Add ability to use variables from grid file in input file expressions by ZedThree · Pull Request #3222 · boutproject/BOUT-dev

ZedThree · 2025-12-02T17:15:33Z

This adds the ability to read fields (2D and 3D) and doubles* from the grid file (mesh:file) and use them in expressions in the input file, for example to use coordinates directly from the grid generator:

[input:grid_variables]
rho = field2d
theta = field2d
scale = boutreal

[mesh]
B = (scale / rho) * cos(theta)

* ints can also be read, but the expression parser only handles doubles

This could also be used for something like region labels to the input file by having hypnotoad make a field which is 1.0 in that region and 0.0 elsewhere:

region_pfr = 1.0 if is_in_pfr else 0.0

and using like

[input:grid_variables]
region_pfr = field2d

[some_source]
function = region_pfr * cos(x) * whatever

We could probably also do this in BOUT++ directly, making fields for each boundary region.

github-actions

clang-tidy made some suggestions

github-actions · 2025-12-02T17:18:18Z

src/field/field_factory.cxx

  return std::make_shared<FieldValuePtr>(ptr);
 }

+BOUT_ENUM_CLASS(GridVariableFunction, field3d, field2d, boutreal);


warning: enum 'GridVariableFunction' uses a larger base type ('int', size: 4 bytes) than necessary for its value set, consider using 'std::uint8_t' (1 byte) as the base type to reduce its size [performance-enum-size]

BOUT_ENUM_CLASS(GridVariableFunction, field3d, field2d, boutreal); ^

github-actions · 2025-12-02T17:18:18Z

src/field/field_factory.cxx

  return std::make_shared<FieldValuePtr>(ptr);
 }

+BOUT_ENUM_CLASS(GridVariableFunction, field3d, field2d, boutreal);


warning: function 'GridVariableFunctionFromString' can be made static or moved into an anonymous namespace to enforce internal linkage [misc-use-internal-linkage]

BOUT_ENUM_CLASS(GridVariableFunction, field3d, field2d, boutreal); ^

expanded from here

github-actions · 2025-12-02T17:18:18Z

src/field/field_factory.cxx

  return std::make_shared<FieldValuePtr>(ptr);
 }

+BOUT_ENUM_CLASS(GridVariableFunction, field3d, field2d, boutreal);


warning: parameter 'UNUSED_similar_to' is unused [misc-unused-parameters]

BOUT_ENUM_CLASS(GridVariableFunction, field3d, field2d, boutreal); ^

Additional context

include/bout/bout_enum_class.hxx:98: expanded from macro 'BOUT_ENUM_CLASS'

inline enumname Options::as<enumname>(const enumname&) const { \ ^

github-actions · 2025-12-02T17:18:18Z

src/field/fieldgenerators.hxx

+class GridVariable : public FieldGenerator {
+public:
+  GridVariable(T var, std::string name)
+      : variable(std::move(var)), name(std::move(name)) {}


warning: no header providing "std::move" is directly included [misc-include-cleaner]

src/field/fieldgenerators.hxx:18:

+ #include <utility>

github-actions · 2025-12-02T17:18:18Z

src/field/fieldgenerators.hxx

+  }
+
+  FieldGeneratorPtr clone(const std::list<FieldGeneratorPtr> args) override {
+    if (args.size() != 0) {


warning: the 'empty' method should be used to check for emptiness instead of 'size' [readability-container-size-empty]

Suggested change

if (args.size() != 0) {

if (!args.empty()) {

Additional context

/usr/include/c++/13/bits/stl_list.h:1142: method 'list'::empty() defined here

empty() const _GLIBCXX_NOEXCEPT ^

dschwoerer

Looks good, thanks for cleaning up 👍

The merge-base changed after approval.

bendudson

Thanks @ZedThree ! This looks really useful. I think this is good as-is, but for future if this becomes widely used:

These expressions are in the global scope rather than being in the mesh namespace.
A nice-to-have might be renaming imports analogous to "import x as y" to avoid name conflicts.
These may over-complicate things, so I think this simplest implementation is good.

The merge-base changed after approval.

The constructor argument `localmesh` can be null but in that case `fieldmesh` is set to `bout::globals::mesh`.

`BoutMesh::load` creates a FieldFactory to read variables such as mesh sizes. This then tries to load grid variables, which in turn starts trying to create coordinates, use regions etc., none of which has been constructed at that point. This defers reading until it's needed, at which point it reads the variable once and re-uses in subsequent calls.

bendudson · 2026-03-26T23:30:21Z

Hey @ZedThree ! This functionality is really useful :) I had to make the loading lazy to avoid issues where a FieldFactory is created during BoutMesh::load and before mesh->get works.

github-actions

clang-tidy made some suggestions

github-actions · 2026-03-26T23:31:48Z

src/field/field_factory.cxx

  return std::make_shared<FieldValuePtr>(ptr);
 }

+BOUT_ENUM_CLASS(GridVariableFunction, field3d, field2d, boutreal);


warning: function 'operator<<' can be made static or moved into an anonymous namespace to enforce internal linkage [misc-use-internal-linkage]

BOUT_ENUM_CLASS(GridVariableFunction, field3d, field2d, boutreal); ^

Additional context

include/bout/bout_enum_class.hxx:102: expanded from macro 'BOUT_ENUM_CLASS'

inline std::ostream& operator<<(std::ostream& out, const enumname& e) { \ ^

github-actions · 2026-03-26T23:31:48Z

src/field/field_factory.cxx

  return std::make_shared<FieldValuePtr>(ptr);
 }

+BOUT_ENUM_CLASS(GridVariableFunction, field3d, field2d, boutreal);


warning: function 'toString' can be made static or moved into an anonymous namespace to enforce internal linkage [misc-use-internal-linkage]

BOUT_ENUM_CLASS(GridVariableFunction, field3d, field2d, boutreal); ^

Additional context

include/bout/bout_enum_class.hxx:70: expanded from macro 'BOUT_ENUM_CLASS'

inline std::string toString(enumname e) { \ ^

github-actions · 2026-03-26T23:31:48Z

src/field/fieldgenerators.hxx

  FieldTanhHat(FieldGeneratorPtr xin, FieldGeneratorPtr widthin,
               FieldGeneratorPtr centerin, FieldGeneratorPtr steepnessin)
-      : X(xin), width(widthin), center(centerin), steepness(steepnessin){};
+      : X(xin), width(widthin), center(centerin), steepness(steepnessin) {};


warning: parameter 'centerin' is passed by value and only copied once; consider moving it to avoid unnecessary copies [performance-unnecessary-value-param]

Suggested change

: X(xin), width(widthin), center(centerin), steepness(steepnessin) {};

: X(xin), width(widthin), center(std::move(centerin)), steepness(steepnessin) {};

github-actions · 2026-03-26T23:31:48Z

src/field/fieldgenerators.hxx

  FieldTanhHat(FieldGeneratorPtr xin, FieldGeneratorPtr widthin,
               FieldGeneratorPtr centerin, FieldGeneratorPtr steepnessin)
-      : X(xin), width(widthin), center(centerin), steepness(steepnessin){};
+      : X(xin), width(widthin), center(centerin), steepness(steepnessin) {};


warning: parameter 'steepnessin' is passed by value and only copied once; consider moving it to avoid unnecessary copies [performance-unnecessary-value-param]

Suggested change

: X(xin), width(widthin), center(centerin), steepness(steepnessin) {};

: X(xin), width(widthin), center(centerin), steepness(std::move(steepnessin)) {};

github-actions · 2026-03-26T23:31:48Z

src/field/fieldgenerators.hxx

  FieldTanhHat(FieldGeneratorPtr xin, FieldGeneratorPtr widthin,
               FieldGeneratorPtr centerin, FieldGeneratorPtr steepnessin)
-      : X(xin), width(widthin), center(centerin), steepness(steepnessin){};
+      : X(xin), width(widthin), center(centerin), steepness(steepnessin) {};


warning: parameter 'widthin' is passed by value and only copied once; consider moving it to avoid unnecessary copies [performance-unnecessary-value-param]

Suggested change

: X(xin), width(widthin), center(centerin), steepness(steepnessin) {};

: X(xin), width(std::move(widthin)), center(centerin), steepness(steepnessin) {};

github-actions · 2026-03-26T23:31:48Z

src/field/fieldgenerators.hxx

  FieldTanhHat(FieldGeneratorPtr xin, FieldGeneratorPtr widthin,
               FieldGeneratorPtr centerin, FieldGeneratorPtr steepnessin)
-      : X(xin), width(widthin), center(centerin), steepness(steepnessin){};
+      : X(xin), width(widthin), center(centerin), steepness(steepnessin) {};


warning: parameter 'xin' is passed by value and only copied once; consider moving it to avoid unnecessary copies [performance-unnecessary-value-param]

Suggested change

: X(xin), width(widthin), center(centerin), steepness(steepnessin) {};

: X(std::move(xin)), width(widthin), center(centerin), steepness(steepnessin) {};

github-actions · 2026-03-26T23:31:48Z

src/field/fieldgenerators.hxx

 public:
  FieldWhere(FieldGeneratorPtr test, FieldGeneratorPtr gt0, FieldGeneratorPtr lt0)
-      : test(test), gt0(gt0), lt0(lt0){};
+      : test(test), gt0(gt0), lt0(lt0) {};


warning: parameter 'gt0' is passed by value and only copied once; consider moving it to avoid unnecessary copies [performance-unnecessary-value-param]

Suggested change

: test(test), gt0(gt0), lt0(lt0) {};

: test(test), gt0(std::move(gt0)), lt0(lt0) {};

github-actions · 2026-03-26T23:31:48Z

src/field/fieldgenerators.hxx

 public:
  FieldWhere(FieldGeneratorPtr test, FieldGeneratorPtr gt0, FieldGeneratorPtr lt0)
-      : test(test), gt0(gt0), lt0(lt0){};
+      : test(test), gt0(gt0), lt0(lt0) {};


warning: parameter 'lt0' is passed by value and only copied once; consider moving it to avoid unnecessary copies [performance-unnecessary-value-param]

Suggested change

: test(test), gt0(gt0), lt0(lt0) {};

: test(test), gt0(gt0), lt0(std::move(lt0)) {};

github-actions · 2026-03-26T23:31:48Z

src/field/fieldgenerators.hxx

 public:
  FieldWhere(FieldGeneratorPtr test, FieldGeneratorPtr gt0, FieldGeneratorPtr lt0)
-      : test(test), gt0(gt0), lt0(lt0){};
+      : test(test), gt0(gt0), lt0(lt0) {};


warning: parameter 'test' is passed by value and only copied once; consider moving it to avoid unnecessary copies [performance-unnecessary-value-param]

Suggested change

: test(test), gt0(gt0), lt0(lt0) {};

: test(std::move(test)), gt0(gt0), lt0(lt0) {};

github-actions · 2026-03-26T23:31:48Z

src/mesh/data/gridfromfile.cxx

  const int myg = (m->LocalNy - (m->yend - m->ystart + 1)) / 2;
-  ///Check that ghost region widths are in fact integers
+  // Check grid has cells
+  ASSERT1(m->LocalNx > 0);


warning: no header providing "ASSERT1" is directly included [misc-include-cleaner]

src/mesh/data/gridfromfile.cxx:1:

- #include "bout/traits.hxx" + #include "bout/assert.hxx" + #include "bout/traits.hxx"

ZedThree and others added 7 commits December 1, 2025 17:04

Remove unused Context constructor

ca7f727

Add cell indices to generator::Context

f9c616b

Add ability to use variables from grid files in input expressions

4034481

tests: Only remove temp files if test passes

aa963d6

Enable reading Field2Ds as well as Field3Ds for expressions

4985b3c

Refactor reading grid variables for input expressions

47b852f

Apply clang-format changes

dc80694

github-actions bot reviewed Dec 2, 2025

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dschwoerer previously approved these changes Feb 4, 2026

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mikekryjak mentioned this pull request Feb 4, 2026

Coordinate index access in input file? #3268

Open

dschwoerer previously approved these changes Feb 4, 2026

View reviewed changes

dschwoerer previously approved these changes Feb 27, 2026

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bendudson previously approved these changes Mar 10, 2026

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bendudson added 3 commits March 26, 2026 15:39

FieldFactory: Use fieldmesh not localmesh

259eff9

The constructor argument `localmesh` can be null but in that case `fieldmesh` is set to `bout::globals::mesh`.

Merge branch 'next' into field-variable-expression

b146562

github-actions bot reviewed Mar 26, 2026

View reviewed changes

	: X(xin), width(widthin), center(centerin), steepness(steepnessin) {};
	: X(xin), width(widthin), center(std::move(centerin)), steepness(steepnessin) {};

	: X(xin), width(widthin), center(centerin), steepness(steepnessin) {};
	: X(xin), width(std::move(widthin)), center(centerin), steepness(steepnessin) {};

	: X(xin), width(widthin), center(centerin), steepness(steepnessin) {};
	: X(std::move(xin)), width(widthin), center(centerin), steepness(steepnessin) {};

	: test(test), gt0(gt0), lt0(lt0) {};
	: test(test), gt0(std::move(gt0)), lt0(lt0) {};

	: test(test), gt0(gt0), lt0(lt0) {};
	: test(test), gt0(gt0), lt0(std::move(lt0)) {};

	: test(test), gt0(gt0), lt0(lt0) {};
	: test(std::move(test)), gt0(gt0), lt0(lt0) {};

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ZedThree commented Dec 2, 2025

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