Math Forum / Math Software / MATLAB / October 2008

Having two matrices a and b like in the example below:

a =

    3     3    -5
    4     5    -6
    4     4    -9
    4     5    -3

b =

    4     4    -2
    4     5    -4
    8     7   -15
    9     0    -1

I need to get an output like this:

    3     3     -5    NaN
    4     4     -9      -2
    4     5     -3      -4
    8     7    NaN   -15
    9     0    NaN   -1

That is, I have to perform a union based on the first two colors of a
and b and keep track of the original indices so as to be able to
retrieve the original values in their third columns, and write them in
the last two colums of the output matrix.

I know the command union can perform the union task on the matrices
but the doc help file says:

"If a value appears in both a and b, union indexes its occurrence in
b. If a value appears more than once in b or in a (but not in b),
union indexes the last occurrence of the value."

Therefore, after union I can know from what rows in the matrix b the
united values come from, but not from matrix a.

Once again, thank you for the cooperation.