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Loren on the Art of MATLAB

July 6th, 2007

Make Code N-D Safe

I've recently been asked some questions about programming safely to account for N-dimensional data. Techniquest that are tried-and-true for two dimensions don't all scale to N dimensions.

Contents

Code That's Not Recommended

The following code doesn't catch dimensions higher than 2.

 if any(any(input))
     output = doAlg(input);
 end

This code is also not recommended because n will be a scalar even if input has more than 2 dimensions. Depending on what is done in the loop, the output might be the wrong shape/dimension (thanks to Jeremy so I could clarify this).

 [m,n] = size(input);
 for i=1:n
     doSomething;
 end

Code That is Safe for Higher Dimensions

This code should work fine for any input.

 sz = size(input);
 if any(input(:))
     output = doAlg(input);
 end
 output = reshape(output,sz);

Benefits

If you consider N-dimensional input when you write functions, you are less likely to find bugs or wrong answers when you go to use it later.

   * The code is easier to understand without so many any statements.
   * Code is robust to N-dimensional inputs.

Your Advice?

What techniques have you used to ensure your code works with all array shapes? List them here.


Get the MATLAB code

Published with MATLAB® 7.4

4 Responses to “Make Code N-D Safe”

  1. Jeremy replied on :

    Actually, if one calls “[m,n] = size(input)” where input has more than 2 dimensions, n is not a vector. It is simply a product of the remaining dimension sizes.

  2. mark replied on :

    I suppose matlab arrays aren’t tensors. However, has there been any discussion about implementing tensor algebra, or can it be done with dimension arguments? For instance, in some circumstances, one might like to consider the column and row dimensions as both being 1 with a new argument to indicate dual space.

  3. Urs (us) Schwarz replied on :

    there are a few helpers, which should not be discarded and often are very useful for preliminary error checking procedures
    help isempty
    help isscalar
    help isvector
    help ndims
    us

  4. Loren replied on :

    Jeremy-

    You are right about size, of course. But it is still possible that your code, if dependent on n for pre-allocation, for instance, will be incorrect for N-dimensional inputs.

    Urs-

    Thanks for the list.

    –Loren

Leave a Reply


Loren Shure works on design of the MATLAB language at The MathWorks. She writes here about once a week on MATLAB programming and related topics.

  • J.B. Brown: Ah, and I am at fault for simply testing collinearity with the origin in the example above.
  • J.B. Brown: Indeed, > collinear( [0 3],[0 8],[0 -1e21+2e-15] ) ans = 1 > collinear( [0 3],[0 8],[0 -1e22+2e-15]...
  • OkinawaDolphin: Loren, thank you for telling me where to download timeit. Here are the two functions I just tested...
  • Loren: JB- It looks to me like Ilya’s solution and therefore yours are equivalent to the determinant. As Tim...
  • Loren: OkinawaDolphin, timeit can be downloaded from the File Exchange. Steve Eddins is the author. It does not ship...
  • OkinawaDolphin: It seems that neither R2007a nor R2007b have the function timeit, but I investigated computation time...
  • J.B. Brown: It would appear to me that Ilya Rozenfeld’s solution would be the cleanest. Just to help those who...
  • Loren: Markus- Congratulations on winning! And a nice illustration of how the size matters. Small enough, and the...
  • Markus: Hi Loren, which version is fastest also depends very much on the matrix dimensions. Look at my test function:...
  • Duncan: OkinawaDolphin, Regarding why your third example is slower than your second example, the result is in fact...

These postings are the author's and don't necessarily represent the opinions of The MathWorks.

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