This (note the modified first line):
intger=[2,6,8,9]
flag(intger)=1
uses linear indexing to assign to elements 2, 6, 8, and 9 of the flag array. Normally you'd want to preallocate the flag array to some known size before assigning to those specific elements. For instance, if you used the Hilbert matrix example from that blog post, you may want to make flag a logical matrix the same size as the Hilbert matrix.
A = hilb(5)
flag = false(size(A))
intger=[2,6,8,9]
flag(intger)=1
elements2_6_8_and_9 = A(flag)
Why is element 9 the element in the fourth row, second column of A rather than the second row, fourth column? MATLAB is column major. The linear order of elements in A is:
orderOfElements = reshape(1:numel(A), size(A))
