search for: monsd

Hello, I am a new user of R. I am coming from SAS and do statistics on stock market data, economic data, and social data. My question is this: How can you get the mean, standard dev, etc. of a variable based on a conditional statement on either the same variable or a different variable in the same data set? So if I had the closing prices of the S&P from 01/01/1990-12/31/1990, how could I get