src/swipl-devel/packages/xpce/man/refmanual/md/eq.md

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class eq {#class-eq}

Binary_condition object that succeeds if <-left and <-right evaluate to the same numeric value. See class binary_condition for details.

Besides acting as a simple test, it can also solve equations with a single variable. See <-var.

@see topic Conditions

Send methods {#class-eq-send}

• eq->_execute Evaluate both expressions to a numeric value and succeed if they evaluate to the same value.

Get methods {#class-eq-get}

• eq<-var Regards the = object as an equation to be solved. It can determine the value of a variable occuring only once in the equation if all other variables are given. This facility is used by class spatial to express spatial relations between graphical objects. Below are some examples:

  ?- get(4+x=7, var, x, X)        ==> X = 3
  ?- get(5*x=y, var, y, x=3, Y)   ==> Y = 15
  

  Note that x, y, w, h and some more are the names of predefined var objects and thus x is translated to this var object by PCE's type-checking system. The binding of the var objects in an = object are not affected by this method.

  See also class var and class spatial.

  @see class spatial

rswipl documentation built on June 16, 2026, 5:07 p.m.