Arjen Markus (11 february 2003) I had a question from a customer about numbers that were displayed with too many digits, like "28.000" instead of "28.0". So, this raised the question: what can we do to make the appearance of (floating-point) numbers more elegant?
As it turns out, the format command has a lot of options, one of them is the # attribute to %g. Now, if all you have is the textual description of the effects, you will be very puzzled indeed.
So, here is a little script that examines the effects:
# Check the effects of the various formats for floating-point numbers
#
foreach value {1.0 -1.0 1.01 -1.01 0.00001 -0.00001 1000000.0 -1000000.0} {
set result ""
foreach form {%g %-g %6g %6.4g %#.4g %06g %-6g} {
append result [format " >$form<" $value]
}
puts "Value $value: $result"
}I have reformatted the output a bit, to get the following table:
Formats: %g %-g %6g %6.4g %#.4g %06g %-6g
Value 1.0: >1< >1< > 1< > 1< >1.000< >000001< >1 <
Value -1.0: >-1< >-1< > -1< > -1< >-1.000< >-00001< >-1 <
Value 1.01: >1.01< >1.01< > 1.01< > 1.01< >1.010< >001.01< >1.01 <
Value -1.01: >-1.01< >-1.01< > -1.01< > -1.01< >-1.010< >-01.01< >-1.01 <
Value 0.00001: >1e-05< >1e-05< > 1e-05< > 1e-05< >1.000e-05< >01e-05< >1e-05 <
Value -0.00001: >-1e-05< >-1e-05< >-1e-05< >-1e-05< >-1.000e-05< >-1e-05< >-1e-05<
Value 1000000.0: >1e+06< >1e+06< > 1e+06< > 1e+06< >1.000e+06< >01e+06< >1e-06 <
Value -1000000.0: >-1e+06< >-1e+06< >-1e+06< >-1e+06< >-1.000e+06< >-1e+06< >-1e+06<
However, there's more! The full format specification is huge and very, very complex. Here it is, bit by bit, aimed at people doing float formatting...
- Start with a % symbol.
- Add any optional positional specifier:
- If you don't want to use the next value off the argument list, add argListIndex and $ here. This is mainly useful for localized formats.
- If you are going to make the space taken up by the field (potentially) larger than required to always hold the value and you want the field left-justified, add a - here.
- If you always want a sign, add a + here.
- If you want a space when the number isn't negative, add a space character here. Don't mix with the +.
- If you want to pad out with zeroes, not spaces, add a 0 here.
- If you want to use the alternate form, add a # here. This guarantees you get a decimal point and, for the g conversion guarantees you a full set of zeroes after the decimal point.
- Add any optional field width:
- If you want to specify a minimum field width (as used in many of the examples above) put the (decimal) width of the field here. You can also take this value as an argument to [format] using a * instead.
- Add any optional precision in either the form .number or .* (the latter if you want to take the value as an argument to [format], as with the field width above):
- If you are using the g conversion, you can say the maximum total number of digits to appear (excluding the exponent)
- If you are using the e or f conversions, you can say the number of digits to appear after the decimal point.
- Floats don't use length modifiers in Tcl. They're only used for integer values.
- Finally, add the conversion specifier which marks the end of the format.
- f uses normal notation.
- e uses scientific notation.
- E is just like e but uses a capital letter for the exponent char.
- g uses either normal or scientific notation, depending on the scale of the number.
- G is just like g but uses a capital letter for the exponent char (if printed).
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