[R] dataframe: how to select an element from a row

[ikuzar]

Thank you Jorge and Florent for your responses.

Now, I 'd like to get the date *(and its index) *where myvalue < 2000 for
the first time. 
I expect for a result like (index, date) = (3, 2012-01-07 )

This way does not work:
ind = match(df$myvalue <2000, df$myvalue)
res = df$DateTime[ind]

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