  In this paper, we compute the number of z-classes (conjugacy classes of
centralizers of elements) in the symmetric group S_n, when n is greater or
equal to 3 and alternating group A_n, when n is greater or equal to 4. It turns
out that the difference between the number of conjugacy classes and the number
of z-classes for S_n is determined by those restricted partitions of n-2 in
which 1 and 2 do not appear as its part. And, in the case of alternating
groups, it is determined by those restricted partitions of n-3 which has all
its parts distinct, odd and in which 1 (and 2) does not appear as its part,
along with an error term. The error term is given by those partitions of n
which have each of its part distinct, odd and perfect square. Further, we prove
that the number of rational-valued irreducible complex characters for A_n is
same as the number of conjugacy classes which are rational.
