
I have a table that looks like this:

structure(list(speed = c(3,9,4,8,7,6), C = c(0.697, 0.011, 0.015, 0.012, 0.018, 0.019), house = c(1, 
1, 1, 1, 1, 1), date = c(719, 1027, 1027, 1027, 1030, 1030), 
    hour = c(18, 8, 8, 8, 11, 11), id = c("1000", "10000", 
    "10001", "10002", "10003", "10004")), .Names = c("speed", 
"C", "house", "date", "hour", "id"), class = "data.frame", row.names = c("1000", 
"10000", "10001", "10002", "10003", "10004"))

I want to determine the minimum speed for each date, and the C that corresponds to that lowest speed.Then I want to make a table that contains all speeds and the difference between C and the lowest C.
 
For example, on the date 1027, the minimum speed is 4 and the C that corresponds to that is 0.015.  The new table should contain:
speed 8 and C -0.003
speed 9 and C -0.004
speed 7 and C -0.001How do you do this?

Thanks,
Jeffrey
 		 	   		  
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