[R] Replacing N.A values in a data frame

[Santosh Srinivas]

Hello,  I have a data frame as below ... in cases where I have N.A. I want
to use an average of the past date and next date .. any help?

13/10/2010	A	23
13/10/2010	B	12
13/10/2010	C	124
14/10/2010	A	43
14/10/2010	B	54
14/10/2010	C	65
15/10/2010	A	43
15/10/2010	B	N.A.
15/10/2010	C	65

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Thanks R-Helpers.