Mark,

This is actually very hard -- there is no simple way to do it. I posted a similar question here in my post but got no replies and even had tech support look at it and had little resolution. I came up with an approach that works by wrapping the numbers in a temporary function and after turning everything into CForm, doing a string replace. Here is some sample code. mapReplace goes through an expression and replaces the Real numbers with a dummy function acf[ paddedform ]. You can edit the paddedForm below to get your desired format.

mapReplace[exp_] := 
 MapAt[acf[
    ToString[
     PaddedForm[#, {8, 3}, 
      NumberFormat -> (Row[{#1, If[#3 != "", "e", ""], #3}] &)]]] &, 
  exp, Position[exp, _Real]]

after this you do a stringReplace to clean it all up:

CNumberReformat[expr_] := 
 StringReplace[ ToString[mapReplace[expr], CForm], 
  Shortest["acf(\"" ~~ x__ ~~ "\")"] :> "(" <> x <> ")"]

To use it:

equation = 25.32032 - 4.55 x + 16.0006 z + 2 y + 234.2341*^8
CNumberReformat[equation]

to get

2y + z( 16.001) + ( 2.342e10) + x*( -4.550)

I hope this helps.

Thank you for your suggestion. What I was really hoping for was something like the C/Python "%e" format, that always produces a string of the same length, so it could be used, for example, to print data in nicely aligned columns:

 1.000e12
-1.000e9 
 1.000e3 
-1.000e1 
 1.000e0 
-1.000e-1 

If I understand it correctly, your proposal for an individual number would be something like

CFormat[x_,n_,f_]:= ToString[
 PaddedForm[x, {n, f}, 
  NumberFormat -> (Row[{#1, If[#3 != "", "e", ""], #3}] &) 
 ]
]

but that doesn't create strings of a fixed length, e,g,

"<" <> CFormat[1.0*^6,9,3] <> ">" 
<      1.000e6>
"<" <> CFormat[10,9,3] <> ">"
<     10.000>

I still don't understand how PaddedForm works. In the example above, the "9" that gets passed to PaddedForm doesn't cause it to produce a string of length 9: the actual output is much longer.