use a variable in format

Hi

how can I use a number in a format, such as

n = 3;
m = 123;

and something like printf("%.nf", m);

---------
result should be 123.000

I tried with printf("%.%df\n",n,m); but never worked,any idea?
Thanks

http://faq.cprogramming.com/cgi-bin/...&id=1043284385

thanks, I got it.