void add_vector(float * a, mat b, int dim){
petuum::RowAccessor row_acc;
b.Get(0, &row_acc);
const petuum::DenseRow<float>& r = row_acc.Get<petuum::DenseRow<float> >();
for(int i=0;i<dim;i++)
a[i]+=r[i];
}
//multiplication W * a, size of W dim1 * dim2, assume a and b have been allocated
void matrix_vector_multiply(mat W, float * a, float * b, int dim1, int dim2){
petuum::RowAccessor row_acc;
for(int i=0;i<dim1;i++){
W.Get(i, &row_acc);
const petuum::DenseRow<float>& r = row_acc.Get<petuum::DenseRow<float> >();
float sum=0;
for(int j=0;j<dim2;j++)
sum+=r[j]*a[j];
b[i]=sum;
}
}