Dividing tick-data into 5 min intervals

Hello,

Im currently writing my bachelor thesis in statistical finance and i have run into a small problem. I want to evaluate forcasts from my GARCH with realized intraday volatility. The intraday data is Tick-data over a certain period. The date column is presented as for example 2011-11-01 09:24:41 for different points in time. The other column is with the stock prices at that same time. What I want to do is to recieve the end courses of certain time intervals. For example i want to know what the closing course if for every five minute or ten minute interval in the sample. In other words, i want to transform the tick-data into k-minute-interval data.

I have been trying to this in the following way:

The data has been converted to a time serie and look likes:

price
2011-11-01 08:00:00 0.000000000
2011-11-01 08:00:00 0.000000000
2011-11-01 08:02:00 0.000000000
2011-11-01 08:03:00 -0.017033339
2011-11-01 08:24:00 0.000000000
2011-11-01 08:24:00 0.000000000
2011-11-01 08:29:00 0.000000000
2011-11-01 08:29:00 0.000000000
2011-11-01 08:29:00 0.000000000
2011-11-01 08:29:00 0.000000000
2011-11-01 08:29:00 0.002166062
2011-11-01 08:44:00 0.000000000
2011-11-01 08:44:00 -0.002166062
2011-11-01 08:44:00 0.004321374
2011-11-01 10:36:00 0.010618976
2011-11-01 15:59:00 0.002092990
2011-11-01 16:21:00 0.000000000
2011-11-01 16:30:00 0.004155960
2011-11-02 08:00:00 0.000000000
2011-11-02 11:50:00 0.000000000
2011-11-02 13:38:00 -0.002073009

and so on for 108 days (this stock is a small cap company, and therefore the infrequent trading)…