Hey,
When I do a(:,3:4) = 2, the 3rd and 4th columns turn into 2 as should be expected. The other command a(3:4,:) = 2 , changes the 1s into 2s in the 3rd and 4th rows. This happens since a = ones(6) is a matrix 6x6 with six rows and six columns.
By convention, each element of the matrix is found writing a(i,j) where i is the row and j is the column. So if you say a(2,3) = 2 , then the element in the 2nd row and 3rd column will be replaced by 2. From here, it it is obvious the role of the commas.
The colons, for example in a(:,2) = 1 , basically say that "for all the rows" (notice that we have : in the rows argument) put in the 2nd column the number 1.
When you use something like a(:,3:4) = 2 , we still have the same case as "for all the rows". The difference here is that we establish a columns range (between the 3rd and the 4th).
As an example a(1:2,3:4) = 3 puts the number 3 in the interval selected: the intersection between the 1st and 2nd rows and 3rd and 4th columns.
In addition, the command a(:,:) = 2 basically changes all the matrix elements to 2.
Greets.
