Splitting a matrix into smaller matrices...

I have a matrix 1000x3 matrix P which can be generated with the following code: (I call the columns of P: X, Y and Z)

P = [];
for X = 0.1:.1:1; 
for Y = 0.01:.01:1;
Z = Y^2 + X;
P = vertcat(P, [X, Y, Z]);
end
end

I now want to create a matrix which contains all of the rows for which X = 0.5, say.

(This will be a 100x3 matrix in this example)

How do I do this?

Many thanks

D Howard

Answer 1

[Walter Roberson]

M2 = P( P(:,1)  == 0.5, : );

However, you need to read: http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F

[Doron]

Thanks Walter,

Will this ever be an issue when I am running a code which explicitly successively goes through the unique values at the immediately previous step?

What I mean to say is, my code will be going through values directly from the matrix, rather than some other calculations:

typically:

PU = unique(P(:,1))

PUC = size(PU, 1)

for i = 1:PUC

x = PU(i)

Q{i} = P( P(:, 1) == x)

end

Thanks again

D Howard

[Walter Roberson]

Use unique() more directly.

[PU, PI, PJ] = unique(P(:,1));

PUC = length(PU);

for i = 1 : PUC

Q{i} = P(PJ == i, :);

end

Notice no floating point comparisons were used.

[Doron]

Thanks for this tip