STL multi-dimensional vector computations on rows and columns using iterators

[Kamil]

I want to sum elements of a multi-dimensional vector, across each
vector, or across each dimension... this is my old code:

#define v_ldouble vector<ldouble>
#define vv_ldouble vector<vector<ldouble> >
....
void vect_sum_old(const vv_ldouble& x, v_ldouble& sum, const int dir)
{
int v,nV,d,nD;

nV=x.size();
nD=x[0].size();

if(dir!=1)
{
sum = v_ldouble(nV,0.0);
for(v=0; v<nV; v++)
for(d=0; d<nD; d++)
sum[v]+=x[v][d];
}
else
{
sum = v_ldouble(nD,0.0);
for(d=0; d<nD; d++)
for(v=0; v<nV; v++)
sum[d]+=x[v][d];
}
}

Yeah I know... excuse my brute-force engineering approach. I am new to
STL -- I switched from C to C++ because STL vectors make my life
easier. Now I want to utilize more of what STL/C++ has to offer. I can
compute the sum of each row vector using iterators:

vv_ldouble::iterator iter;
v_ldouble::iterator iter1;
vv_ldouble mdv = vv_ldouble(3,v_ldouble(4,0.0));
v_ldouble sumrow = v_ldouble(mdv.size());
v_ldouble sumcol = v_ldouble(mdv[0].size());
....
//Sum each vector across its elements...
for(iter = mdv.begin(), iter1=sumrow.begin(); iter!=mdv.end(); iter++,
iter1++)
*iter1 = accumulate((*iter).begin(), (*iter).end(), 0.0);

How do I go about computing sum of each column using iterators
(efficiently)?