Comparing elements in a vector

Hello!

I need to check if a squared element exists in the vector as another element.

for example x=[2 3 4]

so the squared value of 2 would be 4 which is in the vector and would result in a returnation of y=true.

This vector is quite small and I need a solution to work for bigger vectors but you get the idea of what I need.

I would love to find some documentation on this if you have it available, been searching for hours without finding anything that I can make work. I tried using find() among other things but couldn't make it work.

[Steven Lord]

This sounds like a homework assignment. If it is, show us the code you've written to try to solve the problem and ask a specific question about where you're having difficulty and we may be able to provide some guidance.

If you aren't sure where to start because you're not familiar with how to write MATLAB code, I suggest you start with the MATLAB Onramp tutorial (https://www.mathworks.com/support/learn-with-matlab-tutorials.html) to quickly learn the essentials of MATLAB.

If you aren't sure where to start because you're not familiar with the mathematics you'll need to solve the problem, I recommend asking your professor and/or teaching assistant for help.

I will give one hint: ismember may be useful.

[Joel Bovin]

Sure!

My idea is to start of squaring the elements in x and then to check if any element in x equals any element in sqr. I dont know if the if-statement is the way to go or if its easier/better to use a for-loop. But then the problem is to check each element in x against all in sqr.

I seem to run into the problem that sqr is of class double and not a vector. Tried converting it to a vector using these methods but nothing worked. https://www.mathworks.com/matlabcentral/answers/44156-conversion-of-2d-array-in-to-vector?w.mathworks.com

x=[2 3 4]
sqr=x.^2

if x(x==sqr)
    y=true;
else
    y=false;
end

okay so this is the solution I came up with after giving up on the if-statement;

function y = isItSquared(x)
  y=false;
for i = 1:length(x)
    for a=1:length(x)
        if x(a).^2==x(i)
            y=true;
    end
end 
end

The idea here is that I presume that there is no element in x that is another element squared, and then run it through 2 for loops that checks if the squared element in x exists in x. I all have to do is to find one such case for y to turn true.

[David Hill]

Look at this:

x=[2 3 4];
a=sqrt(x);
logical(nnz(a==floor(a)));

[Joel Bovin]

Not sure I really follow this solution, I need to see if a squared element in x exists as another element in x.

so with x=[2 3 4]

and the squared elements being sqr= 4 9 16 the value i need to find in sqr is 4 since it also appears in x.

[James Tursa]

@Joel: Your double for-loop solution looks good. But note that this logic can be short-circuited. As soon as you get your first y=true condition, you can return from the function.

        if x(a).^2==x(i)
            y=true;
            return
        end

[James Tursa]

And yet another method to consider is to stack x and x.^2 into one variable and then see if the unique( ) function drops any elements of this variable.

[David Hill]

Misread that one. How about:

logical(nnz(ismember(x,x.^2)));

[David Goodmanson]

any(x' == x.^2, 'all')

uses more memory than most techniques but still for a vector of length 1000 it's just 8 MBytes

Answer 1

[per isakson]

"My idea is to start of squaring the elements in x and then to check if any element in x equals any element in sqr." That's a good start.

Are all elements of the vector, x, whole numbers?

Steven hinted about ismember. Thus, look it up in the documentation. Type ismember in the command window, select it by double-clicking, right-click and chose "Help on Selection". Reading documentation is a large part of using Matlab.

"any element in x equals any element in sqr" That's what ismember does.

%%
x   = 2:12;
sqr = x.^2;
% 
[ism,loc] = ismember( sqr, x ); 
if any( ism )
    x(loc(ism))
else
    disp('None found')
end
%%

In the bottom of the documetation page of ismember you'll find See Also: ... intersect ... . It's documentation says: "C = intersect(A,B) returns the data common to both A and B, " Worth trying.

intersect( sqr, x )

[Joel Bovin]

I read about ismembet but ran into trouble when it returned a logical array and I didn't know how to handle the logical values in it. So it worked for me but got stuck on how to extrapolate the values from the array, though I guess I only need to check if there is a 1 in it.

I've got several answers that all look good so I'm accepting the answer

Thank you all for the help!

[per isakson]

"but ran into trouble when it returned a logical array" The logical array is true for those elements of sqr, which are members of x. Thus

>> sqr(ism)
ans =
     4     9
>> 

The best way of indexing isn't always the first that comes to mind, which I unwittingly illustrated in my answer.

However, I think intersect is the best alternative. The name makes sense if one knows a little about set theory.