Code only works sometimes when run

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Tom Seath
Tom Seath 2018-9-3
编辑： dpb 2018-9-3
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
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Tom Seath
Tom Seath 2018-9-3
编辑：Tom Seath 2018-9-3
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
Image Analyst 2018-9-3
It's also a good idea not to use i and j because they are the built in imaginary variables.

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Stephen23
Stephen23 2018-9-3
编辑：Stephen23 2018-9-3
"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?

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Stephan
Stephan 2018-9-3
编辑：Stephan 2018-9-3
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

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