[O] was this intentional with the default header ":results value"

[Eric Luo]

Hi, 
Why in the following code block, c1 was printed as scientific notation
rather then characters.
,----
| 
| #+BEGIN_SRC R :results value
|   df <- data.frame(c1="123456789123456789",c2=2)
| #+END_SRC
| 
| #+RESULTS:
| | 1.2345678912345678e+17 | 2 |
`----
But if the header :results output, the result was expected.
,----
| 
| #+BEGIN_SRC R :results output 
|   df <- data.frame(c1="123456789123456789",c2=2)
|   df
| #+END_SRC
| 
| #+RESULTS:
| :                   c1 c2
| : 1 123456789123456789  2
`----

Thanks