tcl formatting floating point with fixed precision - tcl

Question

# the unit of period is picosecond
set period 625000.0
set period_sec [format %3.6g [expr $period * 1e-12]]
puts $period_sec
result: 6.25e-07
Is there a way to force tcl to get results like 625e-09

Answer 1

Assuming that you want to format it to the nearest exponent, you could use a proc which formats it like this:
proc fix_sci {n} {
# Not a sci-fmt number with negative exponent
if {![string match "*e-*" $n]} {return $n}
# The set of exponents
set a 9
set b 12
# Grab the number (I called it 'front') and the exponent (called 'exp')
regexp -- {(-?[0-9.]+)e-0*([0-9]+)} $n - front exp
# Check which set of exponent is closer to the exponent of the number
if {[expr {abs($exp-$a)}] < [expr {abs($exp-$b)}]} {
# If it's the first, get the difference and adjust 'front'
set dif [expr {$exp-$a}]
set front [expr {$front/(10.0**$dif)}]
set exp $a
} else {
# If it's the first, get the difference and adjust 'front'
set dif [expr {$exp-$b}]
set front [expr {$front/(10.0**$dif)}]
set exp $b
}
# Return the formatted numbers, front in max 3 digits and exponent in 2 digits
return [format %3ge-%.2d $front $exp]
}
Note that your original code returns 6.25e-007 (3 digits in the exponent).
If you need to change the rule or rounding the exponent, you will have to change the if part (i.e. [expr {abs($exp-$a)}] < [expr {abs($exp-$b)}]). For example $exp >= $a could be used to format if the exponent is 9 or below.
ideone demo of above code for 'closest' exponent.
For Tcl versions before 8.5, use pow(10.0,$dif) instead of 10.0**$dif

Answer 2

I do not think there is anything in the format command that will help you directly. However, if you consider a slight variation on the format code, then it may be a lot easier to get what you want (with a bit of string manipulation):
format %#3.6g $number
gives a number like: 6.25000e-007
This can be parsed more easily:
Extract the exponent
Determine the number of positions to shift the decimal point
Shift it and replace the exponent
It is not entirely straightforward, I am afraid, but it should be doable. Wiki page http://wiki.tcl.tk/5000 may give you some inspiration.