I have weather data that was recorded every hour, and location data  (X,Y) that 
was recorded every 4 hours.  I want to know what the  temperature was at 
location X,Y.  The  weather data isn't exactly at the  same time.  So, I have 
written this loop for every location to scan  through the weather data looking 
for the "closest" in Date/TIME and  extracting the data from that time.  The 
problem is the way Ive written  it, for location #2, it scans through the 
weather data but will not  allow the closest time information to be assigned 
that was assigned for  location#1.  Say location #1 & 2 are taken within 10 
minutes at 6pm  and 6:10pm, the closest weather time is 6pm.  I can't get it to 
allow  the weather data at 6pm as an option for location #2.  I kind of set it 
up like this  because 200 locations into my location data set (say 3 months into 
it), I  do not want it starting at time 1 from the weather data, when I know  
that the closest weather data was just calculated for the last location  and 
that happens to be 3 months into that data set too.  Below is some  sample data 
and my code.  I don't know if this makes sense.  I am also concerned about the 
search if
the location dates extend past the weather data 

Location data

X   Y   DateTime 
1   2   4/2/2003    18:01:01
3   2   4/4/2003    17:01:33
2   3   4/6/2003    16:03:07
5   6   4/8/2003    15:03:08
3   7   4/10/2003   14:03:06
4   5   4/2/2003    13:02:00
4   5   4/4/2003    12:14:43
4   3   4/6/2003    11:00:56
3   5   4/8/2003    10:02:06
2   4   4/10/2003   9:02:19

Weather Data
DateTime        WndSp   WndDir  Hgt
4/2/2003 17:41:00   8.17    102.86  3462.43
4/2/2003 20:00:00   6.70    106.00  17661.00
4/2/2003 10:41:00   6.18    106.00  22000.00
4/2/2003 11:41:00   5.78    106.00  22000.00
4/2/2003 12:41:00   5.48    104.00  22000.00
4/4/2003 17:53:00   7.96    104.29  6541.00
4/4/2003 20:53:00   6.60    106.00  22000.00
4/4/2003 19:41:00   7.82    105.00  7555.00
4/4/2003 7:41:00    6.62    105.00  14767.50
4/4/2003 8:41:00    6.70    106.00  17661.00
4/4/2003 9:41:00    6.60    106.00  22000.00
4/5/2003 20:41:00   7.38    106.67  11156.67
4/6/2003 18:07:00   7.82    105.00  7555.00
4/6/2003 21:53:00   6.18    106.00  22000.00
4/6/2003 21:41:00   6.62    105.00  14767.50
4/6/2003 4:41:00    7.96    104.29  6541.00
4/6/2003 5:41:00    7.82    105.00  7555.00
4/6/2003 6:41:00    7.38    106.67  11156.67
4/8/2003 18:53:00   7.38    106.67  11156.67
4/8/2003 22:53:00   5.78    106.00  22000.00
4/8/2003 1:41:00    5.78    106.00  22000.00
4/8/2003 2:41:00    5.48    104.00  22000.00
4/8/2003 3:41:00    8.17    102.86  3462.43
4/10/2003 19:53:00  6.62    105.00  14767.50
4/10/2003 23:53:00  5.48    104.00  22000.00
4/10/2003 22:41:00  6.70    106.00  17661.00
4/10/2003 23:41:00  6.60    106.00  22000.00
4/10/2003 0:41:00   6.18    106.00  22000.00
4/11/2003 17:41:00  8.17    102.86  3462.43
4/12/2003 18:41:00  7.96    104.29  6541.0

#####code:

SortLoc = loc[order(loc$DateTime),]   #sorting the location data
SortWeath = weather[order(weather$DateTime),] #sorting the weather data

####

weathrow =1

for (i in 1:nrow(SortLoc)) { 

    t = 0
    while (t < 1){
        timedif1 = difftime(SortLoc$DateTime[i], SortWeath$DateTime[weathrow], 
units="auto")
        timedif2 =  difftime(SortLoc$DateTime[i], 
SortWeath$DateTime[weathrow+1], units="auto") 

            if (timedif2<0){
                if (abs(timedif1)

            } else {
            SortLoc$WndSp[i]=SortWeath$WndSp[weathrow+1]
            SortLoc$WndDir[i]=SortWeath$WndDir[weathrow+1]
            SortLoc$Hgt[i]=SortWeath$Hgt[weathrow+1]

            }
        t = 1

        }
###THIS IS MY ATTEMPT TO GET IT TO USE THE PREVIOUS WEATHER INFORMATION OR TO GO 
ONTO THE NEXT DATA POINT
   #if (abs(SortLoc$DateTime[i] - SortLoc$DateTime[i+1] < 50)){
    #    weathrow=weathrow
    #} else {
    #weathrow = weathrow+1
        #if(weathrow = nrow(SortWeath)){t=1} HOW TO GET IT TO STOP????
    }
} #end while

}
}

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