MATLAB: What does the operator “~=” mean

Image Processing Toolboxnon-maximum suppression

max2 = @(x) max(max(x));
immax = nlfilter(im, [siz siz], max2);
im(im ~= immax) = 0;
result = im;

This is the code take from a non-maximum suppression function. What does the 3rd line mean? Why can we use "something(something) = 0;" as a statement?

Thank you.

Best Answer

something(somthing_else)=0; In MATLAB refers to logical indexing and assignment.

In this case somthing_else has to be logical (think boolean) and

im ~= immax

basically means values where those to are not equal will be true and the rest will be false. When that is followed by:

im(im ~= immax) = 0;

you are equating to zero all the values in im where the above condition is not satisfied.

you will find this information in the documentation under logical indexing.

Related Solutions

MATLAB: What is the difference between “<>“, “!=” and “~=” in a Stateflow chart

The "<>" symbol is indeed equivalent to "!=". Nevertheless, it would be more robust to replace both of these expressions with "~=".

This is because, both "<>" and "!=" are binary operations which are only supported when using C as the action language of the chart:

https://www.mathworks.com/help/stateflow/ug/supported-operations-on-chart-data.html

On the other hand, "~=" is supported both when using MATLAB or C as the action language.

For more information about using MATLAB or C as an action language see:

https://www.mathworks.com/help/stateflow/ug/differences-between-matlab-and-stateflow-action-language.html

MATLAB: How to say if (A & B) or C in Matlab

| operates elementwise on arrays. You might want to check the size of A, B, and C. If then operates on the array (See @Stephen's answer).

>> help |
|   Logical or.
  A | B performs a logical or of arrays A and B and returns an array
  containing elements set to either logical 1 (TRUE) or logical 0
  (FALSE)....
    Note that there are two logical or operators in MATLAB.  The |
    operator performs an element-by-element or between matrices, while 
    the || operator performs a short-circuit or between scalar values.

I prefer using && and | | for a range of personal reasons:

Certainly when I was starting out, I didn't consider the possibility of the behavior of if operating on a vector. Keeping statements to the scalar operators adds implicit error-checking when you hand your code to someone else - they'll now get an error (&& doesn't operate on vectors) instead of a possibly ill-defined result. Of course, if you intend to operate on vectors, you can ignore this comment.
As @Stephen mentioned below, it doesn't have to evaluate the rest of the expression once the condition is met. Just make sure all of your () are in the right places.
It forces you to think a bit more about about the intent of the condition, for example do you want: all(A) to be true, or would you be happy if any(A) are true? Even though the default for "if all(A)" is equivalent to "if A", you've just made your intent explicitly clear.
People coming from other languages may interpret the intent differently. For example in python;

>> if [0, 0, 0]:
       print("yes")
>> yes

(because [0,0,0] is not None.) whereas;

>> if all([0, 0, 0]):
       print("yes")
>>

Best Answer

Related Solutions

MATLAB: What is the difference between “<>“, “!=” and “~=” in a Stateflow chart

MATLAB: How to say if (A & B) or C in Matlab

Related Question