casacore::Slicer Class Reference

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#include <Slicer.h>

Public Types
enum	{ MimicSource }
	Define the "MimicSource" value which defines the open start or end. More...
enum	LengthOrLast {   endIsLength ,   endIsLast }
	Define the possible interpretations of the end-value. More...

Public Member Functions
	Slicer ()
	Construct a 1-dimensional Slicer. More...
	Slicer (const IPosition &start, const IPosition &end, const IPosition &stride, LengthOrLast endInterpretation=endIsLength)
	The member function inferShapeFromSource (invoked as a callback by the source array) will use the shape of the source array for the unspecified values: IPosition elements with the value Slicer::MimicSource. More...
	Slicer (const IPosition &start, const IPosition &end, LengthOrLast endInterpretation=endIsLength)
	Slicer (const IPosition &start)
	Slicer (const Slice &x, LengthOrLast endInterpretation=endIsLength)
	Create a Slicer object from Slice objects. More...
	Slicer (const Slice &x, const Slice &y, LengthOrLast endInterpretation=endIsLength)
	Create a Slicer for a 2-dim array. More...
	Slicer (const Slice &x, const Slice &y, const Slice &z, LengthOrLast endInterpretation=endIsLength)
	Create a Slicer for a 3-dim array. More...
bool	operator== (const Slicer &) const
	Equality. More...
size_t	ndim () const
	Return the number of dimensions of the Slicer. More...
IPosition	inferShapeFromSource (const IPosition &shape, IPosition &startResult, IPosition &endResult, IPosition &strideResult) const
	This function checks all of the start, length (or end), and stride IPositions, and fills in missing values by getting the corresponding values from the shape of the source array. More...
const IPosition &	start () const
	Report the defined starting position. More...
const IPosition &	end () const
	Report the defined ending position. More...
const IPosition &	stride () const
	Report the defined stride. More...
const IPosition &	length () const
	Report the length of the resulting axes. More...
bool	isFixed () const
	Are all values fixed (i.e., no MimicSource given)? More...
void	setStart (const IPosition &start)
	Set the start and end positions. More...
void	setEnd (const IPosition &end)

Private Member Functions
void	fillEndLen ()
	Define a private constructor taking an ssize_t. More...
void	fillSlice (const Slice &, ssize_t &start, ssize_t &length, ssize_t &stride)
	Fill in start, len and stride from a Slice. More...
void	fillFixed ()
	Fill the fixed flag. More...

Private Attributes
LengthOrLast	asEnd_p
IPosition	start_p
IPosition	end_p
IPosition	stride_p
IPosition	len_p
bool	fixed_p

Detailed Description

Specify which elements to extract from an n-dimensional array

Review Status

Reviewed By:

Paul Shannon

Date Reviewed:

1994/07/07

Test programs:

tSlicer

The review and modification of this class were undertaken, in part, with the aim of making this class header an example – this is what the Casacore project thinks a class header should look like.

Prerequisite

• IPosition
• Array
• Slice

Etymology

The class name "Slicer" may be thought of as a short form of "n-Dimensional Slice Specifier." Some confusion is possible between class "Slice" and this class.

Synopsis

If you need to extract or operate upon a portion of an array, the Slicer class is the best way to specify the subarray you are interested in.

Slicer has many constructors. Of these, some require that the programmer supply a full specification of the array elements he wants to extract; other constructors make do with partial information. In the latter case, the constructor will assume sensible default values or, when directed, infer missing information from the array that's getting sliced (hereafter, the "source" array).

Constructing With Full Information

To fully specify a subarray, you must supply three pieces of information for each axis of the subarray:

1. where to start
2. how many elements to extract
3. what stride (or "increment" or "interval") to use: a stride of "n" means pick extract only every "nth" element along an axis

The most basic constructor for Slicer illustrates this. To create an Slicer for getting selected elements from a 3D array:

IPosition start (3,0,0,0), length (3,10,10,10), stride (3,3,3,3);
Slicer slicer (start, length, stride);
// assume proper declarations, and meaningful values in the source array
subArray = sourceArray (slicer);

It gets elements 0,3,6,9,12,15,18,21,24,27 for each dimension.

Caution: If you wish to extract elements from the array at intervals, these intervals must be regular; The interval is one constant integer for each dimension of the array: it cannot be a function;

Caution: "length", the second parameter to the Slicer constructor above, may actually be used in two ways; In normal (and default) use, it specifies how many elements to select from the source; In the alternative use, it specifies the index of the last element to extract from the source array; This ambiguity (does "end" mean "length" or does it mean "last index"?) is handled by a default fourth parameter to the constructor; This code fragment will extract the same subarray as the example above:

IPosition start (3,0,0,0), end (3,27,27,27), stride (3,3,3,3);
Slicer slicer (start, end, stride, Slicer::endIsLast);
subArray = sourceArray (slicer);

Note that in this example end(3,28,29,28) gives the same result; (We use "end" as the name of the formal parameter because it supports both meanings – "last index" or "length;" You may wish to use a clarifying name for the actual parameter in your code, as we have above when we used "length";)
Similar to Python it is possible to address the start and/or end value from the end by giving a negative value (-1 means the last value). However, a length and stride cannot be negative. Unlike Python the end value is inclusive (as discussed above). For example,

Slicer slicer (IPosition(1,-4), IPosition(1,-2), Slicer::endIsLast)
Slicer slicer (IPosition(1,6),  IPosition(1,8),  Slicer::endIsLast)

Both Slicers give the same result when used on a Vector with length 10.

Constructing with Partial Information

Some of the constructors don't require complete information: Slicer either calculates sensible default values or deduces them from the source array. If you do not specify a "stride" argument, for example, a value of 1 will be used for all dimensions. If you specify a "start" but nothing else, a stride of 1, and (perhaps against expectation) a length of 1 will be used.

Note that using a negative start or end is also partial information. The actual array shape is needed to derive the exact start or end value.

To instruct the Slicer to get otherwise unspecified information from the source array, you can create an IPosition like "end" as shown here:

IPosition start (3,0,0,0), stride (3,3,3,3);
IPosition end   (3,Slicer::MimicSource, Slicer::MimicSource, 
Slicer::MimicSource);
Slicer smartSlicer (start, end, stride);
// assume proper declarations..\.
subArray = sourceArray (smartSlicer)

If you are a library programmer, and write a class that can be sliced by the Slicer class, you need to understand the mechanism for completing the information which the application programmer, in using your class, specified incompletely. (If you are an application programmer, who wants to slice a library class, this explanation will be only of academic interest.)

When the source array (the library class you provide) gets the Slicer – which typically comes when the source array is asked to return a reference to a subarray – the source does a callback to the Slicer object. The source array passes its own shape as one of the arguments to the Slicer callback and asks the Slicer to fill in the missing values from that shape.

In use, and with an imagined class "MyVector", code would look like this:

// first, a fragment from the application program:
IPosition start (1,10), end (1, Slicer::MimicSource);
Slicer slicer (start, end);
MyVector <int> v0 (100);
MyVector <int> v1 = v0 (slicer);
//...\.
// second, a fragment from a constructor of the library class "MyVector":
// the MyVector class will construct v1 as a reference to
// selected elements of v0, using (among other things) a
// callback to the slicer it was passed (above, in the
// construction of v1.
// 
IPosition start, end, stride;
fullSliceInformation = 
slicer.inferShapeFromSource (MyVector::shape(), start, end, stride);
// now the MyVector instance knows everything it needs to
// construct the instance.

Please note that v1 will have a length of 90, and refer to elements 10-99 of v0.

Warning: An exception will be thrown if the positions defined in the Slicer exceed the source array's shape;

Example

Given a large image, 4k on a side, extract (by sampling) an image 1k on a side, but covering the same region as the original.

Image <float>  image ("N5364.fits");   // a 4-d VLA map, 4096 x 4096 x 3 x 1
IPosition start (4,0,0,0,0), stride (4,4,4,1,1);
IPosition end   (4, Slicer::MimicSource, Slicer::MimicSource, 
Slicer::MimicSource, Slicer::MimicSource);
Slicer smartSlicer (start, end, stride);
// assume proper declarations..\.
Image <float> subImage = image (smartSlicer);

Motivation

Slicer is particularly convenient for designers of other library classes: Array and Image, for example. (In fact, this convenience was the original motivation for the class.) The benefit is this: the application programmer, who needs a slice of an Array, may provide slicing specifications in many different ways, but the Array class author needs to provide only one member function to return the slice. The Slicer class, in effect, and with its many constructors, provides a way to funnel all of the variety into a single member function call to the array or image class.

For example, imagine a 100 x 100 x 100 array from which you want to extract various subarrays. Here are some of the ways you might specify the the subarray in the -absence- of Slicer.

// preliminaries: create a cube and assign values to all elements --
//  this will be "source" array
Cube <int> bigCube (IPosition (3, 100, 100, 100));
assignValues (bigCube);
// declare a smaller cube, the destination array.
Cube <int> smallCube (IPosition (3, 10, 10, 10));
 
//  example 1: use Slice objects to extract a subcube -- the first
//   ten elements along each axis
Slice xIndices (0,10,1), yIndices (0,10,1), zIndices (0,10,1);
smallCube = bigCube (xIndices, yIndices, zIndices);
 
// example 2: get the same subcube using three IPosition arguments
IPosition start (3,0,0,0), end (3,10,10,10), stride (3,1,1,1);
smallCube = bigCube (start, end, stride);
 
// example 3: use 2 IPositions, letting the 3rd (stride) default to
//            IPosition (3,1,1,1)
smallCube = bigCube (start, end);

So the Cube class (together with its base class) must define three separate member functions for the essentially identical operation of extracting a subcube. The same replication is also required of Image, Array, and the other Array subclasses (Matrix and Vector).

The Slicer class collapses all of this into a single member function per class:

Slicer slicer = (call the constructor that best suits your problem)
smallCube = bigCube (slicer);

Since there are many constructors available for Slicer, you can still specify the subarray that you may want in a number of different ways, by constructing the Slicer in the way most natural to your circumstances. You then pass the Slicer to the array, and you will get back the slice you want.

This class also offers the application programmer considerable flexibility by allowing the shape of the source array to determine some of the slice specification. This benefit is explained and demonstrated above.

To Do

• This class, and the TableArray, Array and Image classes, could allow for the extraction of a subarray with fewer axes than the source array. At present, for example, you cannot, directly slice a matrix from a cube.

Definition at line 288 of file Slicer.h.

Member Enumeration Documentation

anonymous enum

anonymous enum

Define the "MimicSource" value which defines the open start or end.

This value should be different from MIN_INT in IPosition.h. It should also not be the lowest possible value, since that will probably be used as an undefined value. It must be a negative number.

Enumerator
MimicSource

Definition at line 297 of file Slicer.h.

LengthOrLast

enum casacore::Slicer::LengthOrLast

Define the possible interpretations of the end-value.

Enumerator
endIsLength	The end-values given in the constructor define the lengths.
endIsLast	The end-values given in the constructor define the trc.

Definition at line 300 of file Slicer.h.

Constructor & Destructor Documentation

Slicer() [1/7]

casacore::Slicer::Slicer

(

)

Construct a 1-dimensional Slicer.

Start and end are inferred from the source; stride=1. "endIsLength" and "endIsLast" are identical here, so there's no need to discriminate between them by using a default parameter.

Slicer() [2/7]

casacore::Slicer::Slicer	(	const IPosition &	start,
		const IPosition &	end,
		const IPosition &	stride,
		LengthOrLast	endInterpretation = endIsLength
	)

The member function inferShapeFromSource (invoked as a callback by the source array) will use the shape of the source array for the unspecified values: IPosition elements with the value Slicer::MimicSource.

Thrown Exceptions

• ArraySlicerError

Create a Slicer with a given start, end (or length), and stride. An exception will be thrown if a negative length or non-positive stride is given or if the IPositions start, end, and stride do not have the same dimensionality. If length or stride is not given, they default to 1.
It is possible to leave values in start and end undefined by giving the value MimicSource. They can be filled in later with the actual array shape using function inferShapeFromSource.

Slicer() [3/7]

casacore::Slicer::Slicer	(	const IPosition &	start,
		const IPosition &	end,
		LengthOrLast	endInterpretation = endIsLength
	)

Slicer() [4/7]

casacore::Slicer::Slicer ( const IPosition &  start )

explicit

Slicer() [5/7]

casacore::Slicer::Slicer	(	const Slice &	x,
		LengthOrLast	endInterpretation = endIsLength
	)

Create a Slicer object from Slice objects.

In a Slice object one defines the start, length, and stride for one axis. The default Slice constructor (called with no arguments) creates a Slice with start and length equal to zero, and an undefined stride.

Create a Slicer for a 1-dimensional array.

Slicer() [6/7]

casacore::Slicer::Slicer	(	const Slice &	x,
		const Slice &	y,
		LengthOrLast	endInterpretation = endIsLength
	)

Create a Slicer for a 2-dim array.

Slicer() [7/7]

casacore::Slicer::Slicer	(	const Slice &	x,
		const Slice &	y,
		const Slice &	z,
		LengthOrLast	endInterpretation = endIsLength
	)

Create a Slicer for a 3-dim array.

Member Function Documentation

end()

const IPosition & casacore::Slicer::end ( ) const

inline

Report the defined ending position.

Definition at line 455 of file Slicer.h.

References end_p.

Referenced by setEnd().

fillEndLen()

void casacore::Slicer::fillEndLen ( )

private

Define a private constructor taking an ssize_t.

This is to prevent the user from the unexpected and meaningless Slicer that would result when the ssize_t argument is promoted to an IPosition. Slicer (ssize_t);

Check the given start, end/length and stride. Fill in the length or end. It also calls fillFixed to fill the fixed flag.

fillFixed()

void casacore::Slicer::fillFixed ( )

private

Fill the fixed flag.

fillSlice()

void casacore::Slicer::fillSlice ( const Slice &  , ssize_t &  start, ssize_t &  length, ssize_t &  stride  )

private

Fill in start, len and stride from a Slice.

inferShapeFromSource()

IPosition casacore::Slicer::inferShapeFromSource	(	const IPosition &	shape,
		IPosition &	startResult,
		IPosition &	endResult,
		IPosition &	strideResult
	)		const

This function checks all of the start, length (or end), and stride IPositions, and fills in missing values by getting the corresponding values from the shape of the source array.

These will first be resized, if necessary. If, for a given axis, (end < start), it means that a length of zero was specified. An exception is thrown if the start, end, or length exceeds the array shape or if the dimensionality of the array and Slicer do not conform.

Thrown Exceptions

• ArraySlicerError

isFixed()

bool casacore::Slicer::isFixed ( ) const

inline

Are all values fixed (i.e., no MimicSource given)?

Definition at line 464 of file Slicer.h.

References fixed_p.

length()

const IPosition & casacore::Slicer::length ( ) const

inline

Report the length of the resulting axes.

Definition at line 461 of file Slicer.h.

References len_p.

ndim()

size_t casacore::Slicer::ndim ( ) const

inline

Return the number of dimensions of the Slicer.

Definition at line 449 of file Slicer.h.

References casacore::IPosition::nelements(), and start_p.

operator==()

bool casacore::Slicer::operator==

(

const Slicer &

)

const

Equality.

setEnd()

void casacore::Slicer::setEnd ( const IPosition &  end )

inline

Definition at line 408 of file Slicer.h.

References end(), and end_p.

setStart()

void casacore::Slicer::setStart ( const IPosition &  start )

inline

Set the start and end positions.

No explicit checking is done that the input parameters make sense, so you must be certain if you call these. These are useful if you have a loop with many iterations and you do not wish the overhead of creating a new Slicer object for each iteration if the only thing you are doing is adjusting the start and end positions. Other than for performance reasons, these methods should not be called and you should prefer the error checking provided by constructing a new Slicer object. Note that the length is not updated, so in principle care should be taken that the length does not change.

Definition at line 406 of file Slicer.h.

References start(), and start_p.

start()

const IPosition & casacore::Slicer::start ( ) const

inline

Report the defined starting position.

Definition at line 452 of file Slicer.h.

References start_p.

Referenced by setStart().

stride()

const IPosition & casacore::Slicer::stride ( ) const

inline

Report the defined stride.

Definition at line 458 of file Slicer.h.

References stride_p.

Member Data Documentation

asEnd_p

LengthOrLast casacore::Slicer::asEnd_p

private

Definition at line 414 of file Slicer.h.

end_p

IPosition casacore::Slicer::end_p

private

Definition at line 416 of file Slicer.h.

Referenced by end(), and setEnd().

fixed_p

bool casacore::Slicer::fixed_p

private

Definition at line 419 of file Slicer.h.

Referenced by isFixed().

len_p

IPosition casacore::Slicer::len_p

private

Definition at line 418 of file Slicer.h.

Referenced by length().

start_p

IPosition casacore::Slicer::start_p

private

Definition at line 415 of file Slicer.h.

Referenced by ndim(), setStart(), and start().

stride_p

IPosition casacore::Slicer::stride_p

private

Definition at line 417 of file Slicer.h.

Referenced by stride().

---

The documentation for this class was generated from the following file:

• casa/Arrays/Slicer.h