[R] Extracting values from rows which meet a condition in R 3.0.2

Jim Lemon jim at bitwrit.com.au
Tue Apr 15 01:53:35 CEST 2014

On 04/15/2014 06:24 AM, umair durrani wrote:
> Hi, I have a big data frame with millions of rows and more than 20 columns. Let me first describe what the data is to make question more clear. The original data frame consists of locations, velocities and accelerations of 2169 vehicles during a 15 minute period. Each vehicle has a unique Vehicle.ID, an ID of the time frame in which it was observed i.e. Frame.ID, the velocity of vehicle in that frame i.e. svel, the acceleration of vehicle in that frame i.e. sacc and the class of that vehicle, vehicle.class, i.e. 1= motorcycle, 2= car, 3 = truck. These variables were recorded after every 0.1 seconds i.e. each frame is 0.1 seconds. Here are the first 6 rows:
>> dput(head(df))structure(list(Vehicle.ID = c(2L, 2L, 2L, 2L, 2L, 2L), Frame.ID = 133:138,Vehicle.class = c(2L, 2L, 2L, 2L, 2L, 2L), Lane = c(2L, 2L, 2L, 2L, 2L, 2L), svel = c(37.29, 37.11, 36.96, 36.83, 36.73,36.64), sacc = c(0.07, 0.11, 0.15, 0.19, 0.22, 0.25)), .Names = c("Vehicle.ID", "Frame.ID", "Vehicle.class", "Lane", "svel", "sacc"), row.names = 7750:7755, class = "data.frame")
> There are some instances in vehicles' journey during the 15 minute recording period that they completely stop i.e. svel==0. This continues for some frames and then vehicles gain speed again. For the purpose of reproduciblity I am creating an example data set as follows:
> x<- data.frame(Vehicle.ID = c(rep(10,5), rep(20,5), rep(30,5), rep(40,5), rep(50,5)),vehicle.class = c(rep(2,10), rep(3,10),rep(1,5)), svel = rep(c(1,0,0,0,3),5),           sacc = rep(c(0.3,0.001,0.001,0.002,0.5),5))
> As described above some vehicles stop and have zero velocity for some time but later accelerate to get up to speed. I want to find the acceleration, sacc they apply after having zero velocity for some time (moving from standstill position). This means that I should be able to look at the FIRST row AFTER the last frame in which svel==0. In the example data this means that the car (vehicle.class==2) having a Vehicle.ID==10 had a velocity, svel equal to 1 as seen in the first row. Later, it stopped for 3 frames (3 consecutive rows) and then accelerated to velocity, svel, equal to 3. I want the acceleration sacc it applied in those 2 frames (rows 4 and 5 for vehicle 10, which come out to be 0.002 and 0.500). This means that for example data, following should be the output by vehicle.class:
> output<- data.frame(Vehicle.ID = c(10,10,20,20,30,30,40,40,50, 50),vehicle.class = c(2,2,2,2,3,3,3,3,1,1), xf = rep(c('l','f'),10),sacc = rep(c(0.002,0.500),5))
> xf identifies the last row l in which svel==0 and f is the first one after that. I have tried using plyr and for loop to split by vehicle.class but am not sure how to extract the sacc. Please note that xf should be a part of output. It is not in given data. The original data frame df has 2169 vehicles, some stopped and some did not so not all vehicles had svel==0. The vehicles which did stop didn't do it at the same time. Also, the number of rows in which svel==0 is different vehicle to vehicle.
> Thanks,
> Umair Durrani
> Master's candidate
> Civil and Environmental Engineering
> University of Windsor 		 	   		
> 	[[alternative HTML version deleted]]
Hi Umair,
This may be a bit slow, but I think it will do what you want:

initacc<-function(x) {
  for(drow in 2:dim(x)[1]) {
   if(x[drow-1,"svel"] == 0 && x[drow,"svel"] > 0) {
    if(!is.na(xout[1,1])) {
    else {


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