[R] Update data in text file with data in dataframe

[Nic]

Hello all,

I'm a novice R programmer and I don't have a clue as to how to approach 
this. I have tried many different approaches only to fail and I can't 
seem to wrap my head around it.  So I turn to the experts.

I have a text file with patient data that has incorrect dates of 
service.  I need to update the dates of service in the text file with 
data that's in a dataframe.

Data in the text file looks like this (call it PatRecs):

PatientID #:  12345
Date of Service:  8/1/2017
.

.

.

PatientID #:  12346
Date of Service: 8/2/2017

.

.

.

The dots are other data.  I'm just focused on the PatientID and DOS.

The data in the data frame (call it DF) looks like

Id2    DOS

12345, 8/3/2017
12346, 8/4/2017

etc.

The lists for both are longer than this.

I know that in programming for this that there should be readLines and 
writeLines and greps - but I'm far too new to using R to figure out the 
usage of these commands.

How can I update the text file (PatRecs) with the incorrect Date of 
Service, with the correct DOS in the dataframe (DF)???

This is not a homework assignment, I promise.  This is just my attempt 
to automate data quality using R, and so that I can learn something 
new.  In any event, providing you with anything I've done so far would 
be meaningless.  In an effort to minimize email space, please email me 
directly.  If there is any interest I'll compile the responses and 
forward them.  I humbly thank you all for any assistance with this.

Best,

Nic