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14 | <title>uBLAS operations overview</title> | |
15 | </head> | |
16 | <body> | |
17 | <h1><img src="../../../../boost.png" align="middle" />Overview of Matrix and Vector Operations</h1> | |
18 | <div class="toc" id="toc"></div> | |
19 | ||
20 | <dl> | |
21 | <dt>Contents:</dt> | |
22 | <dd><a href="#blas">Basic Linear Algebra</a></dd> | |
23 | <dd><a href="#advanced">Advanced Functions</a></dd> | |
24 | <dd><a href="#sub">Submatrices, Subvectors</a></dd> | |
25 | <dd><a href="#speed">Speed Improvements</a></dd> | |
26 | </dl> | |
27 | ||
28 | <h2>Definitions</h2> | |
29 | ||
30 | <table style="" summary="notation"> | |
31 | <tr><td><code>A, B, C</code></td> | |
32 | <td> are matrices</td></tr> | |
33 | <tr><td><code>u, v, w</code></td> | |
34 | <td>are vectors</td></tr> | |
35 | <tr><td><code>i, j, k</code></td> | |
36 | <td>are integer values</td></tr> | |
37 | <tr><td><code>t, t1, t2</code></td> | |
38 | <td>are scalar values</td></tr> | |
39 | <tr><td><code>r, r1, r2</code></td> | |
40 | <td>are <a href="range.html">ranges</a>, e.g. <code>range(0, 3)</code></td></tr> | |
41 | <tr><td><code>s, s1, s2</code></td> | |
42 | <td>are <a href="range.html#slice">slices</a>, e.g. <code>slice(0, 1, 3)</code></td></tr> | |
43 | </table> | |
44 | ||
45 | <h2><a name="blas">Basic Linear Algebra</a></h2> | |
46 | ||
47 | <h3>standard operations: addition, subtraction, multiplication by a | |
48 | scalar</h3> | |
49 | ||
50 | <pre><code> | |
51 | C = A + B; C = A - B; C = -A; | |
52 | w = u + v; w = u - v; w = -u; | |
53 | C = t * A; C = A * t; C = A / t; | |
54 | w = t * u; w = u * t; w = u / t; | |
55 | </code></pre> | |
56 | ||
57 | <h3>computed assignments</h3> | |
58 | ||
59 | <pre><code> | |
60 | C += A; C -= A; | |
61 | w += u; w -= u; | |
62 | C *= t; C /= t; | |
63 | w *= t; w /= t; | |
64 | </code></pre> | |
65 | ||
66 | <h3>inner, outer and other products</h3> | |
67 | ||
68 | <pre><code> | |
69 | t = inner_prod(u, v); | |
70 | C = outer_prod(u, v); | |
71 | w = prod(A, u); w = prod(u, A); w = prec_prod(A, u); w = prec_prod(u, A); | |
72 | C = prod(A, B); C = prec_prod(A, B); | |
73 | w = element_prod(u, v); w = element_div(u, v); | |
74 | C = element_prod(A, B); C = element_div(A, B); | |
75 | </code></pre> | |
76 | ||
77 | <h3>transformations</h3> | |
78 | ||
79 | <pre><code> | |
80 | w = conj(u); w = real(u); w = imag(u); | |
81 | C = trans(A); C = conj(A); C = herm(A); C = real(A); C = imag(A); | |
82 | </code></pre> | |
83 | ||
84 | <h2><a name="advanced">Advanced functions</a></h2> | |
85 | ||
86 | <h3>norms</h3> | |
87 | ||
88 | <pre><code> | |
89 | t = norm_inf(v); i = index_norm_inf(v); | |
90 | t = norm_1(v); t = norm_2(v); | |
91 | t = norm_inf(A); i = index_norm_inf(A); | |
92 | t = norm_1(A); t = norm_frobenius(A); | |
93 | </code></pre> | |
94 | ||
95 | <h3>products</h3> | |
96 | ||
97 | <pre><code> | |
98 | axpy_prod(A, u, w, true); // w = A * u | |
99 | axpy_prod(A, u, w, false); // w += A * u | |
100 | axpy_prod(u, A, w, true); // w = trans(A) * u | |
101 | axpy_prod(u, A, w, false); // w += trans(A) * u | |
102 | axpy_prod(A, B, C, true); // C = A * B | |
103 | axpy_prod(A, B, C, false); // C += A * B | |
104 | </code></pre> | |
105 | <p><em>Note:</em> The last argument (<code>bool init</code>) of | |
106 | <code>axpy_prod</code> is optional. Currently it defaults to | |
107 | <code>true</code>, but this may change in the future. Setting the | |
108 | <code>init</code> to <code>true</code> is equivalent to calling | |
109 | <code>w.clear()</code> before <code>axpy_prod</code>. | |
110 | There are some specialisation for products of compressed matrices that give a | |
111 | large speed up compared to <code>prod</code>.</p> | |
112 | <pre><code> | |
113 | w = block_prod<matrix_type, 64> (A, u); // w = A * u | |
114 | w = block_prod<matrix_type, 64> (u, A); // w = trans(A) * u | |
115 | C = block_prod<matrix_type, 64> (A, B); // C = A * B | |
116 | </code></pre> | |
117 | <p><em>Note:</em> The blocksize can be any integer. However, the | |
118 | actual speed depends very significantly on the combination of blocksize, | |
119 | CPU and compiler. The function <code>block_prod</code> is designed | |
120 | for large dense matrices.</p> | |
121 | <h3>rank-k updates</h3> | |
122 | <pre><code> | |
123 | opb_prod(A, B, C, true); // C = A * B | |
124 | opb_prod(A, B, C, false); // C += A * B | |
125 | </code></pre> | |
126 | <p><em>Note:</em> The last argument (<code>bool init</code>) of | |
127 | <code>opb_prod</code> is optional. Currently it defaults to | |
128 | <code>true</code>, but this may change in the future. This function | |
129 | may give a speedup if <code>A</code> has less columns than rows, | |
130 | because the product is computed as a sum of outer products.</p> | |
131 | ||
132 | <h2><a name="sub">Submatrices, Subvectors</a></h2> | |
133 | <p>Accessing submatrices and subvectors via <b>proxies</b> using <code>project</code> functions:</p> | |
134 | <pre><code> | |
135 | w = project(u, r); // the subvector of u specifed by the index range r | |
136 | w = project(u, s); // the subvector of u specifed by the index slice s | |
137 | C = project(A, r1, r2); // the submatrix of A specified by the two index ranges r1 and r2 | |
138 | C = project(A, s1, s2); // the submatrix of A specified by the two index slices s1 and s2 | |
139 | w = row(A, i); w = column(A, j); // a row or column of matrix as a vector | |
140 | </code></pre> | |
141 | <p>Assigning to submatrices and subvectors via <b>proxies</b> using <code>project</code> functions:</p> | |
142 | <pre><code> | |
143 | project(u, r) = w; // assign the subvector of u specifed by the index range r | |
144 | project(u, s) = w; // assign the subvector of u specifed by the index slice s | |
145 | project(A, r1, r2) = C; // assign the submatrix of A specified by the two index ranges r1 and r2 | |
146 | project(A, s1, s2) = C; // assign the submatrix of A specified by the two index slices s1 and s2 | |
147 | row(A, i) = w; column(A, j) = w; // a row or column of matrix as a vector | |
148 | </code></pre> | |
149 | <p><em>Note:</em> A range <code>r = range(start, stop)</code> | |
150 | contains all indices <code>i</code> with <code>start <= i < | |
151 | stop</code>. A slice is something more general. The slice | |
152 | <code>s = slice(start, stride, size)</code> contains the indices | |
153 | <code>start, start+stride, ..., start+(size-1)*stride</code>. The | |
154 | stride can be 0 or negative! If <code>start >= stop</code> for a range | |
155 | or <code>size == 0</code> for a slice then it contains no elements.</p> | |
156 | <p>Sub-ranges and sub-slices of vectors and matrices can be created directly with the <code>subrange</code> and <code>sublice</code> functions:</p> | |
157 | <pre><code> | |
158 | w = subrange(u, 0, 2); // the 2 element subvector of u | |
159 | w = subslice(u, 0, 1, 2); // the 2 element subvector of u | |
160 | C = subrange(A, 0,2, 0,3); // the 2x3 element submatrix of A | |
161 | C = subslice(A, 0,1,2, 0,1,3); // the 2x3 element submatrix of A | |
162 | subrange(u, 0, 2) = w; // assign the 2 element subvector of u | |
163 | subslice(u, 0, 1, 2) = w; // assign the 2 element subvector of u | |
164 | subrange(A, 0,2, 0,3) = C; // assign the 2x3 element submatrix of A | |
165 | subrange(A, 0,1,2, 0,1,3) = C; // assigne the 2x3 element submatrix of A | |
166 | </code></pre> | |
167 | <p>There are to more ways to access some matrix elements as a | |
168 | vector:</p> | |
169 | <pre><code>matrix_vector_range<matrix_type> (A, r1, r2); | |
170 | matrix_vector_slice<matrix_type> (A, s1, s2); | |
171 | </code></pre> | |
172 | <p><em>Note:</em> These matrix proxies take a sequence of elements | |
173 | of a matrix and allow you to access these as a vector. In | |
174 | particular <code>matrix_vector_slice</code> can do this in a very | |
175 | general way. <code>matrix_vector_range</code> is less useful as the | |
176 | elements must lie along a diagonal.</p> | |
177 | <p><em>Example:</em> To access the first two elements of a sub | |
178 | column of a matrix we access the row with a slice with stride 1 and | |
179 | the column with a slice with stride 0 thus:<br /> | |
180 | <code>matrix_vector_slice<matrix_type> (A, slice(0,1,2), | |
181 | slice(0,0,2)); | |
182 | </code></p> | |
183 | ||
184 | <h2><a name="speed">Speed improvements</a></h2> | |
185 | <h3><a name='noalias'>Matrix / Vector assignment</a></h3> | |
186 | <p>If you know for sure that the left hand expression and the right | |
187 | hand expression have no common storage, then assignment has | |
188 | no <em>aliasing</em>. A more efficient assignment can be specified | |
189 | in this case:</p> | |
190 | <pre><code>noalias(C) = prod(A, B); | |
191 | </code></pre> | |
192 | <p>This avoids the creation of a temporary matrix that is required in a normal assignment. | |
193 | 'noalias' assignment requires that the left and right hand side be size conformant.</p> | |
194 | ||
195 | <h3>Sparse element access</h3> | |
196 | <p>The matrix element access function <code>A(i1,i2)</code> or the equivalent vector | |
197 | element access functions (<code>v(i) or v[i]</code>) usually create 'sparse element proxies' | |
198 | when applied to a sparse matrix or vector. These <em>proxies</em> allow access to elements | |
199 | without having to worry about nasty C++ issues where references are invalidated.</p> | |
200 | <p>These 'sparse element proxies' can be implemented more efficiently when applied to <code>const</code> | |
201 | objects. | |
202 | Sadly in C++ there is no way to distinguish between an element access on the left and right hand side of | |
203 | an assignment. Most often elements on the right hand side will not be changed and therefore it would | |
204 | be better to use the <code>const</code> proxies. We can do this by making the matrix or vector | |
205 | <code>const</code> before accessing it's elements. For example:</p> | |
206 | <pre><code>value = const_cast<const VEC>(v)[i]; // VEC is the type of V | |
207 | </code></pre> | |
208 | <p>If more then one element needs to be accessed <code>const_iterator</code>'s should be used | |
209 | in preference to <code>iterator</code>'s for the same reason. For the more daring 'sparse element proxies' | |
210 | can be completely turned off in uBLAS by defining the configuration macro <code>BOOST_UBLAS_NO_ELEMENT_PROXIES</code>. | |
211 | </p> | |
212 | ||
213 | ||
214 | <h3>Controlling the complexity of nested products</h3> | |
215 | ||
216 | <p>What is the complexity (the number of add and multiply operations) required to compute the following? | |
217 | </p> | |
218 | <pre> | |
219 | R = prod(A, prod(B,C)); | |
220 | </pre> | |
221 | <p>Firstly the complexity depends on matrix size. Also since prod is transitive (not commutative) | |
222 | the bracket order affects the complexity. | |
223 | </p> | |
224 | <p>uBLAS evaluates expressions without matrix or vector temporaries and honours | |
225 | the bracketing structure. However avoiding temporaries for nested product unnecessarly increases the complexity. | |
226 | Conversly by explictly using temporary matrices the complexity of a nested product can be reduced. | |
227 | </p> | |
228 | <p>uBLAS provides 3 alternative syntaxes for this purpose: | |
229 | </p> | |
230 | <pre> | |
231 | temp_type T = prod(B,C); R = prod(A,T); // Preferable if T is preallocated | |
232 | </pre> | |
233 | <pre> | |
234 | prod(A, temp_type(prod(B,C)); | |
235 | </pre> | |
236 | <pre> | |
237 | prod(A, prod<temp_type>(B,C)); | |
238 | </pre> | |
239 | <p>The 'temp_type' is important. Given A,B,C are all of the same type. Say | |
240 | matrix<float>, the choice is easy. However if the value_type is mixed (int with float or double) | |
241 | or the matrix type is mixed (sparse with symmetric) the best solution is not so obvious. It is up to you! It | |
242 | depends on numerical properties of A and the result of the prod(B,C). | |
243 | </p> | |
244 | ||
245 | <hr /> | |
246 | <p>Copyright (©) 2000-2007 Joerg Walter, Mathias Koch, Gunter | |
247 | Winkler, Michael Stevens<br /> | |
248 | Use, modification and distribution are subject to the | |
249 | Boost Software License, Version 1.0. | |
250 | (See accompanying file LICENSE_1_0.txt | |
251 | or copy at <a href="http://www.boost.org/LICENSE_1_0.txt"> | |
252 | http://www.boost.org/LICENSE_1_0.txt | |
253 | </a>). | |
254 | </p> | |
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