Difference between revisions of "cpp/numeric/valarray/gslice"
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(Example: print 3D matrix horizontally to reduce vertical space in Output) |
m (Fixed the link to std::valarray::operator[].) |
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{{cpp/title|gslice}} | {{cpp/title|gslice}} | ||
{{cpp/numeric/valarray/navbar}} | {{cpp/numeric/valarray/navbar}} | ||
| − | {{ddcl | header=valarray | | + | {{ddcl|header=valarray| |
class gslice; | class gslice; | ||
}} | }} | ||
| − | {{tt|std::gslice}} is the selector class that identifies a subset of {{lc|std::valarray}} indices defined by a multi-level set of strides and sizes. Objects of type {{tt|std::gslice}} can be used as indices with valarray's {{ | + | {{tt|std::gslice}} is the selector class that identifies a subset of {{lc|std::valarray}} indices defined by a multi-level set of strides and sizes. Objects of type {{tt|std::gslice}} can be used as indices with valarray's {{c/core|operator[]}} to select, for example, columns of a multidimensional array represented as a {{tt|valarray}}. |
Given the starting value {{math|s}}, a list of strides {{math|i{{su|b=j}}}} and a list of sizes {{math|d{{su|b=j}}}}, a {{tt|std::gslice}} constructed from these values selects the set of indices {{math|k{{su|b=j}}{{=}}s+Σ{{su|b=j}}(i{{su|b=j}}d{{su|b=j}})}}. | Given the starting value {{math|s}}, a list of strides {{math|i{{su|b=j}}}} and a list of sizes {{math|d{{su|b=j}}}}, a {{tt|std::gslice}} constructed from these values selects the set of indices {{math|k{{su|b=j}}{{=}}s+Σ{{su|b=j}}(i{{su|b=j}}d{{su|b=j}})}}. | ||
| Line 23: | Line 23: | ||
3 + 1*19 + 3*4 + 2*1 = 36}} | 3 + 1*19 + 3*4 + 2*1 = 36}} | ||
| − | It is possible to construct {{tt|std::gslice}} objects that select some indices more than once: if the above example used the strides {{tt|{1,1,1} }}, the indices would have been {{tt|{3, 4, 5, 4, 5, 6, ...} }}. Such gslices may only be used as arguments to the const version of {{ | + | It is possible to construct {{tt|std::gslice}} objects that select some indices more than once: if the above example used the strides {{tt|{1,1,1}<!---->}}, the indices would have been {{tt|{3, 4, 5, 4, 5, 6, ...}<!---->}}. Such gslices may only be used as arguments to the const version of {{rlpt|operator at|std::valarray::operator[]}}, otherwise the behavior is undefined. |
===Member functions=== | ===Member functions=== | ||
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc mem ctor | cpp/numeric/valarray/gslice | inlinemem=true | constructs a generic slice}} | + | {{dsc mem ctor|cpp/numeric/valarray/gslice|inlinemem=true|constructs a generic slice}} |
| − | {{dsc mem fun | cpp/numeric/valarray/gslice | title=start<br | + | {{dsc mem fun|cpp/numeric/valarray/gslice|title=start<br>size<br>stride|inlinemem=true|returns the parameters of the slice}} |
{{dsc end}} | {{dsc end}} | ||
| − | {{member | {{small|std::gslice::}}gslice | | + | {{member|{{small|std::gslice::}}gslice| |
{{dcl begin}} | {{dcl begin}} | ||
| − | {{dcl | | + | {{dcl|num=1| |
gslice() | gslice() | ||
}} | }} | ||
| − | {{dcl | | + | {{dcl|num=2| |
gslice( std::size_t start, const std::valarray<std::size_t>& sizes, | gslice( std::size_t start, const std::valarray<std::size_t>& sizes, | ||
const std::valarray<std::size_t>& strides ); | const std::valarray<std::size_t>& strides ); | ||
}} | }} | ||
| − | {{dcl | | + | {{dcl|num=3| |
gslice( const gslice& other ); | gslice( const gslice& other ); | ||
}} | }} | ||
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@1@ Default constructor. Equivalent to {{c|gslice(0, std::valarray<std::size_t>(), std::valarray<std::size_t>())}}. This constructor exists only to allow construction of arrays of slices. | @1@ Default constructor. Equivalent to {{c|gslice(0, std::valarray<std::size_t>(), std::valarray<std::size_t>())}}. This constructor exists only to allow construction of arrays of slices. | ||
| − | @2@ Constructs a new slice with parameters {{ | + | @2@ Constructs a new slice with parameters {{c|start}}, {{c|sizes}}, {{c|strides}}. |
| − | @3@ Constructs a copy of {{ | + | @3@ Constructs a copy of {{c|other}}. |
===Parameters=== | ===Parameters=== | ||
{{par begin}} | {{par begin}} | ||
| − | {{par | start | the position of the first element}} | + | {{par|start|the position of the first element}} |
| − | {{par | sizes | an array that defines the number of elements in each dimension}} | + | {{par|sizes|an array that defines the number of elements in each dimension}} |
| − | {{par | strides | an array that defines the number of positions between successive elements in each dimension}} | + | {{par|strides|an array that defines the number of positions between successive elements in each dimension}} |
| − | {{par | other | another slice to copy}} | + | {{par|other|another slice to copy}} |
{{par end}} | {{par end}} | ||
}} | }} | ||
| − | {{member | {{small|std::slice::}}start, size, stride | | + | {{member|{{small|std::slice::}}start, size, stride| |
{{dcl begin}} | {{dcl begin}} | ||
| − | {{dcl | num=1 | | + | {{dcl|num=1| |
std::size_t start() const; | std::size_t start() const; | ||
}} | }} | ||
| − | {{dcl | num=2 | | + | {{dcl|num=2| |
std::valarray<std::size_t> size() const; | std::valarray<std::size_t> size() const; | ||
}} | }} | ||
| − | {{dcl | num=3 | | + | {{dcl|num=3| |
std::valarray<std::size_t> stride() const; | std::valarray<std::size_t> stride() const; | ||
}} | }} | ||
| Line 90: | Line 90: | ||
===Example=== | ===Example=== | ||
{{example | {{example | ||
| − | + | |Demonstrates the use of gslices to address columns of a 3D array: | |
| − | + | |code= | |
#include <iostream> | #include <iostream> | ||
#include <valarray> | #include <valarray> | ||
| + | |||
void test_print(std::valarray<int>& v, int planes, int rows, int cols) | void test_print(std::valarray<int>& v, int planes, int rows, int cols) | ||
{ | { | ||
| − | for(int r=0; r<rows; ++r) { | + | for (int r = 0; r < rows; ++r) |
| − | for(int z=0; z<planes; ++z) { | + | { |
| − | for(int c=0; c<cols; ++c) | + | for (int z = 0; z < planes; ++z) |
| − | std::cout << v[z*rows*cols + r*cols + c] << ' '; | + | { |
| + | for (int c = 0; c < cols; ++c) | ||
| + | std::cout << v[z * rows * cols + r * cols + c] << ' '; | ||
std::cout << " "; | std::cout << " "; | ||
} | } | ||
| Line 105: | Line 108: | ||
} | } | ||
} | } | ||
| + | |||
int main() | int main() | ||
{ | { | ||
std::valarray<int> v = // 3d array: 2 x 4 x 3 elements | std::valarray<int> v = // 3d array: 2 x 4 x 3 elements | ||
| − | + | {111,112,113 , 121,122,123 , 131,132,133 , 141,142,143, | |
| − | + | 211,212,213 , 221,222,223 , 231,232,233 , 241,242,243}; | |
// int ar3d[2][4][3] | // int ar3d[2][4][3] | ||
std::cout << "Initial 2x4x3 array:\n"; | std::cout << "Initial 2x4x3 array:\n"; | ||
test_print(v, 2, 4, 3); | test_print(v, 2, 4, 3); | ||
| − | + | ||
// update every value in the first columns of both planes | // update every value in the first columns of both planes | ||
| − | v[std::gslice(0, {2, 4}, {4*3, 3})] = 1; // two level one strides of 12 elements | + | v[std::gslice(0, {2, 4}, {4 * 3, 3})] = 1; // two level one strides of 12 elements |
| − | + | // then four level two strides of 3 elements | |
| − | + | ||
// subtract the third column from the second column in the 1st plane | // subtract the third column from the second column in the 1st plane | ||
| − | v[std::gslice(1, {1, 4}, {4*3, 3})] -= v[std::gslice(2, {1, 4}, {4*3, 3})]; | + | v[std::gslice(1, {1, 4}, {4 * 3, 3})] -= v[std::gslice(2, {1, 4}, {4 * 3, 3})]; |
| − | + | ||
std::cout << "\n" "After column operations:\n"; | std::cout << "\n" "After column operations:\n"; | ||
test_print(v, 2, 4, 3); | test_print(v, 2, 4, 3); | ||
} | } | ||
| − | + | |output= | |
Initial 2x4x3 array: | Initial 2x4x3 array: | ||
111 112 113 211 212 213 | 111 112 113 211 212 213 | ||
| Line 137: | Line 141: | ||
1 -1 143 1 242 243 | 1 -1 143 1 242 243 | ||
}} | }} | ||
| + | |||
| + | ===Defect reports=== | ||
| + | {{dr list begin}} | ||
| + | {{dr list item|wg=lwg|dr=543|std=C++98|before=it was unclear whether a default constructed generic slice is usable|after=it is usable (as an empty subset)}} | ||
| + | {{dr list end}} | ||
===See also=== | ===See also=== | ||
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc inc | cpp/numeric/valarray/dsc operator_at}} | + | {{dsc inc|cpp/numeric/valarray/dsc operator_at}} |
| − | {{dsc inc | cpp/numeric/valarray/dsc slice}} | + | {{dsc inc|cpp/numeric/valarray/dsc slice}} |
| − | {{dsc inc | cpp/numeric/valarray/dsc gslice_array}} | + | {{dsc inc|cpp/numeric/valarray/dsc gslice_array}} |
{{dsc end}} | {{dsc end}} | ||
{{langlinks|de|es|fr|it|ja|pt|ru|zh}} | {{langlinks|de|es|fr|it|ja|pt|ru|zh}} | ||
Latest revision as of 01:12, 27 February 2023
| Defined in header <valarray>
|
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| class gslice; |
||
std::gslice is the selector class that identifies a subset of std::valarray indices defined by a multi-level set of strides and sizes. Objects of type std::gslice can be used as indices with valarray's operator[] to select, for example, columns of a multidimensional array represented as a valarray.
Given the starting value s, a list of strides ij and a list of sizes dj, a std::gslice constructed from these values selects the set of indices kj=s+Σj(ijdj).
For example, a gslice with starting index 3, strides {19,4,1} and lengths {2,4,3} generates the following set of 24=2*4*3 indices:
3 + 0*19 + 0*4 + 0*1 = 3,
3 + 0*19 + 0*4 + 1*1 = 4,
3 + 0*19 + 0*4 + 2*1 = 5,
3 + 0*19 + 1*4 + 0*1 = 7,
3 + 0*19 + 1*4 + 1*1 = 8,
3 + 0*19 + 1*4 + 2*1 = 9,
3 + 0*19 + 2*4 + 0*1 = 11,
...
3 + 1*19 + 3*4 + 1*1 = 35,
3 + 1*19 + 3*4 + 2*1 = 36
It is possible to construct std::gslice objects that select some indices more than once: if the above example used the strides {1,1,1}, the indices would have been {3, 4, 5, 4, 5, 6, ...}. Such gslices may only be used as arguments to the const version of std::valarray::operator[], otherwise the behavior is undefined.
Contents |
[edit] Member functions
| (constructor) |
constructs a generic slice (public member function) |
| startsizestride |
returns the parameters of the slice (public member function) |
std::gslice::gslice
| gslice() |
(1) | |
| gslice( std::size_t start, const std::valarray<std::size_t>& sizes, const std::valarray<std::size_t>& strides ); |
(2) | |
| gslice( const gslice& other ); |
(3) | |
Constructs a new generic slice.
1) Default constructor. Equivalent to gslice(0, std::valarray<std::size_t>(), std::valarray<std::size_t>()). This constructor exists only to allow construction of arrays of slices.
2) Constructs a new slice with parameters start, sizes, strides.
3) Constructs a copy of other.
Parameters
| start | - | the position of the first element |
| sizes | - | an array that defines the number of elements in each dimension |
| strides | - | an array that defines the number of positions between successive elements in each dimension |
| other | - | another slice to copy |
std::slice::start, size, stride
| std::size_t start() const; |
(1) | |
| std::valarray<std::size_t> size() const; |
(2) | |
| std::valarray<std::size_t> stride() const; |
(3) | |
Returns the parameters passed to the slice on construction - start, sizes and strides respectively.
Parameters
(none)
Return value
The parameters of the slice -- start, sizes and strides respectively.
Complexity
Constant.
[edit] Example
Demonstrates the use of gslices to address columns of a 3D array:
Run this code
#include <iostream> #include <valarray> void test_print(std::valarray<int>& v, int planes, int rows, int cols) { for (int r = 0; r < rows; ++r) { for (int z = 0; z < planes; ++z) { for (int c = 0; c < cols; ++c) std::cout << v[z * rows * cols + r * cols + c] << ' '; std::cout << " "; } std::cout << '\n'; } } int main() { std::valarray<int> v = // 3d array: 2 x 4 x 3 elements {111,112,113 , 121,122,123 , 131,132,133 , 141,142,143, 211,212,213 , 221,222,223 , 231,232,233 , 241,242,243}; // int ar3d[2][4][3] std::cout << "Initial 2x4x3 array:\n"; test_print(v, 2, 4, 3); // update every value in the first columns of both planes v[std::gslice(0, {2, 4}, {4 * 3, 3})] = 1; // two level one strides of 12 elements // then four level two strides of 3 elements // subtract the third column from the second column in the 1st plane v[std::gslice(1, {1, 4}, {4 * 3, 3})] -= v[std::gslice(2, {1, 4}, {4 * 3, 3})]; std::cout << "\n" "After column operations:\n"; test_print(v, 2, 4, 3); }
Output:
Initial 2x4x3 array: 111 112 113 211 212 213 121 122 123 221 222 223 131 132 133 231 232 233 141 142 143 241 242 243 After column operations: 1 -1 113 1 212 213 1 -1 123 1 222 223 1 -1 133 1 232 233 1 -1 143 1 242 243
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 543 | C++98 | it was unclear whether a default constructed generic slice is usable | it is usable (as an empty subset) |
[edit] See also
| get/set valarray element, slice, or mask (public member function) | |
| BLAS-like slice of a valarray: starting index, length, stride (class) | |
| proxy to a subset of a valarray after applying a gslice (class template) |
