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5.5 Previous Result

You can insert the result of the previous conversion using the underscore (‘_’). It is useful when you want to convert the same input to several different units, for example

     You have: 2.3 tonrefrigeration
     You want: btu/hr
             * 27600
             / 3.6231884e-005
     You have: _
     You want: kW
             * 8.0887615
             / 0.12362832

Suppose you want to do some deep frying that requires an oil depth of 2 inches. You have 1/2 gallon of oil, and want to know the largest-diameter pan that will maintain the required depth. The nonlinear unit ‘circlearea’ gives the radius of the circle (see Other Nonlinear Units, for a more detailed description) in SI units; you want the diameter in inches:

     You have: 1|2 gallon / 2 in
     You want: circlearea
             0.10890173 m
     You have: 2 _
     You want: in
             * 8.5749393
             / 0.1166189

In most cases, surrounding white space is optional, so the previous example could have used ‘2_’. If ‘_’ follows a non-numerical unit symbol, however, the space is required:

     You have: m_
     Parse error

When ‘_’ is followed by a digit, the operation is multiplication rather than exponentiation, so that ‘_2’, is equivalent to ‘_ * 2 rather than ‘_^2’.

You can use the ‘_’ symbol any number of times; for example,

     You have: m
     You want:
             Definition: 1 m
     You have: _ _
     You want:
             Definition: 1 m^2

Using ‘_’ before a conversion has been performed (e.g., immediately after invocation) generates an error:

     You have: _
     No previous result; '_' not set

Accordingly, ‘_’ serves no purpose when units is invoked non-interactively.

If units is invoked with the --verbose option (see Invoking Units), the value of ‘_’ is not expanded:

     You have: mile
     You want: ft
             mile = 5280 ft
             mile = (1 / 0.00018939394) ft
     You have: _
     You want: m
             _ = 1609.344 m
             _ = (1 / 0.00062137119) m

You can give ‘_’ at the ‘You want: prompt, but it usually is not very useful.