From 9e1685211b1f3c6295758d7f413b14b8c801c7b3 Mon Sep 17 00:00:00 2001
From: Axel Kohlmeyer <akohlmey@gmail.com>
Date: Fri, 15 Jul 2022 05:39:17 -0400
Subject: [PATCH] allow ramp(x,y) to be used in between runs (returning x) and
 avoid division by zero on run 0

---
 doc/src/variable.rst |  4 +++-
 src/variable.cpp     | 37 +++++++++++++++++++++++++------------
 2 files changed, 28 insertions(+), 13 deletions(-)

diff --git a/doc/src/variable.rst b/doc/src/variable.rst
index 9f76f78f45..ceaa4d7245 100644
--- a/doc/src/variable.rst
+++ b/doc/src/variable.rst
@@ -685,7 +685,9 @@ of a run, according to this formula:
 The run begins on startstep and ends on stopstep.  Startstep and
 stopstep can span multiple runs, using the *start* and *stop* keywords
 of the :doc:`run <run>` command.  See the :doc:`run <run>` command for
-details of how to do this.
+details of how to do this.  If called in between runs or during a
+:doc:`run 0 <run>` command, the ramp(x,y) function will return the
+value of x.
 
 The stagger(x,y) function uses the current timestep to generate a new
 timestep.  X,y > 0 and x > y are required.  The generated timesteps
diff --git a/src/variable.cpp b/src/variable.cpp
index 878066f466..574aa26c86 100644
--- a/src/variable.cpp
+++ b/src/variable.cpp
@@ -2561,9 +2561,14 @@ double Variable::collapse_tree(Tree *tree)
     arg2 = collapse_tree(tree->second);
     if (tree->first->type != VALUE || tree->second->type != VALUE) return 0.0;
     tree->type = VALUE;
-    double delta = update->ntimestep - update->beginstep;
-    if (delta != 0.0) delta /= update->endstep - update->beginstep;
-    tree->value = arg1 + delta*(arg2-arg1);
+    if (update->whichflag == 0) {
+      tree->value = arg1;
+    } else {
+      double delta = update->ntimestep - update->beginstep;
+      if ((delta != 0.0) && (update->beginstep != update->endstep))
+        delta /= update->endstep - update->beginstep;
+      tree->value = arg1 + delta*(arg2-arg1);
+    }
     return tree->value;
   }
 
@@ -2935,9 +2940,14 @@ double Variable::eval_tree(Tree *tree, int i)
   if (tree->type == RAMP) {
     arg1 = eval_tree(tree->first,i);
     arg2 = eval_tree(tree->second,i);
-    double delta = update->ntimestep - update->beginstep;
-    if (delta != 0.0) delta /= update->endstep - update->beginstep;
-    arg = arg1 + delta*(arg2-arg1);
+    if (update->whichflag == 0) {
+      arg = arg1;
+    } else {
+      double delta = update->ntimestep - update->beginstep;
+      if ((delta != 0.0) && (update->beginstep != update->endstep))
+        delta /= update->endstep - update->beginstep;
+      arg = arg1 + delta*(arg2-arg1);
+    }
     return arg;
   }
 
@@ -3445,14 +3455,17 @@ int Variable::math_function(char *word, char *contents, Tree **tree, Tree **tree
   } else if (strcmp(word,"ramp") == 0) {
     if (narg != 2)
       print_var_error(FLERR,"Invalid math function in variable formula",ivar);
-    if (update->whichflag == 0)
-      print_var_error(FLERR,"Cannot use ramp in variable formula between runs",ivar);
     if (tree) newtree->type = RAMP;
     else {
-      double delta = update->ntimestep - update->beginstep;
-      if (delta != 0.0) delta /= update->endstep - update->beginstep;
-      double value = value1 + delta*(value2-value1);
-      argstack[nargstack++] = value;
+      if (update->whichflag == 0) {
+        argstack[nargstack++] = value1;
+      } else {
+        double delta = update->ntimestep - update->beginstep;
+        if ((delta != 0.0) && (update->beginstep != update->endstep))
+          delta /= update->endstep - update->beginstep;
+        double value = value1 + delta*(value2-value1);
+        argstack[nargstack++] = value;
+      }
     }
 
   } else if (strcmp(word,"stagger") == 0) {