# Construct Sparse Arrays¶

## From coordinates and data¶

You can construct `COO`

arrays from coordinates and value data.

The `coords`

parameter contains the indices where the data is nonzero,
and the `data`

parameter contains the data corresponding to those indices.
For example, the following code will generate a \(5 \times 5\) diagonal
matrix:

```
>>> import sparse
>>> coords = [[0, 1, 2, 3, 4],
... [0, 1, 2, 3, 4]]
>>> data = [10, 20, 30, 40, 50]
>>> s = sparse.COO(coords, data, shape=(5, 5))
>>> s.todense()
array([[10, 0, 0, 0, 0],
[ 0, 20, 0, 0, 0],
[ 0, 0, 30, 0, 0],
[ 0, 0, 0, 40, 0],
[ 0, 0, 0, 0, 50]])
```

In general `coords`

should be a `(ndim, nnz)`

shaped
array. Each row of `coords`

contains one dimension of the
desired sparse array, and each column contains the index
corresponding to that nonzero element. `data`

contains
the nonzero elements of the array corresponding to the indices
in `coords`

. Its shape should be `(nnz,)`

.

If `data`

is the same across all the coordinates, it can be passed
in as a scalar. For example, the following produces the \(4 \times 4\)
identity matrix:

```
>>> import sparse
>>> coords = [[0, 1, 2, 3],
... [0, 1, 2, 3]]
>>> data = 1
>>> s = sparse.COO(coords, data, shape=(4, 4))
```

You can, and should, pass in `numpy.ndarray`

objects for
`coords`

and `data`

.

In this case, the shape of the resulting array was determined from
the maximum index in each dimension. If the array extends beyond
the maximum index in `coords`

, you should supply a shape
explicitly. For example, if we did the following without the
`shape`

keyword argument, it would result in a
\(4 \times 5\) matrix, but maybe we wanted one that was actually
\(5 \times 5\).

```
coords = [[0, 3, 2, 1], [4, 1, 2, 0]]
data = [1, 4, 2, 1]
s = COO(coords, data, shape=(5, 5))
```

`COO`

arrays support arbitrary fill values. Fill values are the “default”
value, or value to not store. This can be given a value other than zero. For
example, the following builds a (bad) representation of a \(2 \times 2\)
identity matrix. Note that not all operations are supported for operations
with nonzero fill values.

```
coords = [[0, 1], [1, 0]]
data = [0, 0]
s = COO(coords, data, fill_value=1)
```

## From Scipy sparse matrices¶

To construct `COO`

array from `spmatrix`

objects, you can use the `COO.from_scipy_sparse`

method. As an
example, if `x`

is a `scipy.sparse.spmatrix`

, you can
do the following to get an equivalent `COO`

array:

```
s = COO.from_scipy_sparse(x)
```

## From Numpy arrays¶

To construct `COO`

arrays from `numpy.ndarray`

objects, you can use the `COO.from_numpy`

method. As an
example, if `x`

is a `numpy.ndarray`

, you can
do the following to get an equivalent `COO`

array:

```
s = COO.from_numpy(x)
```

## Generating random `COO`

objects¶

The `sparse.random`

method can be used to create random
`COO`

arrays. For example, the following will generate
a \(10 \times 10\) matrix with \(10\) nonzero entries,
each in the interval \([0, 1)\).

```
s = sparse.random((10, 10), density=0.1)
```

## Building `COO`

Arrays from `DOK`

Arrays¶

It’s possible to build `COO`

arrays from `DOK`

arrays, if it is not
easy to construct the `coords`

and `data`

in a simple way. `DOK`

arrays provide a simple builder interface to build `COO`

arrays, but at
this time, they can do little else.

You can get started by defining the shape (and optionally, datatype) of the
`DOK`

array. If you do not specify a dtype, it is inferred from the value
dictionary or is set to `dtype('float64')`

if that is not present.

```
s = DOK((6, 5, 2))
s2 = DOK((2, 3, 4), dtype=np.uint8)
```

After this, you can build the array by assigning arrays or scalars to elements or slices of the original array. Broadcasting rules are followed.

```
s[1:3, 3:1:-1] = [[6, 5]]
```

DOK arrays also support fancy indexing assignment if and only if all dimensions are indexed.

```
s[[0, 2], [2, 1], [0, 1]] = 5
s[[0, 3], [0, 4], [0, 1]] = [1, 5]
```

At the end, you can convert the `DOK`

array to a `COO`

array, and
perform arithmetic or other operations on it.

```
s3 = COO(s)
```

In addition, it is possible to access single elements and slices of the `DOK`

array
using normal Numpy indexing, as well as fancy indexing if and only if all dimensions are indexed.
Slicing and fancy indexing will always return a new DOK array.

```
s[1, 2, 1] # 5
s[5, 1, 1] # 0
s[[0, 3], [0, 4], [0, 1]] # <DOK: shape=(2,), dtype=float64, nnz=2, fill_value=0.0>
```

## Converting `COO`

objects to other Formats¶

`COO`

arrays can be converted to Numpy arrays,
or to some `spmatrix`

subclasses via the following
methods:

`COO.todense`

: Converts to a`numpy.ndarray`

unconditionally.`COO.maybe_densify`

: Converts to a`numpy.ndarray`

based oncertain constraints.

`COO.to_scipy_sparse`

: Converts to a`scipy.sparse.coo_matrix`

ifthe array is two dimensional.

`COO.tocsr`

: Converts to a`scipy.sparse.csr_matrix`

ifthe array is two dimensional.

`COO.tocsc`

: Converts to a`scipy.sparse.csc_matrix`

ifthe array is two dimensional.