Question

suppose i have A=ones(10,1)*10
then what does this do eye(10)(A,:) ??

i know eye gives a 10,10 identity matrix but i am not really sure what the above code is doing.
and what is the equivalent code in Matlab?

Answer 1

A=ones(10,1)*10 just makes a column matrix (10 rows, 1 column) of 10s.

eye(10) makes a 10×10 identity matrix.

The (A,:) indexes into the identity matrix you just made. The A here acts as a numeric index into the eye(10) matrix. Suppose A was just 10. Then the myMatrix(A,:) would select the 10th row of myMatrix. Since A is 10 rows of 10, myMatrix(A,:) selects the 10th row of myMatrix, 10 times. So you get back out the 10th row of eye(10) 10 times.

It may be less confusing if you do A=ones(5,1)*10 and have a look at eye(10)(A,:); notice that you now only get the 10th row of A 5 times.

You can’t do this as such in Matlab because it doesn’t let you do the one-hit eye(10)(A,:). You have to do it in two steps:

A = ones(10,1)*10
I = eye(10)
I(A,:)

More on various types of matrix indexing in Matlab.