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compute reduce command

Syntax:

compute ID reduce mode input1 input2 ... keyword args ... 

Examples:

compute 1 reduce sum c_grid[*]
compute 2 reduce min f_ave v_myKE
compute 3 reduce max c_mine[1] c_mine[2] c_temp replace 1 3 replace 2 3 

These commands will include the average grid cell temperature, across all grid cells, in the stats output:

compute 1 temp
compute	2 grid all all temp
compute 3 reduce ave c_2[1]
stats_style step c_temp c_3 

Description:

Define a calculation that "reduces" one or more vector inputs into scalar values, one per listed input. The inputs can be per-particle or per-grid or per-surf quantities; they cannot be global quantities. Particle attributes are per-particle quantities, computes may generate per-particle or per-grid quantities, fixes may generate any of the three kinds of quantities, and particle-style or grid-style variables generate per-particle or per-grid quantities. See the variable command and its special functions which can perform the same operations as the compute reduce command on global vectors.

The reduction operation is specified by the mode setting. The sum option adds the values in the vector into a global total. The min or max options find the minimum or maximum value across all vector values. The ave setting adds the vector values into a global total, then divides by the number of values in the vector. The sumsq option sums the square of the values in the vector into a global total. The avesq setting does the same as sumsq, then divdes the sum of squares by the number of values. The last two options can be useful for calculating the variance of some quantity, e.g. variance = sumsq - ave^2.

Each listed input is operated on independently.

Each listed input can be a particle attribute or can be the result of a compute or fix or the evaluation of a variable.

Note that for values from a compute or fix, the bracketed index I can be specified using a wildcard asterisk with the index to effectively specify multiple values. This takes the form "*" or "*n" or "n*" or "m*n". If N = the size of the vector (for mode = scalar) or the number of columns in the array (for mode = vector), then an asterisk with no numeric values means all indices from 1 to N. A leading asterisk means all indices from 1 to n (inclusive). A trailing asterisk means all indices from n to N (inclusive). A middle asterisk means all indices from m to n (inclusive).

Using a wildcard is the same as if the individual columns of the array had been listed one by one. E.g. these 2 compute reduce commands are equivalent, since the compute grid command creates a per-grid array with 3 columns:

compute myGrid grid all all u v w
compute 2 all reduce min c_myGrid[*]
compute 2 all reduce min c_myGrid[1] c_myGrid[2] c_myGrid[3] 

The particle attributes x,y,z,vx,vy,vz are position and velocity components. The ke,erot,evib attributes are for kinetic, rotational, and vibrational energy of particles.

If a value begins with "c_", a compute ID must follow which has been previously defined in the input script. Computes can generate per-particle or per-grid quantities. See the individual compute doc page for details. If no bracketed integer is appended, the vector calculated by the compute is used. If a bracketed integer is appended, the Ith column of the array calculated by the compute is used. Users can also write code for their own compute styles and add them to SPARTA. See the discussion above for how I can be specified with a wildcard asterisk to effectively specify multiple values.

IMPORTANT NOTE: A compute which generates per-surf quantities cannot be used as input. This is because its values have not yet been combined across processors to sum the contributions from all processors whose particles collide with the same surface element. The combining is performed by the fix ave/surf command, at each of its Nfreq timesteps. Thus to use this compute on per-surf values, specify a fix ID for a fix ave/surf and insure the fix outputs its values when they are needed.

If a value begins with "f_", a fix ID must follow which has been previously defined in the input script. Fixes can generate per-particle or per-grid or per-surf quantities. See the individual fix doc page for details. Note that some fixes only produce their values on certain timesteps, which must be compatible with when this compute references the values, else an error results. If no bracketed integer is appended, the vector calculated by the fix is used. If a bracketed integer is appended, the Ith column of the array calculated by the fix is used. Users can also write code for their own fix style and add them to SPARTA. See the discussion above for how I can be specified with a wildcard asterisk to effectively specify multiple values.

If a value begins with "v_", a variable name must follow which has been previously defined in the input script. It must be a particle-style or grid-style variable. Both styles define formulas which can reference stats keywords or invoke other computes, fixes, or variables when they are evaluated. Particle-style variables can also reference various per-particle attributes (position, velocity, etc). So these variables are a very general means of creating per-particle or per-grid quantities to reduce.


If the replace keyword is used, two indices vec1 and vec2 are specified, where each index ranges from 1 to the # of input values. The replace keyword can only be used if the mode is min or max. It works as follows. A min/max is computed as usual on the vec2 input vector. The index N of that value within vec2 is also stored. Then, instead of performing a min/max on the vec1 input vector, the stored index is used to select the Nth element of the vec1 vector.

Here is an example which prints out both the grid cell ID and number of particles for the grid cell with the maximum number of particles:

compute 1 property/grid id
compute	2 grid all n
compute	3 reduce max c_1 c_2[1] replace 1 2
stats_style step c_temp c_3[1] c_3[2] 

The first two input values in the compute reduce command are vectors with the ID and particle count of each grid cell. Instead of taking the max of the ID vector, which does not yield useful information in this context, the replace keyword will extract the ID for the grid cell which has the maximum number of particles. This ID and the cell's particle count will be printed with the statistical output.


If a single input is specified this compute produces a global scalar value. If multiple inputs are specified, this compute produces a global vector of values, the length of which is equal to the number of inputs specified.


Output info:

This compute calculates a global scalar if a single input value is specified or a global vector of length N where N is the number of inputs, and which can be accessed by indices 1 to N. These values can be used by any command that uses global scalar or vector values from a compute as input. See Section 4.4 for an overview of SPARTA output options.

The scalar or vector values will be in whatever units the quantities being reduced are in.

Restrictions: none

Related commands:

compute, fix, variable

Default: none