Expresiones

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Resumen

Es posible definir propiedades utilizando expresiones matemáticas. En la interfaz gráfica de usuario, los cuadros de giro o campos de entrada que están vinculados a las propiedades contienen un icono azul Bound-expression.svg. Haciendo clic en el icono o escribiendo el signo igual = aparece el editor de expresiones para esa propiedad en particular.

Es una expresión matemática que sigue la notación de los operadores y funciones matemáticas estándar, como se describe a continuación. Además, la expresión puede hacer referencia a otras propiedades, y también utilizar condicionales. Los números en una expresión pueden tener una unidad opcional adjunta.

Los número pueden usar ya sea una coma , o un punto decimal . para separar los dígitos enteros de los decimales. Cuando se usa el marcador decimal, "debe" ser seguido por al menos un dígito. Por lo tanto, las expresiones 1.+2. y 1,+2, son inválidas, pero 1.0+2.0 y 1,0+2,0 son válidas.

Los operadores y las funciones son conscientes de la unidad y requieren combinaciones válidas de unidades, si se suministran. Por ejemplo, 2mm+4mm es una expresión válida, mientras que 2mm+4 no lo es (la razón es que una expresión como 1in+4 probablemente será interpretada como 1in+4in por los humanos, pero todas las unidades se convierten al sistema métrico internamente, y el sistema no puede adivinar esto). Estas unidades son reconocidas actualmente.

Puedes usar constantes predefinidas y funciones.

Argumentos de la función

Múltiples argumentos a una función pueden estar separados por un punto y coma ; o una coma seguida por un espacio , . En el último caso, la coma se convierte a un punto y coma después de la entrada. Cuando se usa un punto y coma, no es necesario un espacio final.

Los argumentos pueden incluir referencias a las celdas en una hoja de cálculo. Una referencia de celda consiste en la letra mayúscula de fila de la celda seguida de su número de columna, por ejemplo A1. También se puede hacer referencia a una celda utilizando el alias de la celda, por ejemplo, Spreadsheet.MyPartWidth.

Referencia de objetos

Puede hacer referencia a un objeto por su Datos Name o por su Datos Label. En el caso de un Datos Label, debe estar encerrado en doble símbolos << y >>, como <<Label>>.

Puede referenciar cualquier propiedad de un objeto. Por ejemplo, para referenciar la altura de un Cilindro puede usar Cylinder.Height o <<Long_name_of_cylinder>>.Height. Para referenciar al objeto mismo use la pseudo propiedad _self. Por ejemplo, puede usar Cylinder._self o <<Label_of_cylinder>>._self.

Para referenciar una lista de objetos use <<object_label>>.list[list_index] or object_name.list[list_index]. Si quiere por ejemplo referenciar una restricción de un croquis use <<MySketch>>.Constraints[16]. Si está en el mismo croquis puede omitir su nombre y solo usar Constraints[16].
Nota: El índice inicia en 0, por lo que la restricción 17 tiene el índice 16.

Para más información de como referenciar objetos, vea Referencia a los datos CAD.

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Constantes soportadas

Las siguientes constantes están soportadas.

Constant Description
e Número de Euler
pi Pi

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Operadores soportados

Los siguientes operadores son soportados.

Operator Description
+ Suma
- Resta
* Multiplicación
/ Division de punto flotante
% Residuo
^ Potenciación

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Funciones soportadas

Funciones matemáticas generales

Las siguientes funciones matemáticas son soportadas:

Funciones trigonométricas

Trigonometric functions use degree as their default unit. For radian measure, add rad following the first value in an expression. So e.g. cos(45) is the same as cos(pi rad / 4). Expressions in degrees can use either deg or °, e.g. 360deg - atan2(3; 4) or 360° - atan2(3; 4). If an expression is without units and needs to be converted to degrees or radians for compatibility, multiply by 1 deg, 1 ° or 1 rad as appropriate, e.g. (360 - X) * 1deg; (360 - X) * 1°; (0.5 + pi / 2) * 1rad.

Function Description Value range
acos(x) Arc cosine -1 <= x <= 1
asin(x) Arc sine -1 <= x <= 1
atan(x) Arc tangent all
atan2(x; y) Arc tangent of x/y all, except y = 0
cos(x) Cosine all
cosh(x) Hyperbolic cosine all
sin(x) Sine all
sinh(x) Hyperbolic sine all
tan(x) Tangent all, except x = n*90 with n = uneven integer
tanh(x) Hyperbolic tangent all
hypot(x; y) Pythagorean addition (hypotenuse). E.g. hypot(4; 3) = 5. x and y > 0
cath(x; y) Given hypotenuse, and one side, returns other side of triangle. E.g. cath(5; 3) = 4. x and y > 0, x >= y

Exponential and logarithmic functions

Function Description Value range
exp(x) Exponential function all
log(x) Natural logarithm x > 0
log10(x) Common logarithm x > 0
pow(x; y) Exponentiation all
sqrt(x) Square root x >= 0

Rounding, truncation and remainder functions

Function Description Value range
abs(x) Absolute value all
ceil(x) Ceiling function, smallest integer value greater than or equal to x all
floor(x) Floor function, largest integer value less than or equal to x all
mod(x; y) Remainder after dividing x by y all, except y = 0
round(x) Rounding to the nearest integer all
trunc(x) Truncation to the nearest integer in the direction of zero all

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Funciones estadísticas/agregadas

Aggregate functions take one or more arguments.

Individual arguments to aggregate functions may consist of ranges of cells. A range of cells is expressed as two cell references separated by a colon :, for example average(B1:B8) or sum(A1:A4; B1:B4). The cell references may also use cell aliases, for example average(StartTemp:EndTemp).

The following aggregate functions are supported:

Function Description Value range
average(a; b; c; ...) Average value of the arguments; same as sum(a; b; c; ...) / count(a; b; c; ...) all
count(a; b; c; ...) Count of the arguments; typically used for cell ranges all
max(a; b; c; ...) Maximum value of the arguments all
min(a; b; c; ...) Minimum value of the arguments all
stddev(a; b; c; ...) Standard deviation of the values of the arguments all
sum(a; b; c; ...) Sum of the values of the arguments; typically used for cell ranges all

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String manipulation

String identification

Strings are identified in expressions by surrounding them with opening/closing double chevrons (as are labels).

In following example, "TEXT" is recognized as a string : <<TEXT>>

String concatenation

Strings can be concatenated using the '+' sign.

Following example <<MY>> + <<TEXT>> will be concatenated to "MYTEXT".

String conversion

Numerical values can be converted to strings with the str function:

str(Box.Length.Value)

String formatting

String formatting is supported using the (old) %-style Python way.

All %-specifiers as defined in Python documentation.

As an example, supposing you have a default 10mm-side cube named 'Box' (default FreeCAD naming), the following expression <<Cube length : %s>> % Box.Length will expand to "Cube length : 10.0 mm"

For more than one %-specifier use the following syntax: <<Cube length is %s and width is %s>> % tuple(Box.Length; Box.Width). Or use concatenation: <<Cube length is %s>> % Box.Length + << and width is %s>> % Box.Width. Both will expand to "Cube length is 10.0 mm and width is 10.0 mm".

A FreeCAD sample file using string formatting is available in the forum

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Create function

The following objects may be created in expressions via the create function:

  • Vector
  • Matrix
  • Rotation
  • Placement

The create function passes subsequent arguments to the underlying Python constructor when creating the object.

Various mathematical operations such as multiplication, addition, and subtraction are supported via standard mathematical operators (e.g. *, +, -).

Vector

When create is passed <<vector>> as the 1st argument, the next 3 arguments are the X, Y, and Z coordinates for the Vector respectively.

Example:

create(<<vector>>; 2; 1; 2)

Matrix

When create is passed <<matrix>> as the 1st argument, the next 16 arguments are the elements for the Matrix in row-major order.

Example:

create(<<matrix>>; 1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16)

Rotation

When create is passed <<rotation>> as the 1st argument, there are two ways to create a Rotation:

1. Specify an axis vector and a rotation angle.

Example:

create(<<rotation>>; create(<<vector>>; 0; 1; 0); 45)

2. Specify 3 rotations about the X, Y, and Z axes as Euler angles.

Example:

create(<<rotation>>; 30; 30; 30)

Placement

When create is passed <<placement>> as the 1st argument, there are five ways to create a Placement.

These possible combinations are documented in the below table and are based on the Placement API page.

Number of arguments Description
2 create(<<placement>>; Placement)
2 create(<<placement>>; Matrix)
3 create(<<placement>>; Base; Rotation)
4 create(<<placement>>; Base; Rotation; Center)
4 create(<<placement>>; Base; Axis; Angle)

The following example shows the syntax for creating a Placement from a Base (vector) and a Rotation:

create(<<placement>>; create(<<vector>>; 2; 1; 2); create(<<rotation>>; create(<<vector>>; 0; 1; 0); 45))

For readability, you can define vectors and rotations in separate cells, and then reference the cells in your expression.

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Matrix functions

mscale

Scale a Matrix with a given Vector.

mscale(Matrix; Vector)

mscale(Matrix; x; y; z)

minvert

Invert the given Matrix, Rotation, or Placement.

minvert(Matrix)

minvert(Rotation)

minvert(Placement)

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Tuple & list

You can create Python tuple or list objects via their respective functions.

tuple(2; 1; 2)

list(2; 1; 2)

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Expresiones condicionales

Conditional expressions are of the form condition ? resultTrue : resultFalse. The condition is defined as an expression that evaluates to either 0 (false) or non-zero (true). Note that enclosing the conditional expression in parentheses is currently considered an error.

The following relational operators are defined:

Unit Description
== equal to
!= not equal to
> greater than
< less than
>= greater than or equal to
<= less than or equal to

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Unidades

Units can be used directly in expressions. The parser connects them to the previous value. So 2mm or 2 mm is valid while mm is invalid because there is no preceding value.

All values must have a unit. Therefore you must in general use a unit for values in spreadsheets.
In some cases it works even without a unit, for example if you have e.g. in spreadsheet cell B1 just the number 1.5 and refer to it for a pad height. This only works because the pad height predefines the unit mm that is used if no unit is given. It will nevertheless fail if you use for the pad height e.g. Sketch1.Constraints.Width - Spreadsheet.B1 because Sketch1.Constraints.Width has a unit and Spreadsheet.B1 has not.

Units with exponents can directly be entered. So e.g. mm^3 will be recognized as mm³ and m^3 will be recognized as m³.

If you have a variable whose name is that of a unit you must put the variable between << >> to prevent it from being recognized as a unit. For example if you have the dimension Sketch.Constraints.A it would be recognized as the unit ampere. Therefore you must write it in the expression as Sketch.Constraints.<<A>>.

The following units are recognized by the expression parser:

Amount of substance

Unit Description
mol Mole

Angle

Unit Description
° Degree; alternative to the unit deg
deg Degree; alternative to the unit °
rad Radian
gon Gradian
S Second of arc; alternative to the unit ″
Second of arc; alternative to the unit S
M Minute of arc; alternative to the unit ′
Minute of arc; alternative to the unit M

Current

Unit Description
mA Milliampere
A Ampere
kA Kiloampere
MA Megaampere

Energy/work

Unit Description
J Joule
Ws Watt second; alternative to the unit Joule
VAs Volt-ampere-second; alternative to the unit Joule
CV Coulomb-volt; alternative to the unit Joule

Force

Unit Description
mN Millinewton
N Newton
kN Kilonewton
MN Meganewton
lbf Pound of force

Length

Unit Description
nm Nanometer
um Micrometer; alternative to the unit µm
µm Micrometer; alternative to the unit um
mm Millimeter
cm Centimeter
dm Decimeter
m Meter
km Kilometer
mil Thousandth of an inch; alternative to the unit thou
thou Thousandth of an inch; alternative to the unit mil
in Inch; alternative to the unit "
" Inch; alternative to the unit in
ft Foot; alternative to the unit '
' Foot; alternative to the unit ft
yd Yard
mi Mile

Luminous intensity

Unit Description
cd Candela

Mass

Unit Description
ug Microgram; alternative to the unit µg
µg Microgram; alternative to the unit ug
mg Milligram
g Gram
kg Kilogram
t Tonne
oz Ounce
lb Pound; alternative to the unit lbm
lbm Pound; alternative to the unit lb
st Stone
cwt Hundredweight

Power

Unit Description
W Watt
VA Volt-ampere

Pressure

Unit Description
Pa Pascal
kPa Kilopascal
MPa Megapascal
GPa Gigapascal
uTorr Microtorr; alternative to the unit µTorr
µTorr Microtorr; alternative to the unit uTorr
mTorr Millitorr
Torr Torr; 1 Torr = 133.32 Pa
psi Pound-force per square inch; 1 psi = 6.895 kPa
ksi Kilopound-force per square inch

Temperature

Unit Description
uK Microkelvin; alternative to the unit µK
µK Microkelvin; alternative to the unit uK
mK Millikelvin
K Kelvin

Time

Unit Description
s Second
min Minute
h Hour

Volume

Unit Description
l Liter

Unsupported units

The following commonly used units are not yet supported, for some an alternative is provided:

Unit Description Alternative
°C Celsius [°C] + 273.15 K
°F Fahrenheit; ([°F] + 459.67) × ​5/9
u Atomic mass unit; alternative to the unit Da 1.66053906660e-27 kg
Da Dalton; alternative to the unit u 1.66053906660e-27 kg
sr Steradian not directly
lm Lumen not directly
lx Lux not directly
px Pixel not directly

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Caracteres y nombres no válidos

The expression feature is very powerful but to achieve this power it has some limitations concerning some characters. To overcome this, FreeCAD offers to use labels and reference them instead of the object names. In labels you can use almost all special characters.

In cases where you cannot use a label, such as the name of a sketch's constraints, you must be aware what characters are not allowed.

Etiquetas

For labels there are no invalid characters, however some characters need to be escaped:

Characters Description
', \, " Need to be escaped by adding \ in front of them.

For example, the label Sketch\002 must be referenced as <<Sketch\\002>>.

Nombres

Names of objects like dimensions, sketches, etc. may not have the characters or character sequences listed below, otherwise the name is invalid:

Characters / Character sequences Description
+, -, *, /, ^, _, <, >, (, ), {, }, [, ], ., ,, = Characters that are math operators or part of mathematical constructs
A, kA, mA, MA, J, K, ' , ft , °, and many more! Characters and character sequences that are units (see the Units paragraph)
#, !, ?, §, $, %, &, :, ;, \, |, ~, , ¿, and many more! Characters used as placeholder or to trigger special operations
pi, e Mathematical constants
´, `, ' , " Characters used for accents
space A space defines the end of a name and can therefore not be used

For example, the following name is valid: <<Sketch>>.Constraints.T2üßµ@. While these are invalid names: <<Sketch>>.Constraints.test\result_2 (\r means "carriage return") or <<Sketch>>.Constraints.mol (mol is a unit).

Since shorter names (especially if they have only one or two characters) can easily result in invalid names, consider using longer names and/or establishing a suitable naming convention.

Cell aliases

See Spreadsheet SetAlias.

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Referencia a los datos CAD

It is possible to use data from the model itself in an expression. To reference a property useobject.property. If the property is a compound of fields, the individual fields can be accessed as object.property.field.

The following table shows some examples:

CAD data Call in expression Result
Parametric Length of a Part-Workbench Cube Cube.Length Length with units mm
Volume of the Cube Cube.Shape.Volume Volume in mm³ without units
Type of the Cube-shape Cube.Shape.ShapeType String: Solid
Label of the Cube Cube.Label String: Label
X-coordinate of center of mass of the Cube Cube.Shape.CenterOfMass.x X-coordinate in mm without units
Full Cube object Cube._self Object of type <Part::PartFeature>
Value of constraint in a sketch Constraints.Width Numeric value of the named constraint Width in the sketch, if the expression is used in the sketch itself.
Value of constraint in a sketch MySketch.Constraints.Width Numeric value of the named constraint Width in the sketch, if the expression is used outside of the sketch.
Value of a spreadsheet alias Spreadsheet.Depth Value of the alias Depth in the spreadsheet Spreadsheet
Value of a local property Length Value of the DatosLength property in e.g a Pad object, if the expression is used in e.g DatosLength2 in the same object.

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Variables globales de todo el documento

There is no concept of global variables in FreeCAD at the moment. Instead, arbitrary variables can be defined as cells in a spreadsheet using the Spreadsheet workbench, and then be given a name using the alias property for the cell (right-click on cell). Then they can be accessed from any expression just as any other object property.

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Enlace entre documentos

It is possible (with limitations) to define a Property of an object in your current document (".FCstd" file) by using an Expression to reference a Property of an object contained in a different document (".FCstd" file). For example, a cell in a spreadsheet or the DatosLength of a Part Cube, etc. in one document can be defined by an Expression that references the X Placement value or another Property of an object contained in a different document.

A document's name is used to reference it from other documents. When saving a document the first time, you choose a file name; this is usually different from the initial default "Unnamed1" (or its translated equivalent). To prevent links being lost when the master document is renamed upon saving, it is recommended that you first create the master document, create a spreadsheet inside it, and save it. Subsequently, you can still make changes to the file and its spreadsheet but you should not rename it.

Once the master document with the spreadsheet is created and saved (named), it is safe to create dependent documents. For example, assuming you name the master document master, the spreadsheet modelConstants, and give a cell an alias-name Length, you can then access the value as:

master#modelConstants.Length

Note: that the master document must be loaded for the values in the master to be available to the dependent document.

Unfortunately, the integrated checker sometimes claims that a valid name doesn't exist. Continue typing anyway. When you have completed the full reference, the OK button will become active.

Of course, it's up to you to load the corresponding documents later when you want to change anything.

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Problemas conocidos / tareas pendientes

  • The dependency graph is based on the relationship between document objects, not properties. This means that you cannot provide data to an object and query that same object for results. For example, even though there are no cyclic dependencies when the properties themselves are considered, you may not have an object which gets its dimensions from a spreadsheet and then display the volume of that object in the same spreadsheet. As a work-around use multiple spreadsheets, one to drive your model and the other for reporting.
  • The expression parser does not handle parentheses well, and is unable to properly parse some expressions. For example: = (A1 > A2) ? 1 : 0 results in an error, while = A1 > A2 ? 1 : 0 is accepted. The expression = 5 + ((A1>A2) ? 1 : 0) cannot be entered in any form.
  • As stated above, unfortunately, the integrated checker sometimes claims that a valid name doesn't exist. Continue typing anyway. When you have completed the full reference, the OK button will become active.
  • FreeCAD does not yet have a built-in expression manager where all expressions in a document are listed, and can be created, deleted, queried, etc. But an addon is available: fcxref expression manager.
  • Open bugs/tickets for Expressions can be found in the FreeCAD Bugtracker Expressions category

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