Select rows from Data Frame with conditions

Something along the lines of

top100 <- A[match(B,A[,1]),]

Please provide R code with sample data and desired output. See http://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example
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Hi all,

I have two dataframes. The first (A) contains all the stock prices for
today including today. So the first column is the stock Symbol and the
second column is the stock price. The second (B) is the symbol list in
the
top 100 stocks.

I want to pick out from dataframe A only the rows containing the
symbols
from B. i.e. something like:

           prices <- A[A[,1]==B,2]

is there any way to do this without using a for loop, I have to do this
365
times (i.e. for one year).

Thanks,
Sachin

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