CODEWITHSUNDEEP
rfor-loopvector
Brad
Brad

Reputation: 29

Adding values to an R vector based on a for loop

I have a data frame that contains wifi download bandwidth and GPS data (latitude and longitude) on a transportation system. I want to determine from the data what the average bandwidth is when the vehicle is moving north, and what it is when it is moving south.

(bandwidth and latitude values from df)

bandwidth <- df$bandwidth

latitude <-df$latitude

(These both have 2800 entries)

(create empty vectors to fill with bandwidth values depending on whether the vehicle is moving north or south)

movingnorth <- vector('numeric')

movingsouth <- vector('numeric')

(If the train is moving north, fill the moving north vector with data from bandwidth vector)

for(y in latitude){

  if(latitude[y]>= latitude[y+1]){
  movingnorth <- c(movingnorth, received[y])}
  }

Here, I am basically saying if the latitude value is going up, then the vehicle is moving north, and therefore enter the bandwidth value from that location into the movingnorth vector. I would expect only a portion of the values from bandwidth vector to be added to the movingnorth vector, but all 2800 values are added. What am I doing wrong here?

Upvotes: 0

Views: 254

Answers (2)

Luis
Luis

Reputation: 511

As lmo said, you want to use seq_along(latitude) or 1:length(latitude), which return the index instead of the actual element in latitude.

Also, you may want to double check that latitude[y+1] is correct. The current syntax assumes that the order of the latitude values in the data goes from the latest to the oldest. It is not possible to know if this is correct from the information you provide, but it may be the reverse.

As pointed out by Frank, you are growing your vector in a loop and that is bad practice (since it does not scale well and becomes very slow for large objects). Nathan Werth's answer suggests a vectorized implementation.

Upvotes: 0

Nathan Werth
Nathan Werth

Reputation: 5263

Take advantage of R's vectorized operations. First we use diff to find the change between successive elements of latitude

latitude_change <- diff(df$latitude)

Now we have a vector whose length is 1 less than the length of latitude. Direction happens between the measurements, so that makes sense. Let's say we won't determine direction for the first measurement. So that means if latitude_change[i] > 0, then the train's northbound at time i - 1.

df$movingnorth <- c(FALSE, latitude_change > 0)

I'm keeping this part of df because it's related information, so a table's the perfect place for it.

Upvotes: 1

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