Previous Result (Units: A Unit Conversion Program and Scientific Calculator)

### 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

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.