[R] Comparing dates in dataframes

[Stephan Kolassa]

Hi,

it looks like when you read in your data.frames, you didn't tell R to 
expect dates, so it treats the Date columns as factors. Judicious use of 
  something along these lines before doing your comparisons may help:

arr$Date <- as.Date(as.character(arr$Date),format=something)

Then again, it may be possible to do the actual merging using merge().

HTH
Stephan


James Rome schrieb:
> I have two data frames. One (arr) has all arrivals to an airport for a
> year, and the other (gw) has the dates and quarter hour of the day when
> the weather is good. arr has a Date and quarter hour column.
> 
>> names(arr)
>  [1] "Date"     "weekday"      "hour"         "month"        "minute"     
>  [6] "quarter"      "ICAO"         "Flight"       "AircraftType"
> "Tail"       
> [11] "Arrived"      "STA"          "Runway"       "FromTo"      
> "Delay"      
> [16] "Operator"     "gw" 
> 
> I added the gw column to arr and initialized it to all FALSE
> 
>> names(gw)
>  [1] "Date"           "minute"         "hour"           "quarter"      
>  [5] "Efficiency.Val" "Weekly.Avg"     "Arrival.Val"    "Weekly.Avg.1" 
>  [9] "Departure.Val"  "Weekly.Avg.2"   "Num.of.Hold"    "Runway"       
> [13] "Weather" 
> 
> First point of confusion:
>> gw[1,1]
> [1] 1/1/09
> 353 Levels: 1/1/09 1/1/10 1/10/09 1/10/10 1/11/09 1/11/10 1/12/09 ... 9/9/09
> Why do I get 353 levels?
> 
> I am trying to identify the quarter hours with good weather in the arr
> data frame. What I want to do is to go through the rows in gw, and to
> set arr$gw to TRUE if arr$Date and arr$quarter match those in the gw row.
> 
> So I tried
> gooddates = function(all, good) {
>    la = length(all)   # All the flights
>   lw = length(good)  # The good 15-minute periods
>   for(j in 1:lw) {
>     d=good$Date[j]
>     q=good$quarter[j]
>     all[all$DateTime==d && all$quarter==q,17]=TRUE
>   }
> }
> 
> but when I run this, I get
> "Error in Ops.factor(all$DateTime, d) :
>   level sets of factors are different"
> 
> I know the level sets are different, that is what I am trying to find.
> But I think I am comparing single elements from the data frames.
> 
> So what am I doing wrong? And there ought to be a better way to do this.
> 
> Thanks in advance,
> Jim Rome
> 
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