It is possible to obtain any sub-vector or sub-matrix of a fully interfaced object. There are four types of sub indices:

```
gmm::sub_interval(first, length);
```

represents an interval whose first index is `first` and length is `length` ( for instance `gmm::sub_interval(10, 3);` represents the indices `{10, 11, 12}`).

```
gmm::sub_slice(first, length, step);
```

represents also an interval in which one index over `step` is taken. ( for instance `gmm::sub_slice(10, 3, 2);` represents the indices `{10, 12, 14}`)

```
gmm::sub_index(CONT c);
```

represents the sub-index which is the collection of index contained in the container `c`. For instance:

```
std::vector<size_t> c(3);
c[0] = 1; c[1] = 3; c[2] = 16;
gmm::sub_index(c);
```

represents the indices `{1, 3, 16}`.

VERY IMPORTANT : the container `c` has to be sorted from the smaller index to the greater one (i.e. with increasing order) and no repetition is allowed.

For unsorted index such as permutation, a special type of sub index is defined:

```
gmm::unsorted_sub_index(CONT c);
```

Some algorithms are a little bit slower with unsorted sub indices.

Now `gmm::sub_vector(V, subi)` gives a reference to a sub-vector:

```
gmm::vsvector<double> V(10);
V[5] = 3.0;
std::cout << gmm::sub_vector(V, gmm::sub_interval(2, 3)) << std::endl;
```

prints to the standard output `V[2], V[3]` and `V[4]`.

`gmm::sub_matrix(V, subi1, subi2)` gives a reference to a sub-matrix. For instance:

```
gmm::col_matrix< gmm::wsvector<double> > M(5, 20);
M(3, 2) = 5.0;
std::cout << gmm::sub_matrix(M, gmm::sub_interval(2, 3), gmm::sub_interval(2, 3))
<< std::endl;
```

prints to the output a sub-matrix. If the two sub-indices are equal, it is possible to omit the second. For instance:

```
gmm::col_matrix< gmm::wsvector<double> > M(5, 20);
M(3, 2) = 5.0;
std::cout << gmm::sub_matrix(V, gmm::sub_interval(2, 3)) << std::endl;
```

The reference on sub_matrix is writable if the corresponding matrix is writable (so you can copy on a sub_matrix, add sub-matrices ...).

`gmm::mat_row(M, i)` gives a (possibly writable) reference to the row `i` of matrix `M`, and `gmm::mat_col(M, i)` gives a (possibly writable) reference to the column `i`. It is not possible to access to the rows if `M` is a column matrix and to the columns if it is a row matrix. It is possible to use `gmm::mat_const_row(M, i)` and `gmm::mat_const_col(M, i)` to have constant references.