How do I duplicate in knitr the output from the R console?

Question

If I write in the R console

> 1.000000000000000123456 - 1

I get:

[1] 2.220446e-16

If I type this in an .Rnw file, knitr gives the following output in the PDF:

1 - 1

with result

0

How do I get knitr to reproduce the console in a particular block of code (preferably with the output not in scientific notation)?