Class Slice¶
Defined in File Slice.hpp
Nested Relationships¶
Nested Types¶
Class Documentation¶
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class Slice¶
A class to hold the bounds of a general slice, not tied to any particular array.
The Slice class allows the definition of a slice of an array. The format used mirrors that of the Python numpy library as far as possible. The slicing admits single element indexing with the syntax {{{i}}}. This defines a slice that accesses only a single element of a one dimensional array. The index can also be absent, as {{{}}}, which is used to designate the entire one-dimensional array. This can also be explicitly defined using an empty initializer list, as {{{{}}}}.
Beyond indexing a single element the slice syntax can specify a range. This consists of a start index, an optional stop index and an optional step index. If both stop and step are absent, then the slice reverts to single element addressing. The stop index is the index of the element one past the end of the desired slice. The number of elements in the slice is given by stop - start if the step is 1 (its default value). This means that a slice with a start of 2 and a stop of 7 will have five elements at indices 2, 3, 4, 5 & 6. The syntax for a slice with both a start and a stop is {{{start, stop}}}. Either can also be replaced by an empty initializer list, as {{{{}, stop}}}, {{{start, {}}}} and {{{{}, {}}}}. A default start runs from the start of the array and a default stop runs to the end of the array. If both are default empty initializer lists, then the slice spans the entire array.
A step value can also be present. When traversing the slice, the index of the location in the array will increase by the step value. Accessing the start=2 and stop=7 slice described above with a step of 2 would be written as {{{2, 7, 2}}} and access the elements at indices 2, 4 & 6. The syntax for a slice with a start, stop and step is {{{start, stop, step}}}. Both start and stop must be present to define a step using the default initializer list syntax to give {{{{}, {}, step}}}. The step value can be negative. This requires the start value to be greater than the stop value to avoid an empty slice. The slice written as {{{2, 7, -2}} defines an empty slice. The slice written as {{{7, 2, -2}}} would access the indices 7, 5 & 3.
The start and stop indices can also be negative, which are interpreted as counting backwards from the end of the array. The element -1 is the last element in the array, -2 is the second last all the way to -n, the first element of an array of length n.
The above description details how indexing works on a one dimensional array. A slice can also be defined over a multidimensional array of arbitrarily high dimension. This is done by adding additional bounds initializers to the second set of braces. If the suffix _n is applied to the start, stop and step indices in dimension n, then a fully specified multidimensional slice is written as {{{start_0, stop_0, step_0}, {start_1, stop_1, step_1}…}}. The bounds term for each dimensional can be specified in the same way as for the one dimensional bounds described above, including default initializer list values, negative indices and negative step values.