Linking variables

Linking variables together allows you to:

  • Chain formulas together so that the result of solving one formula is fed as input to some other formula
  • Avoid re-typing quantities that are used as input values in multiple places

The values of all variables in a link always match - if any value is changed, the other variables are automatically updated.

Restrictions on linking variables

Any number of variables can be in a link as long as these restrictions are adhered to:

  1. All variables in the link must have compatible units
  • For example, you can link a variable with ‘inches’ units to one with ‘centimeters’, but not to ‘kilograms’
  1. Links cannot contain input and output variables that would cause an endless loop when solving a formula:

For example,

  • Formulas may not be cross-linked so that their solved (output) variables are linked to each other’s input variables
    • This would result in an endless loop if the 1st formula were solved. The 2nd formula’s linked input variables would also be updated, causing the 2nd formula to be solved. This would in turn update the 1st formula’s input variables, causing it to be solved again, and so on.
  • A formula’s output variable may not be linked to any of its input variables.
  • Only one output variable may be in a link.

Note

The Solver will prevent you from making any links that would violate these constraints by disabling the corresponding ‘link’ button.

Note on formulas with ‘free’ units

Certain formulas have what is known as free units.

For instance, a simple multiplication formula such as a = bc has no units intrinsically assigned to its variables.

There are a few things to be aware of when linking such variables:

  1. While the free-unit variable is in a link, its units can only be re-assigned so that they are compatible with the other variables in the link
  2. Once removed from the link, the variable re-gains complete free-units status, i.e. it can have any units assigned to it