s not a tail call.
Identify what work is being done between that call and its return statement.
But I do not understand how can the "work" be identified in the case when it is happening from within internal loop.
So, my problem is that all examples above are providing cases when the "work can be easily identified" because there are no control instructions from within the function. I understand the concept behind recursion on a compilation level, but what I want to avoid is code repetition. So,
My question: how to approach transformation of the pseudo-code above which does not mean code repetitions for simulating stack frames?
It looks like an algorithm to descend a nested data structure (lists of lists) and flatten it out into a single list. It would have been good to have a simple description like that in the question.
To do that, you need to keep track of multiple indices / iterators / cursors, one at each level that you've descended through. A recursive implementation does that by using the call stack. A non-recursive implementation will need a manually-implemented stack data structure where you can push/pop stuff.
Since you don't have to save context (registers) and a return address on the call stack, just the actual iterator (e.g. array index), this can be a lot more space efficient.
When you're looping over the result of Y and need to call X or Z, push the current state onto the stack. Branch back to the beginning of the foreach, and call Y on the new entity. When you get to the end of a loop, pop the old state if there is any, and pick up in the middle of that loop.
I have homeowork to write pseudo code to check if a valid binary tree is a search binary tree.
I created an array to hold the in-order values of the tree. if the in-order values are in decreasing order it means it is indeed BST. However I've got some problem with the recursion in the method InOverArr.
I need to update the index of the array in order to submit the values to the array in the order they are at the tree.
I'm not sure the index is really updated properly during the recursion.. is it or not? and if you see some problem can you help me fix this? thanks a lot
pseudo code
first function
IsBST(node)
size ← TreeSize(node)
create new array TreeArr of Size number of cells
index ← 0
few comments:
now we use the IN_ORDER procedure with a small variation , I called the new version of the procedure: InOrderArr
the pseudo code of InOrderArr is described below IsBST
InOrderArr(node, TreeArr, index)
for i from 1 to size-1 do
if not (TreeArr[i] > TreeArr[i-1]) return
false
return true
second function
InOrderArr (node, Array, index)
if node = NULL then return
else
InOrderArr (node.left, Array, index)
treeArr[index] = node.key
index ← index + 1
InOrderArr (node.right, Array, index)
Return
Your code is generally correct. Just three notes.
The correctness of the code depends on the implementation, specifically on the way of index handling. Many programming languages pass arguments to subroutines by value. That means the subroutine receives a copy of the value and modifications made to the parameter have no effect on the original value. So incrementing index during execution of InOrderArr (node.left, Array, index) would not affect the position used by treeArr[index] = node.key. As a result only the rightmost path would be stored in the array.
To avoid that you'll have to ensure that index is passed by reference, so that incrementation done by a callee advances the position used later by a caller.
BST is usually defined so that the left subtreee of a node contains keys that are less than that node's key, and the right subtree contains nodes with greater keys – see Wikipedia's article on BST. Then the inorder traversal retrieves keys in ascending order. Why do you expect descending order?
Possibly it would be more efficient to drop the array and just recursively test a definition condition of BST?
Whenever we follow a left link we expect keys which are less than the current one. Whenever we follow the right link we expect keys greater the the current one. So for most subtrees there is some interval of keys values, defined by some ancestor nodes' keys. Just track those keys and test whether the key falls inside the current valid interval. Be sure to handle 'no left end defined' condition on the letfmost path and 'no right end' on the rightmost path of the tree. At the root node there's no ancestor yet, so the root key is not tested at all (any value is OK).
EDIT
C code draft:
// Test a node against its closest left-side and right-side ancestors
boolean isNodeBST(NODE *lt, NODE *node, NODE *rt)
{
if(node == NULL)
return true;
if(lt != NULL && node->key < lt->key)
return false;
if(rt != NULL && node->key > rt->key)
return false;
return
isNodeBST(lt, node->left, node) &&
isNodeBST(node, node->right, rt);
}
boolean isTreeBST(TREE *tree)
{
return isNodeBST( NULL, tree->root, NULL);
}
Using the count(preceding-sibling::*) XPath expression one can obtaining incrementing counters. However, can the same also be accomplished in a two-levels deep sequence?
example XML instance
<grandfather>
<father>
<child>a</child>
</father>
<father>
<child>b</child>
<child>c</child>
</father>
</grandfather>
code (with Saxon HE 9.4 jar on the CLASSPATH for XPath 2.0 features)
Trying to get an counter sequence of 1,2 and 3 for the three child nodes with different kinds of XPath expressions:
XPathExpression expr = xpath.compile("/grandfather/father/child");
NodeList nodes = (NodeList) expr.evaluate(doc, XPathConstants.NODESET);
for (int i = 0 ; i < nodes.getLength() ; i++) {
Node node = nodes.item(i);
System.out.printf("child's index is: %s %s %s, name is: %s\n"
,xpath.compile("count(preceding-sibling::*)").evaluate(node)
,xpath.compile("count(preceding-sibling::child)").evaluate(node)
,xpath.compile("//child/position()").evaluate(doc)
,xpath.compile(".").evaluate(node));
}
The above code prints:
child's index is: 0 0 1, name is: a
child's index is: 0 0 1, name is: b
child's index is: 1 1 1, name is: c
None of the three XPaths I tried managed to produce the correct sequence: 1,2,3. Clearly it can trivially be done using the i loop variable but I want to accomplish it with XPath if possible. Also I need to keep the basic framework of evaluating an XPath expression to get all the nodes to visit and then iterating on that set since that's the way the real application I work on is structured. Basically I visit each node and then need to evaluate a number of XPath expressions on it (node) or on the document (doc); one of these XPAth expressions is supposed to produce this incrementing sequence.
Use the preceding axis with a name test instead.
count(preceding::child)
Using XPath 2.0, there is a much better way to do this. Fetch all <child/> nodes and use the position() function to get the index:
//child/concat("child's index is: ", position(), ", name is: ", text())
You don't say efficiency is important, but I really hate to see this done with O(n^2) code! Jens' solution shows how to do that if you can use the result in the form of a sequence of (position, name) pairs. You could also return an alternating sequence of strings and numbers using //child/(string(.), position()): though you would then want to use the s9api API rather than JAXP, because JAXP can only really handle the data types that arise in XPath 1.0.
If you need to compute the index of each node as part of other processing, it might still be worth computing the index for every node in a single initial pass, and then looking it up in a table. But if you're doing that, the simplest way is surely to iterate over the result of //child and build a map from nodes to the sequence number in the iteration.