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doc/_sources/compute_slice.txt
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doc/_sources/compute_slice.txt
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.. index:: compute slice
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compute slice command
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=====================
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Syntax
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""""""
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.. parsed-literal::
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compute ID group-ID slice Nstart Nstop Nskip input1 input2 ...
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* ID, group-ID are documented in :doc:`compute <compute>` command
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* slice = style name of this compute command
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* Nstart = starting index within input vector(s)
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* Nstop = stopping index within input vector(s)
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* Nskip = extract every Nskip elements from input vector(s)
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* input = c_ID, c_ID[N], f_ID, f_ID[N]
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.. parsed-literal::
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c_ID = global vector calculated by a compute with ID
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c_ID[I] = Ith column of global array calculated by a compute with ID
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f_ID = global vector calculated by a fix with ID
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f_ID[I] = Ith column of global array calculated by a fix with ID
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Examples
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""""""""
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.. parsed-literal::
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compute 1 all slice 1 100 10 c_msdmol[4]
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compute 1 all slice 301 400 1 c_msdmol[4]
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Description
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"""""""""""
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Define a calculation that "slices" one or more vector inputs into
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smaller vectors, one per listed input. The inputs can be global
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quantities; they cannot be per-atom or local quantities.
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:doc:`Computes <compute>` and :doc:`fixes <fix>` may generate any of the
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three kinds of quantities. :doc:`Variables <variable>` do not generate
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global vectors. The group specified with this command is ignored.
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The values extracted from the input vector(s) are determined by the
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*Nstart*, *Nstop*, and *Nskip* parameters. The elements of an input
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vector of length N are indexed from 1 to N. Starting at element
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*Nstart*, every Mth element is extracted, where M = *Nskip*, until
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element *Nstop* is reached. The extracted quantities are stored as a
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vector, which is typically shorter than the input vector.
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Each listed input is operated on independently to produce one output
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vector. Each listed input must be a global vector or column of a
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global array calculated by another :doc:`compute <compute>` or
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:doc:`fix <fix>`.
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If an input value begins with "c_", a compute ID must follow which has
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been previously defined in the input script and which generates a
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global vector or array. See the individual :doc:`compute <compute>` doc
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page for details. If no bracketed integer is appended, the vector
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calculated by the compute is used. If a bracketed integer is
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appended, the Ith column of the array calculated by the compute is
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used. Users can also write code for their own compute styles and :doc:`add them to LAMMPS <Section_modify>`.
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If a value begins with "f_", a fix ID must follow which has been
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previously defined in the input script and which generates a global
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vector or array. See the individual :doc:`fix <fix>` doc page for
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details. Note that some fixes only produce their values on certain
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timesteps, which must be compatible with when compute slice references
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the values, else an error results. If no bracketed integer is
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appended, the vector calculated by the fix is used. If a bracketed
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integer is appended, the Ith column of the array calculated by the fix
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is used. Users can also write code for their own fix style and :doc:`add them to LAMMPS <Section_modify>`.
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If a single input is specified this compute produces a global vector,
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even if the length of the vector is 1. If multiple inputs are
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specified, then a global array of values is produced, with the number
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of columns equal to the number of inputs specified.
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----------
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**Output info:**
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This compute calculates a global vector if a single input value is
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specified or a global array with N columns where N is the number of
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inputs. The length of the vector or the number of rows in the array
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is equal to the number of values extracted from each input vector.
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These values can be used by any command that uses global vector or
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array values from a compute as input. See :ref:`this section <howto_15>` for an overview of LAMMPS output
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options.
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The vector or array values calculated by this compute are simply
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copies of values generated by computes or fixes that are input vectors
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to this compute. If there is a single input vector of intensive
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and/or extensive values, then each value in the vector of values
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calculated by this compute will be "intensive" or "extensive",
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depending on the corresponding input value. If there are multiple
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input vectors, and all the values in them are intensive, then the
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array values calculated by this compute are "intensive". If there are
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multiple input vectors, and any value in them is extensive, then the
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array values calculated by this compute are "extensive".
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The vector or array values will be in whatever :doc:`units <units>` the
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input quantities are in.
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Restrictions
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""""""""""""
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none
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Related commands
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""""""""""""""""
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:doc:`compute <compute>`, :doc:`fix <fix>`, :doc:`compute reduce <compute_reduce>`
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**Default:** none
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Section_commands.html#comm
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