MATLAB: Easy question from a newbie

matrix

This should be a really easy question for some of you…I think.

Is there a function to specifically call out the number of rows in matrix? Length usually works but it takes the "longest side" so if you have more rows than columns, it counts the columns. I know size returns both the # of rows and columns, but I'm tweaking somebody else's more complicated code, so I just need a function to replace length where its reading # of columns instead of # of rows.

Thanks

Best Answer

size(A,1)   % number of rows
size(A,2)   % number of columns
size(A,3)   % number of channels

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MATLAB: Delete a column from an array of uncertain size

The trick needed is to use the fact that cell arrays can be expanded into comma-separated lists, so can represent any number of subscripts. It works like this.

Test data:

x = 1 + randi(9);   % random no. dimensions from 2 to 10
oldarray = rand(repmat(3, 1, x)); % 3 x 3 x 3 ... array

Computation:

% get cell array of subscript arguments representing whole of oldarray
subs = arrayfun(@(s) {1:s}, size(oldarray));
% change second subscript to start from 2
subs{2} = 2:size(oldarray,2);
% create new array by indexing old array
newarray = oldarray(subs{:});

Check newarray is correct size, and look at one column (noting that trailing ones are always OK regardless of the number of dimensions):

disp(size(oldarray));
disp(size(newarray));
disp(oldarray(1, :, 1, 1, 1, 1, 1, 1, 1, 1, 1));
disp(newarray(1, :, 1, 1, 1, 1, 1, 1, 1, 1, 1));

MATLAB: Remove column in matrix

nz = logical(A);
B = A(sum(nz,2) >= size(A,2)/2, sum(nz) >= size(A,1)/2);

You will notice this gives a 3 x 2 array, not a 2 x 2 array. The row 0 1 2 0 does not have more 0 than values, so it needs to be kept.

If you change the rules to say that you are not to keep something unless the number of values exceeds 1/2 of the possible, then you need to eliminate the third column of A, [0; 2; 8; 2; 0; 0] because in that case 3 entries out of 6 are populated and 3 does not exceed (6/2)

Best Answer

Related Solutions

MATLAB: Delete a column from an array of uncertain size

MATLAB: Remove column in matrix

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