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.help tunits Jan99 ttools
.ih
NAME
tunits -- Convert a table column from one set of units to another
.ih
USAGE
tunits table column newunits
.ih
DESCRIPTION
Tunits converts a column in a table from one set of units to
another. It supports both scalar and array columns. You supply the
table and column name and the new units you want the column to be
in. Optionally, you can apply the current column units if the units
string stored in the table is missing or incorrect. Tunits only
supports multiplicative conversions, conversions that involve additive
changes, like Kelvin to Celsius, are not supported. Unit names and
conversions are read from two tables. You can copy these tables and
edit them if you want to add new conversions.
You can find out the current units for a table column by running
tlcol. If a units conversion is not supported by this task and you
know the conversion formula, you can run tcalc.
Tunits must parse the unit strings you pass it, which requires that
they follow a certain set of rules. Units can be a simple units name,
such as ergs, or a units name raised to a power, such as meters^2, or
the product or quotient of units names raised to a power, such as
feet/sec or gm*cm/s^2. If two units names are next to each other
without any operator between them, a multiplication is assumed. If a
units name is followed by a number, the units name is assumed to be
raised to that power.
Powers can also be specified by preceding the unit name with the
string "square", "sq", "cubic", or "cu". For example, "square meter"
is equivalent to "meter^2".
The operators recognized are:
.nf
division / or per
multiplication *
exponentiation ^ or **
.fi
Division has the lowest priority and exponentiation the highest. This
means ergs/sec*angstrom is interpreted as ergs/(sec*angstrom) and not
(ergs/sec)*angstrom, as it would be in most computer languages. The
default priority can be changed by changed by grouping terms with
parentheses.
Each set of units can have several synonymous names. These names are
listed in the abbreviations table. Case is not significant in names
and regular plurals (made by adding an "s") are converted to the
singular. Names should contain only alphabetic characters. Blanks,
underscores and digits are not allowed.
The conversions supported by this task are read from the units
conversion table. The table lists the old and new units and a
conversion factor. Only one conversion is allowed for each set of
units.
.ih
PARAMETERS
.ls table [file]
The name of the table the conversion is applied to.
.le
.ls column [string]
The column to be converted. Both scalar and array columns are
supported.
.le
.ls newunits [string]
The new set of units for the column. The format of this parameter is
described above. This task writes the new units to the units field in
the table column.
.le
.ls (oldunits = " ") [string]
The units that the table column is currently in. If the value of this
parameter is blank, the units will be read from the table.
.le
.ls (abrevtab = "ttools$tunits/abrev.tab") [file]
A table of alternate names for each unit. This table contains two
columns. The first column is the name of the units and the second
column is the standard abbreviation. Because the default table is an
ascii file, columns are read positionally and not by column names
Many units have more than one name or abbreviation. Using a standard
abbreviation allows units to be converted to a standard form, which
simplifies calculations. The standard abbreviation is used internally
when computing the conversion factor. Case is not significant in names
and regular plurals (made by adding an "s") are converted to the
singular before looking them up in the table. Names should contain
only alphabetic characters. Blanks, underscores and digits are not
allowed.
The name of this table is a parameter to allow you to create your own
table of standard abbreviations, with additional units.
.le
.ls (unittab = "ttools$tunits/units.tab") [file]
A table of conversion factors from one set of units into another.
This table contains four columns. The first is the conversion factor,
a double precision number. The second is the units the task tries to
convert from. The third column is the units the task tries to convert
to. The fourth column is contains the boolean variable swap, explained
a little later.
The table is interpreted as "There are <factor> <from> in a <to>."
For example, "There are 100 centimeters in a meter." The last column,
swap, does not change the sense of the sentence but does change the
direction that the conversion is applied, For example, "60 seconds in
a minute" is actually a conversion from minutes to seconds because
swap is yes. Unit conversions should set swap to yes when the desired
conversion is not an exact value, but its inverse is. Only one
conversion is allowed per unit, which simplifies the program logic
considerably. Conversions should be chosen so that they ultimately
resolve to MKS units. To prevent endless loops conversions from the
fundamental units of MKS are checked for and forbidden. However, the
program does not check for other loops, so be careful when adding new
conversions!
As in the case of the abbreviation table, the table name is a
parameter to allow you to create your own table with additional unit
conversions.
.le
.ls (verbose = no) [bool]
If you set this parameter to yes, the task will print a message to
STDERR for each units conversion utilized in computing the conversion
factor.
.le
.ih
EXAMPLES
Convert watts to ergs per second. Print the diagnostic messages:
.nf
tt> tunits source.tab power "ergs/sec" old=watts verb+
.fi
.ih
REFERENCES
This task was written by Bernie Simon
.ih
SEE ALSO
tlcol, tcalc
.endhelp
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