linear search in matlab

Question

javeed miyandad 2011-3-13

1
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i have MxN matrix, i need search elements which are more than value 'T' THAT IS . IF 'T'=5 THEN OUTPUT SHOULD BE CO-ORDINATES OF EACH ELEMENT which have value more than or equal to (>=) '5' IN the MxN MATRIX

example let us we have 3X4 matrix

2 6 1
3 1 0 
0 5 1

the output should be coordinates of element 6-(3,3), coordinates of element 5-(1,3) plz help in coding

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javeed miyandad 2011-3-15

thank you so much i got wat i needed thank you so much sir

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Answer 1

John D'Errico 2011-3-13

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I won't give you the complete answer, though I'm sure someone else will do so. I want you to learn to experiment in MATLAB, to learn the language, to get a feeling for how it works. This is how to learn such a language. So, try a couple of things. I'll give you enough hints to get there very quickly. First, I'll assume that your matrix is rather uninterestingly called "matrix". Thus

matrix = [1 2 6 1;4 3 1 0;1 0 5 1];

What does MATLAB do when you perform the simple test

T = 5;
result = (matrix >= T)

TRY IT! (Don't just look for me to hand anything to you on a silver platter! I won't do so. This is how you learn.) I put in parens above just for purposes of clarity. They are not needed syntactically here.

The result is a boolean matrix, in MATLAB, an array of type "logical". This array will be of the same size and shape as your original matrix, but composed of zeros and ones. The ones will be in the place of those elements that are greater than T.

Next, what is your goal? You wish to FIND those elements of this boolean variable that are 1, i.e., true. In MATLAB, the basic tool you want is called find. TRY IT. Do something like this now...

result = find(matrix >= T)

What does this return? Gosh, it seems like this should work. In fact, you are almost there. Read the help for find

help find
doc find

You will see that if find is called with TWO output arguments, then it gives you an interesting and perhaps useful result. In MATLAB, you put [ and ] around the arguments when there are two such arguments returned.

The point of all this is to get used to breaking down your problems in MATLAB into small, fundamental units that MATLAB can solve. Look for the utilities in MATLAB that will solve these little problems, and learn to combine them into a solution. The tools to do this are help, doc, and lookfor. And, finally, read the tutorials, the getting started stuff they provide. Had you done so, you would have found all of this VERY quickly, and you would have had your answer very much earlier than you are getting it from me.