scipy稀疏矩阵模块

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今天在使用lightfm的时候，输入的数据类型可以是稀疏矩阵，一开始尝试先构造列表然后类型转换为系数矩阵，发现运行到中途进程被kill掉了，才发现这个dok_matrix神奇的类

1.sparse模块初探

python中scipy模块中，有一个模块叫sparse模块，就是专门为了解决稀疏矩阵而生。本文的大部分内容，其实就是基于sparse模块而来的。第一步自然就是导入sparse模块

<span class="hljs-prompt">>>> </span><span class="hljs-keyword">from</span> scipy <span class="hljs-keyword">import</span> sparse

然后help一把，先来看个大概

>>> <span class="hljs-built_in">help</span>(sparse)

直接找到我们最关心的部分：

    Usage information
    =================

    There are <span class="hljs-constant">seven</span> available sparse matrix types:

        <span class="hljs-number">1.</span> csc_matrix: Compressed Sparse Column <span class="hljs-built_in">format</span>
        <span class="hljs-number">2.</span> csr_matrix: Compressed Sparse Row <span class="hljs-built_in">format</span>
        <span class="hljs-number">3.</span> bsr_matrix: Block Sparse Row <span class="hljs-built_in">format</span>
        <span class="hljs-number">4.</span> lil_matrix: List <span class="hljs-operator">of</span> Lists <span class="hljs-built_in">format</span>
        <span class="hljs-number">5.</span> dok_matrix: Dictionary <span class="hljs-operator">of</span> Keys <span class="hljs-built_in">format</span>
        <span class="hljs-number">6.</span> coo_matrix: COOrdinate <span class="hljs-built_in">format</span> (aka IJV, triplet <span class="hljs-built_in">format</span>)
        <span class="hljs-number">7.</span> dia_matrix: DIAgonal <span class="hljs-built_in">format</span>

    To construct <span class="hljs-operator">a</span> matrix efficiently, use either dok_matrix <span class="hljs-operator">or</span> lil_matrix.
    The lil_matrix class supports basic slicing <span class="hljs-operator">and</span> fancy
    indexing <span class="hljs-operator">with</span> <span class="hljs-operator">a</span> similar syntax <span class="hljs-built_in">to</span> NumPy arrays.  As illustrated below,
    <span class="hljs-operator">the</span> COO <span class="hljs-built_in">format</span> may also be used <span class="hljs-built_in">to</span> efficiently construct matrices.

    To perform manipulations such <span class="hljs-keyword">as</span> multiplication <span class="hljs-operator">or</span> inversion, <span class="hljs-keyword">first</span>
    <span class="hljs-built_in">convert</span> <span class="hljs-operator">the</span> matrix <span class="hljs-built_in">to</span> either CSC <span class="hljs-operator">or</span> CSR <span class="hljs-built_in">format</span>. The lil_matrix <span class="hljs-built_in">format</span> is
    row-based, so conversion <span class="hljs-built_in">to</span> CSR is efficient, whereas conversion <span class="hljs-built_in">to</span> CSC
    is less so.

    All conversions <span class="hljs-operator">among</span> <span class="hljs-operator">the</span> CSR, CSC, <span class="hljs-operator">and</span> COO formats are efficient,
    linear-<span class="hljs-built_in">time</span> operations.

通过这段描述，我们对sparse模块就有了个大致的了解。sparse模块里面有7种存储稀疏矩阵的方式。接下来，我们对这7种方式来做个一一介绍。

2.coo_matrix

coo_matrix是最简单的存储方式。采用三个数组row、col和data保存非零元素的信息。这三个数组的长度相同，row保存元素的行，col保存元素的列，data保存元素的值。一般来说，coo_matrix主要用来创建矩阵，因为coo_matrix无法对矩阵的元素进行增删改等操作，一旦矩阵创建成功以后，会转化为其他形式的矩阵。

>>> row = [<span class="hljs-number">2</span>,<span class="hljs-number">2</span>,<span class="hljs-number">3</span>,<span class="hljs-number">2</span>]
>>> col = [<span class="hljs-number">3</span>,<span class="hljs-number">4</span>,<span class="hljs-number">2</span>,<span class="hljs-number">3</span>]
>>> c = sparse.coo_matrix((data,(row,col)),shape=(<span class="hljs-number">5</span>,<span class="hljs-number">6</span>))
>>> <span class="hljs-built_in">print</span> c.toarray()
<span class="hljs-string">[[0 0 0 0 0 0]
 [0 0 0 0 0 0]
 [0 0 0 5 2 0]
 [0 0 3 0 0 0]
 [0 0 0 0 0 0]]</span>

稍微需要注意的一点是，用coo_matrix创建矩阵的时候，相同的行列坐标可以出现多次。矩阵被真正创建完成以后，相应的坐标值会加起来得到最终的结果。

3.dok_matrix与lil_matrix

dok_matrix和lil_matrix适用的场景是逐渐添加矩阵的元素。doc_matrix的策略是采用字典来记录矩阵中不为0的元素。自然，字典的key存的是记录元素的位置信息的元祖，value是记录元素的具体值。

>>> import numpy as np
>>> from scipy.sparse import dok_matrix
>>> S = dok_matrix((<span class="hljs-number">5</span>, <span class="hljs-number">5</span>), dtype=np.float32)
>>> <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">5</span>):
<span class="hljs-keyword">...</span>     <span class="hljs-keyword">for</span> j <span class="hljs-keyword">in</span> range(<span class="hljs-number">5</span>):
<span class="hljs-keyword">...</span>             S[i, j] = i + j
<span class="hljs-keyword">...</span>
>>> print S.toarray()
[[ <span class="hljs-number">0.</span>  <span class="hljs-number">1.</span>  <span class="hljs-number">2.</span>  <span class="hljs-number">3.</span>  <span class="hljs-number">4.</span>]
 [ <span class="hljs-number">1.</span>  <span class="hljs-number">2.</span>  <span class="hljs-number">3.</span>  <span class="hljs-number">4.</span>  <span class="hljs-number">5.</span>]
 [ <span class="hljs-number">2.</span>  <span class="hljs-number">3.</span>  <span class="hljs-number">4.</span>  <span class="hljs-number">5.</span>  <span class="hljs-number">6.</span>]
 [ <span class="hljs-number">3.</span>  <span class="hljs-number">4.</span>  <span class="hljs-number">5.</span>  <span class="hljs-number">6.</span>  <span class="hljs-number">7.</span>]
 [ <span class="hljs-number">4.</span>  <span class="hljs-number">5.</span>  <span class="hljs-number">6.</span>  <span class="hljs-number">7.</span>  <span class="hljs-number">8.</span>]]

lil_matrix则是使用两个列表存储非0元素。data保存每行中的非零元素,rows保存非零元素所在的列。这种格式也很适合逐个添加元素，并且能快速获取行相关的数据。

>>> from scipy.sparse import lil_matrix
>>> l = lil_matrix((<span class="hljs-number">6</span>,<span class="hljs-number">5</span>))
>>> l[<span class="hljs-number">2</span>,<span class="hljs-number">3</span>] = <span class="hljs-number">1</span>
>>> l[<span class="hljs-number">3</span>,<span class="hljs-number">4</span>] = <span class="hljs-number">2</span>
>>> l[<span class="hljs-number">3</span>,<span class="hljs-number">2</span>] = <span class="hljs-number">3</span>
>>> <span class="hljs-built_in">print</span> l.toarray()
<span class="hljs-string">[[ 0.  0.  0.  0.  0.]
 [ 0.  0.  0.  0.  0.]
 [ 0.  0.  0.  1.  0.]
 [ 0.  0.  3.  0.  2.]
 [ 0.  0.  0.  0.  0.]
 [ 0.  0.  0.  0.  0.]]</span>
>>> <span class="hljs-built_in">print</span> l.data
<span class="hljs-string">[[] [] [1.0] [3.0, 2.0] [] []]</span>
>>> <span class="hljs-built_in">print</span> l.rows
<span class="hljs-string">[[] [] [3] [2, 4] [] []]</span>

由上面的分析很容易可以看出，上面两种构建稀疏矩阵的方式，一般也是用来通过逐渐添加非零元素的方式来构建矩阵，然后转换成其他可以快速计算的矩阵存储方式。

4.dia_matrix

这是一种对角线的存储方式。其中，列代表对角线，行代表行。如果对角线上的元素全为0，则省略。如果原始矩阵是个对角性很好的矩阵那压缩率会非常高。找了网络上的一张图，大家就很容易能看明白其中的原理。

5.csr_matrix与csc_matrix

csr_matrix，全名为Compressed Sparse Row，是按行对矩阵进行压缩的。CSR需要三类数据：数值，列号，以及行偏移量。CSR是一种编码的方式，其中，数值与列号的含义，与coo里是一致的。行偏移表示某一行的第一个元素在values里面的起始偏移位置。同样在网络上找了一张图，能比较好反映其中的原理。

看看在python里怎么使用：

>>> from scipy.sparse import csr_matrix
>>> indptr = np.array([<span class="hljs-number">0</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>, <span class="hljs-number">6</span>])
>>> indices = np.array([<span class="hljs-number">0</span>, <span class="hljs-number">2</span>, <span class="hljs-number">2</span>, <span class="hljs-number">0</span>, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>])
>>> data = np.array([<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>, <span class="hljs-number">4</span>, <span class="hljs-number">5</span>, <span class="hljs-number">6</span>])
>>> csr_matrix((data, indices, indptr), shape=(<span class="hljs-number">3</span>, <span class="hljs-number">3</span>)).toarray()
array(<span class="hljs-string">[[1, 0, 2],
       [0, 0, 3],
       [4, 5, 6]]</span>)

怎么样，是不是也不是很难理解。我们再看看文档中是怎么说的

 Notes
 <span class="hljs-string">|  -----</span>
 <span class="hljs-string">|</span>
 <span class="hljs-string">|  Sparse matrices can be used in arithmetic operations: they support</span>
 <span class="hljs-string">|  addition, subtraction, multiplication, division, and matrix power.</span>
 <span class="hljs-string">|</span>
 <span class="hljs-string">|  Advantages of the CSR format</span>
 <span class="hljs-string">|    - efficient arithmetic operations CSR + CSR, CSR * CSR, etc.</span>
 <span class="hljs-string">|    - efficient row slicing</span>
 <span class="hljs-string">|    - fast matrix vector products</span>
 <span class="hljs-string">|</span>
 <span class="hljs-string">|  Disadvantages of the CSR format</span>
 <span class="hljs-string">|    - slow column slicing operations (consider CSC)</span>
 <span class="hljs-string">|    - changes to the sparsity structure are expensive (consider LIL or DOK)</span>

不难看出，csr_matrix比较适合用来做真正的矩阵运算。至于csc_matrix，跟csr_matrix类似，只不过是基于列的方式压缩的，不再单独介绍。