additional small changes to doc page and code comment

doc/src/fix_ave_moments.rst

@@ -53,7 +53,7 @@ Description
 .. versionadded:: TBD
 
 Using one or more values as input, calculate the moments of the underlying
-(population) distribution based on samples collected every few time
+(population) distributions based on samples collected every few time
 steps over a time step window. The definitions of the moments calculated
 are given below.
 
@@ -199,11 +199,12 @@ time 0, so setting *start* to a larger value can avoid including a 0.0
 in a longer series.
 
 The *history* keyword stores the Nrecent most recent outputs on Nfreq
-timesteps, so they can be accessed as global outputs of the fix.  By
-default, only the most recent output is accessible. For example, if
-history 10 is specified and Nfreq = 1000, then on timestep 20000, the
-Nfreq outputs from steps 20000, 19000, ... 11000 are available for
-access.  See below for details on how to access the history values.
+timesteps, so they can be accessed as global outputs of the fix.  Nrecent
+must be >= 1. The default is 1, meaning only the most recent output is
+accessible. For example, if history 10 is specified and Nfreq = 1000,
+then on timestep 20000, the Nfreq outputs from steps 20000, 19000, ...
+11000 are available for access.  See below for details on how to access
+the history values.
 
 For example, this will store the outputs of the previous 10 Nfreq
 time steps, i.e. a window of 10000 time steps:
@@ -214,10 +215,11 @@ time steps, i.e. a window of 10000 time steps:
 
 The previous results can be accessed as values in a global array output
 by this fix. Each column of the array is the vector output of the N-th
-preceding Nfreq timestep.  For example, the most recent result of the
-third input value would be accessed as "f_name[3][1]", "f_name[3][4]"
-is the 4th most recent and so on.  The current vector output is always
-the first column of the array, corresponding to the most recent result.
+preceding Nfreq timestep.  For example, assuming a single moment is
+calculated, the most recent result corresponding to the third input
+value would be accessed as "f_name[3][1]", "f_name[3][4]" is the 4th
+most recent and so on.  The current vector output is always the first
+column of the array, corresponding to the most recent result.
 
 To illustrate the utility of keeping output history, consider using
 this fix in conjunction with :doc:`fix halt <fix_halt>` to stop a run
@@ -250,9 +252,9 @@ accessed on any time step, but may not be current.
 
 A global vector is produced with the # of elements = number of moments *
 number of inputs.  The moments are output in the order given in the fix
-definition.  An array is produced having # of rows = same as vector output,
-using the same ordering and # of columns = value of *history*. There is
-always at least one column.
+definition.  An array is produced having # of rows = length of vector
+output (with an ordering which matches the vector) and # of columns =
+value of *history*. There is always at least one column.
 
 Each element of the global vector or array can be either "intensive" or
 "extensive", depending on whether the values contributing to the element

src/EXTRA-FIX/fix_ave_moments.cpp

@@ -377,8 +377,9 @@ double FixAveMoments::compute_array(int i, int j)
 {
   if (i >= size_vector) return 0.0;
   if (j >= nhistory) return 0.0;
+  // locate the j'th previous result in the ring buffer, relative to the
+  // row before iresult (the current insert cursor)
   int row = (iresult - 1 - j + nhistory) % nhistory;
-  if (row >= nhistory) return 0.0;
   return result_list[row][i];
 }