Code only works sometimes when run

this code works some of the time i run it, but not others.

i.e. i and j will display some of the time when the script is run, but most often they will not

A = randi(5, 4, 4, 3);
size = numel(A);

n = randperm(size, 1);
[i j] = find(A==n);
disp([i,j])

any ideas why? cheers

[dpb]

Ideas? Sure. :)

Hint: What does numel(A) return?

PS. Don't use size as a variable; that aliases the builtin size() function and that will cause consternation elsewhere...

PPS. It works every time; it just illustrates that just having code that doesn't have any syntax error doesn't mean the logic is correct to solve the problem! :) While it is a necessary condition, it isn't sufficient.

We don't know what the actual intent was so we don't know what the actual solution would be.

[Stephen23]

@Tom Seath: do NOT call any variable size, as this will shadow the very important inbuilt size function.

[Tom Seath]

I actually haven't been using the variable size, i just changed it to paste the code here, but thanks for the tip.

[Image Analyst]

It's also a good idea not to use i and j because they are the built in imaginary variables.

Answer 1

[Stephen23]

"any ideas why?"

Because you test for numbers that do not exist in A, and so the output is empty.

Lets have a look at an example:

>> A = randi(5, 4, 4, 3) % 4x4x3 array of values 1-5.
A(:,:,1) =
   1   3   1   3
   4   3   1   1
   1   1   5   4
   5   3   5   3
A(:,:,2) =
   1   2   1   1
   5   3   1   5
   1   5   2   5
   1   1   2   4
A(:,:,3) =
   4   2   3   1
   5   2   2   1
   2   4   2   3
   5   2   5   4
>> S = numel(A) % do NOT use SIZE as a variable name!
S =   48
>> n = randperm(S,1) % could be 1-48.
n =    7

But are any of the values in A equal to 7? Of course not, because you specified that the values of A must be 1-5. So for most random values of n (from 6-48), there will be absolutely no matches:

>> A==n
ans(:,:,1) =
  0  0  0  0
  0  0  0  0
  0  0  0  0
  0  0  0  0
ans(:,:,2) =
  0  0  0  0
  0  0  0  0
  0  0  0  0
  0  0  0  0
ans(:,:,3) =
  0  0  0  0
  0  0  0  0
  0  0  0  0
  0  0  0  0
>> find(A==n)
ans = []

What do you expect to happen when you test for values that do not exist?

[Tom Seath]

Thank you very much for the example. Well explained

Answer 2

[Stephan]

Hi,

in addition to the valueable comments:

What you do is make a Matrix A with the dimensions: 4 x 4 x 3 which means it has 48 elements. These elements are randomly between 1...5

Then you create 1(!) random number n between 1...48

Then you want to get the indicies of all occurences from this one number in A.

The chance that n is in the Matrix = 5/48 ~ 10,42% --> Thats because you will only get indices back from the find function if n = [1...5]

At about 90% of all cases you test this, you will get nothin back, because n = [6...48]

I guess this is not the bahavior you wanted, but it is what you have written in your code.

Best regards

Stephan

[Tom Seath]

Thank you!