I have a 250*2001 matrix. I want to find the location for the maximum value for a(:,i) where i takes 5 different values: i = i + 256
a(:,256)
a(:,512)
a(:,768)
a(:,1024)
a(:,1280)
I tried using MAXLOC, but since I'm new to fortran, I couldn't get it right.
Try this
maxloc(a(:,256:1280:256))
but be warned, this call will return a value in the range 1..5 for the second dimension. The call will return the index of the maxloc in the 2001*5 array section that you pass to it. So to get the column index of the location in the original array you'll have to do some multiplication. And note that since the argument in the call to maxloc is a rank-2 array section the call will return a 2-element vector.
Your question is a little unclear: it could be either of two things you want.
One value for the maximum over the entire 250-by-5 subarray;
One value for the maximum in each of the 5 250-by-1 subarrays.
Your comments suggest you want the latter, and there is already an answer for the former.
So, in case it is the latter:
b(1:5) = MAXLOC(a(:,256:1280:256), DIM=1)
Related
I am trying to solve a simple problem, but at the moment I cannot think of a better solution. I am testing an API that is not documented.
There is an ID used to fetch objects and it has a min and max value with random values missing in-between. I'm trying to test the responses I receive for random objects, but to find objects, I need to have valid IDs.
It would be very inefficient to test random numbers and hope that I get an object back. The best I can do is find a range, get a random number between that range and check if it exists before conducting tests.
A sample list of all of the IDs in the database might look like this:
[1005, 25984, 25986, 29587, 30000, ...]
Assuming the deviation from one value to another will never exceed C, e.g. from the first value to the next value, the difference will never be greater than a pre-defined constant, how would you calculate the min/max of the range given only one value in the range?
Starting from a given value and looping until the last value is found is horrible but that is how it was implemented by previous devs. Below is pseudocode that more or less covers what they do.
// this can be any valid object ID from the database
// assuming the ID's in the database are [1005, 25984, 25986, 29587, 30000]
// "i" could be any one of these values
var i = givenPredefinedObjectId;
var deviation = 100;
// objectWithIdExists() is going to lookup an object with the ID "i" in the database
// if there is no object with the ID "i" , it will return false
// otherwise the object will get tested and return true
while(objectWithIdExists(i)){
i++;
}
for(i; i < i+deviation; i++){
if(objectWithIdExists(i)){
goto while loop;
}
}
endPoint = i - deviation;
Assuming there is no knowledge about the possible values except you can check if they exist and you are given one valid value (there is no array with all possible IDs, that was just an example), how would you find the min/max values?
Unbounded binary search is feasible, with a factor of C slowdown. Given an algorithm for unbounded binary search that, given access to the oracle less_equal(n) for some natural number n, returns n in time O(log n), implement the oracle on input k by querying all of the IDs C*k, C*k+1, ..., C*k+C-1 and reporting that k is less than or equal to n if and only if one ID is found. The running time is O(C*log((max-min)/C)).
I'm trying to use Matlab to implement the MDO algorithm, which requires me to sort an array of objects of a custom-defined mdoVertex class by their degree, and then delete the one with the smallest degree value. My first attempt was this:
for i = 1:m
if graph(i).degree < minDegree
minDegree = graph(i).degree;
elimObject = graph(i);
end
end
Matlab is complaining that elimObject, or the object to be eliminated after the loop executes, is an undefined function or variable. How, then, can I keep track of not only the current smallest degree the loop has encountered, but also which object it corresponded to? 'graph' is the name of the array holding all of my vertex objects.
I suspect that you're somehow trying to call clear on the object returned from your function. Or is it just a few lines of code in a script? I'm guessing here. In any event, calling clear won't work. As you've noticed, clear expects to be given a variable name.
But in this case, you're not trying to delete a variable, you're trying to remove an element from an array. For that, you do arrayname(indextodelete) = [];
So I think that you want...
minDegree = inf; % See what I did there? I defined the variable, and I did it in such a way that I KNOW that the first vertex will satisfy the condition.
for i = 1:length(graph) % Properly loop over the entire graph
if graph(i).degree < minDegree % The first vertex will definitely satisfy this. Maybe another one (or more) will later!
minDegree = graph(i).degree;
minDegreeIndex = i; % Don't record the value, just remember WHERE it is in the array.
end
end
graph(minDegreeIndex) = []; % Now, remove the element that you identified from the array!
(By the way, you never showed us how you tried to eliminate elimObject. I assume that you called clear (the object that you identified)? You shouldn't make us guess; show us.)
I'm attempting to solve http://projecteuler.net/problem=1.
I want to create a method which takes in an integer and then creates an array of all the integers preceding it and the integer itself as values within the array.
Below is what I have so far. Code doesn't work.
def make_array(num)
numbers = Array.new num
count = 1
numbers.each do |number|
numbers << number = count
count = count + 1
end
return numbers
end
make_array(10)
(1..num).to_a is all you need to do in Ruby.
1..num will create a Range object with start at 1 and end at whatever value num is. Range objects have to_a method to blow them up into real Arrays by enumerating each element within the range.
For most purposes, you won't actually need the Array - Range will work fine. That includes iteration (which is what I assume you want, given the problem you're working on).
That said, knowing how to create such an Array "by hand" is valuable learning experience, so you might want to keep working on it a bit. Hint: you want to start with an empty array ([]) instead with Array.new num, then iterate something num.times, and add numbers into the Array. If you already start with an Array of size num, and then push num elements into it, you'll end up with twice num elements. If, as is your case, you're adding elements while you're iterating the array, the loop never exits, because for each element you process, you add another one. It's like chasing a metal ball with the repulsing side of a magnet.
To answer the Euler Question:
(1 ... 1000).to_a.select{|x| x%3==0 || x%5==0}.reduce(:+) # => 233168
Sometimes a one-liner is more readable than more detailed code i think.
Assuming you are learning Ruby by examples on ProjectEuler, i'll explain what the line does:
(1 ... 1000).to_a
will create an array with the numbers one to 999. Euler-Question wants numbers below 1000. Using three dots in a Range will create it without the boundary-value itself.
.select{|x| x%3==0 || x%5==0}
chooses only elements which are divideable by 3 or 5, and therefore multiples of 3 or 5. The other values are discarded. The result of this operation is a new Array with only multiples of 3 or 5.
.reduce(:+)
Finally this operation will sum up all the numbers in the array (or reduce it to) a single number: The sum you need for the solution.
What i want to illustrate: many methods you would write by hand everyday are already integrated in ruby, since it is a language from programmers for programmers. be pragmatic ;)
I am trying to keep the last n elements from a changing list of x elements (where x >> n)
I found out about the deque method, with a fixed length, in other programming languages. I was wondering if there is something similar for VB6
Create a Class that extends an encapsulated Collection.
Add at the end (anonymous), retrieve & remove from the beginning (index 1). As part of adding check your MaxDepth property setting (or hard code it if you like) and if Collection.Count exceeds it remove the extra item.
Or just hard code it all inline if a Class is a stumper for you.
This is pretty routine.
The only thing I can think of is possibly looping through the last 5 values of the dynamic array using something like:
For UBound(Array) - 5 To UBound(Array)
'Code to store or do the desired with these values
Loop
Sorry I don't have a definite answer, but hopefully that might help.
Here's my simplest solution to this:
For i = n - 1 To 1 Step -1
arrayX(i) = arrayX(i - 1)
Next i
arrayX(0) = latestX
Where:
arrayX = array of values
n = # of array elements
latestX = latest value of interest (assumes entire code block is also
within another loop)
I decided to get shuffled values from array. for that i used this function, i got it from net, it works fine. But i don't know, how it's works...
any one can help me to understand this?
my code is :
function rand(ar){
return 0.5-Math.random();
}
var ar = [5,10,15,20,25]
ar.sort(rand);
console.log(ar)
I am using this function for getting new shuffled array values from the declared one.
This code is using the supplied rand function as the comparison operator for the Array.Sort method (http://msdn.microsoft.com/en-us/library/4b4fbfhk(VS.85).aspx). Since the Math.random (http://msdn.microsoft.com/en-us/library/41336409.aspx) function returns a value from 0 (inclusive) to 1 (exclusive), the rand function will return a value from 0.5 (inclusive) to -0.5 (exclusive).
Normally the sortFunction supplied to the Sort method takes 2 arguments that are compared. The sortFunction compares them and returns a value that means:
Negative - The first item is less than the second
Zero - The items are equal
Positive - The first item is greater than the second
As the sort method runs, it uses this comparison to determine which array values should go before the others.
In the case of your code, the rand function's return value is random and has no correlation to the data. This means that, whenever the sort function tries to compare two values in the array, half of the time it will say the first item is less than the second and half the second item will be less than the first. As this is done over the entire length of the array, items are swapped randomly and the whole array becomes randomized.
array.sort() has an optional parameter that is a sorting function, you can pass a function reference to change the order of the array.
Maybe this page can be helpful http://www.javascriptkit.com/javatutors/arraysort.shtml