A way to convert any arbitrary number to engineering notation [L1 ]
# eng - return engineering notation of any given number in a 2 element list as {num eng_unit}
# pico nano micro milli 1 kilo Mega Giga Tera
# use: eng <num> <unit> eg. eng 123.456E6 Hz
# First - scan to check if n is a number, if not then just return with input given {n u}
proc eng {n u} {
if ![scan $n %g res] {return [list $n {} $u]}
if [expr $n>=1E-12 && $n<1E-9] {list [expr $n*1E12] p$u} \
elseif [expr $n>=1E-9 && $n<1E-6] {list [expr $n*1E9] n$u} \
elseif [expr $n>=1E-6 && $n<1E-3] {list [expr $n*1E6] u$u} \
elseif [expr $n>=1E-3 && $n<1] {list [expr $n*1E3] m$u} \
elseif [expr $n>=1 && $n<1E3] {list [format %g $n] $u} \
elseif [expr $n>=1E3 && $n<1E6] {list [expr $n/1E3] k$u} \
elseif [expr $n>=1E6 && $n<1E9] {list [expr $n/1E6] M$u} \
elseif [expr $n>=1E9 && $n<1E12] {list [expr $n/1E9] G$u} \
elseif [expr $n>=1E12 && $n<1E15] {list [expr $n/1E12] T$u} \
else {list [format %g $n] $u}
}
### Example Output
%eng 1.23456E-2 V
12.3456 mV
%eng 209357.209857E5 Hz
20.9357209857 GHz
%eng 0.012e-7 H
1.2 nHCvK (Dec 2006) - is there any way this could be simplified ?
Bryan Oakley notes: that's a lot of unbraced expressions! MJ - even more so if you consider that if and elseif pass the first following argument to expr as well. Always brace your expr-essions.
For that matter, "if [expr ..." is a bit redundant.
AMG: The rampant indenting was hurting my eyes, so I removed it. This version should be a bit easier to read.
MJ - Try the following (with the additional benefit that it is easier to add more powers of 1000):
proc eng {num u} {
array set orders {
-8 y -7 z -6 a -5 f -4 p -3 n -2 u -1 m 0 {} 1 k 2 M 3 G 4 T 5 P 6 E 7 Z 8 Y
}
set numInfo [split [format %e $num] e]
set order [expr {[scan [lindex $numInfo 1] %d] / 3}]
if {[catch {set orders($order)} prefix]} {return [list $num $u]}
set num [expr {$num/pow(10,3*$order)}]
return [list $num $prefix$u]
}CvK - MJ, Thanks for your solution. It looks better and perhaps more readable. I agree that the nested "if [expr .." is redundant. Syntax highlighting helped with the aesthetics of indentation which is lost with the pre-formatting in this wiki. I tried your example but it gave a syntax error ...
syntax error in expression "$num/(10.**(3*$order))": unexpected operator *
MJ - That's what I get for testing under 8.5 only. In 8.5 ** is a new operator. Fixed above. BTW not only is "if [expr .." redundant, it is also slower and invokes triple substitution which can lead to very unexpected or even unsecure behaviour. Example:
% set a 1
% set b {$a}
% set c {$b}
% if [expr $c] {puts "3 times is a charm :-)"} {puts "... or not"}
3 times a charm :-)
% set a 0
% if [expr $c] {puts "3 times is a charm :-)"} {puts "... or not"}
... or notCvK - The speed difference as measured by [clock clicks] is about 5 fold. I measured an average of about 430 [clock clicks] for the "if..elseif" version, while MJ's "pow()..array-lookup" version measured about 80 [clock clicks], approximately 5 times faster. Not sure what units of time [clock clicks] returns, but the measure is a relative difference anyway.
NEM - clock clicks returns a system-dependent value. You can use the time command for benchmarking.
() 3 % time {eng 209357.209857E5 Hz } 1000 ; # ifelse version
222.984 microseconds per iteration
() 5 % time {eng 209357.209857E5 Hz } 1000 ; # array lookup version
88.785 microseconds per iterationBTW - what improvements does Tcl8.5 offer? See Changes in Tcl/Tk 8.5