Split a string column into several dummy variables

Question

As a relatively inexperienced user of the data.table package in R, I've been trying to process one text column into a large number of indicator columns (dummy variables), with a 1 in each column indicating that a particular sub-string was found within the string column. For example, I want to process this:

ID     String  
1       a$b  
2       b$c  
3       c  

into this:

ID     String     a     b     c  
1       a$b       1     1     0  
2       b$c       0     1     1  
3        c        0     0     1  

I have figured out how to do the processing, but it takes longer to run than I would like, and I suspect that my code is inefficient. A reproduceable version of my code with dummy data is below. Note that in the real data, there are over 2000 substrings to search for, each substring is roughly 30 characters long, and there may be up to a few million rows. If need be, I can parallelize and throw lots of resources at the problem, but I want to optimize the code as much as possible. I have tried running Rprof, which suggested no obvious (to me) improvements.

set.seed(10)  
elements_list <- c(outer(letters, letters, FUN = paste, sep = ""))  
random_string <- function(min_length, max_length, separator) {  
    selection <- paste(sample(elements_list, ceiling(runif(1, min_length, max_length))), collapse = separator)  
    return(selection)  
}  
dt <- data.table(id = c(1:1000), messy_string = "")  
dt[ , messy_string := random_string(2, 5, "$"), by = id]  
create_indicators <- function(search_list, searched_string) {  
    y <- rep(0, length(search_list))  
    for(j in 1:length(search_list)) {  
        x <- regexpr(search_list[j], searched_string)  
        x <- x[1]  
        y[j] <- ifelse(x > 0, 1, 0)  
    }  
    return(y)  
}  
timer <- proc.time()  
indicators <- matrix(0, nrow = nrow(dt), ncol = length(elements_list))  
for(n in 1:nrow(dt)) {  
    indicators[n, ] <- dt[n, create_indicators(elements_list, messy_string)]  
}  
indicators <- data.table(indicators)  
setnames(indicators, elements_list)  
dt <- cbind(dt, indicators)  
proc.time() - timer  

user  system elapsed 
13.17    0.08   13.29 

EDIT

Thanks for the great responses--all much superior to my method. The results of some speed tests below, with slight modifications to each function to use 0L and 1L in my own code, to store the results in separate tables by method, and to standardize the ordering. These are elapsed times from single speed tests (rather than medians from many tests), but the larger runs each take a long time.

Number of rows in dt     2K      10K      50K     250K      1M   
OP                       28.6    149.2    717.0   
eddi                     5.1     24.6     144.8   1950.3  
RS                       1.8     6.7      29.7    171.9     702.5  
Original GT              1.4     7.4      57.5    809.4   
Modified GT              0.7     3.9      18.1    115.2     473.9  
GT4                      0.1     0.4      2.26    16.9      86.9

Pretty clearly, the modified version of GeekTrader's approach is best. I'm still a bit vague on what each step is doing, but I can go over that at my leisure. Although somewhat out of bounds of the original question, if anyone wants to explain what GeekTrader and Ricardo Saporta's methods are doing more efficiently, it would be appreciated both by me and probably by anyone who visits this page in the future. I'm particularly interested to understand why some methods scale better than others.

*****EDIT # 2*****

I tried to edit GeekTrader's answer with this comment, but that seems not to work. I made two very minor modifications to the GT3 function, to a) order the columns, which adds a small amount of time, and b) replace 0 and 1 with 0L and 1L, which speeds things up a bit. Call the resulting function GT4. Table above edited to add times for GT4 at different table sizes. Clearly the winner by a mile, and it has the added advantage of being intuitive.

Answer 1

UPDATE : VERSION 3

Found even faster way. This function is also highly memory efficient. Primary reason previous function was slow because of copy/assignments happening inside lapply loop as well as rbinding of the result.

In following version, we preallocate matrix with appropriate size, and then change values at appropriate coordinates, which makes it very fast compared to other looping versions.

funcGT3 <- function() {
    #Get list of column names in result
    resCol <- unique(dt[, unlist(strsplit(messy_string, split="\\$"))])

    #Get dimension of result
    nresCol <- length(resCol)
    nresRow <- nrow(dt)

    #Create empty matrix with dimensions same as desired result
    mat <- matrix(rep(0, nresRow * nresCol), nrow = nresRow, dimnames = list(as.character(1:nresRow), resCol))

    #split each messy_string by $
    ll <- strsplit(dt[,messy_string], split="\\$")

    #Get coordinates of mat which we need to set to 1
    coords <- do.call(rbind, lapply(1:length(ll), function(i) cbind(rep(i, length(ll[[i]])), ll[[i]] )))

    #Set mat to 1 at appropriate coordinates
    mat[coords] <- 1    

    #Bind the mat to original data.table
    return(cbind(dt, mat))

}


result <- funcGT3()  #result for 1000 rows in dt
result
        ID   messy_string zn tc sv db yx st ze qs wq oe cv ut is kh kk im le qg rq po wd kc un ft ye if zl zt wy et rg iu
   1:    1 zn$tc$sv$db$yx  1  1  1  1  1  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
   2:    2    st$ze$qs$wq  0  0  0  0  0  1  1  1  1  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
   3:    3    oe$cv$ut$is  0  0  0  0  0  0  0  0  0  1  1  1  1  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
   4:    4 kh$kk$im$le$qg  0  0  0  0  0  0  0  0  0  0  0  0  0  1  1  1  1  1  0  0  0  0  0  0  0  0  0  0  0  0  0  0
   5:    5    rq$po$wd$kc  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  1  1  1  1  0  0  0  0  0  0  0  0  0  0
  ---                                                                                                                    
 996:  996    rp$cr$tb$sa  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 997:  997    cz$wy$rj$he  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  1  0  0  0
 998:  998       cl$rr$bm  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 999:  999    sx$hq$zy$zd  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
1000: 1000    bw$cw$pw$rq  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  1  0  0  0  0  0  0  0  0  0  0  0  0  0

Benchmark againt version 2 suggested by Ricardo (this is for 250K rows in data) :

Unit: seconds
 expr       min        lq    median        uq       max neval
  GT2 104.68672 104.68672 104.68672 104.68672 104.68672     1
  GT3  15.15321  15.15321  15.15321  15.15321  15.15321     1

---

VERSION 1 Following is version 1 of suggested answer

set.seed(10)  
elements_list <- c(outer(letters, letters, FUN = paste, sep = ""))  
random_string <- function(min_length, max_length, separator) {  
  selection <- paste(sample(elements_list, ceiling(runif(1, min_length, max_length))), collapse = separator)  
  return(selection)  
}  
dt <- data.table(ID = c(1:1000), messy_string = "")  
dt[ , messy_string := random_string(2, 5, "$"), by = ID]  


myFunc <- function() {
  ll <- strsplit(dt[,messy_string], split="\\$")


  COLS <- do.call(rbind, 
                  lapply(1:length(ll), 
                         function(i) {
                           data.frame(
                             ID= rep(i, length(ll[[i]])),
                             COL = ll[[i]], 
                             VAL= rep(1, length(ll[[i]]))
                             )
                           }
                         )
                  )

  res <- as.data.table(tapply(COLS$VAL, list(COLS$ID, COLS$COL), FUN = length ))
  dt <- cbind(dt, res)
  for (j in names(dt))
    set(dt,which(is.na(dt[[j]])),j,0)
  return(dt)
}


create_indicators <- function(search_list, searched_string) {  
  y <- rep(0, length(search_list))  
  for(j in 1:length(search_list)) {  
    x <- regexpr(search_list[j], searched_string)  
    x <- x[1]  
    y[j] <- ifelse(x > 0, 1, 0)  
  }  
  return(y)  
}  
OPFunc <- function() {
indicators <- matrix(0, nrow = nrow(dt), ncol = length(elements_list))  
for(n in 1:nrow(dt)) {  
  indicators[n, ] <- dt[n, create_indicators(elements_list, messy_string)]  
}  
indicators <- data.table(indicators)  
setnames(indicators, elements_list)  
dt <- cbind(dt, indicators)
return(dt)
}



library(plyr)
plyrFunc <- function() {
  indicators = do.call(rbind.fill, sapply(1:dim(dt)[1], function(i)
    dt[i,
       data.frame(t(as.matrix(table(strsplit(messy_string,
                                             split = "\\$")))))
       ]))
  dt = cbind(dt, indicators)
  #dt[is.na(dt)] = 0 #THIS DOESN'T WORK. USING FOLLOWING INSTEAD

  for (j in names(dt))
    set(dt,which(is.na(dt[[j]])),j,0)

  return(dt)  
}

BENCHMARK

system.time(res <- myFunc())
## user  system elapsed 
## 1.01    0.00    1.01

system.time(res2 <- OPFunc())
## user  system elapsed 
## 21.58    0.00   21.61

system.time(res3 <- plyrFunc())
## user  system elapsed 
## 1.81    0.00    1.81 

---

VERSION 2 : Suggested by Ricardo

I'm posting this here instead of in my answer as the framework is really @GeekTrader's -Rick_

  myFunc.modified <- function() {
    ll <- strsplit(dt[,messy_string], split="\\$")

    ## MODIFICATIONS: 
    # using `rbindlist` instead of `do.call(rbind.. )`
    COLS <- rbindlist( lapply(1:length(ll), 
                           function(i) {
                             data.frame(
                               ID= rep(i, length(ll[[i]])),
                               COL = ll[[i]], 
                               VAL= rep(1, length(ll[[i]])), 
  # MODICIATION:  Not coercing to factors                             
                               stringsAsFactors = FALSE
                               )
                             }
                           )
                    )

  # MODIFICATION: Preserve as matrix, the output of tapply
    res2 <- tapply(COLS$VAL, list(COLS$ID, COLS$COL), FUN = length )

  # FLATTEN into a data.table
    resdt <- data.table(r=c(res2))

  # FIND & REPLACE NA's of single column
    resdt[is.na(r), r:=0L]

  # cbind with dt, a matrix, with the same attributes as `res2`  
    cbind(dt, 
          matrix(resdt[[1]], ncol=ncol(res2), byrow=FALSE, dimnames=dimnames(res2)))
  }


 ### Benchmarks: 

orig = quote({dt <- copy(masterDT); myFunc()})
modified = quote({dt <- copy(masterDT); myFunc.modified()})
microbenchmark(Modified = eval(modified), Orig = eval(orig), times=20L)

#  Unit: milliseconds
#        expr      min        lq   median       uq      max
#  1 Modified  895.025  971.0117 1011.216 1189.599 2476.972
#  2     Orig 1953.638 2009.1838 2106.412 2230.326 2356.802

Answer 2

  # split the `messy_string` and create a long table, keeping track of the id
  DT2 <- setkey(DT[, list(val=unlist(strsplit(messy_string, "\\$"))), by=list(ID, messy_string)], "val")

  # add the columns, initialize to 0
  DT2[, c(elements_list) := 0L]
  # warning expected, re:adding large ammount of columns


  # iterate over each value in element_list, assigning 1's ass appropriate
  for (el in elements_list)
     DT2[el, c(el) := 1L]

  # sum by ID
  DT2[, lapply(.SD, sum), by=list(ID, messy_string), .SDcols=elements_list]

Note that we are carrying along the messy_string column since it is cheaper than leaving it behind and then joining on ID to get it back. If you dont need it in the final output, just delete it above.

---

Benchmarks:

Creating the sample data:

# sample data, using OP's exmple
set.seed(10)
N <- 1e6  # number of rows
elements_list <- c(outer(letters, letters, FUN = paste, sep = ""))  
messy_string_vec <- random_string_fast(N, 2, 5, "$")   # Create the messy strings in a single shot. 
masterDT <- data.table(ID = c(1:N), messy_string = messy_string_vec, key="ID")   # create the data.table

Side Note It is significantly faster to create the random strings all at once and assign the results as a single column than to call the function N times and assign each, one by one.

  # Faster way to create the `messy_string` 's
  random_string_fast <- function(N, min_length, max_length, separator) {  
    ints <- seq(from=min_length, to=max_length)
    replicate(N, paste(sample(elements_list, sample(ints)), collapse=separator))
  }

Comparing Four Methods:

• this answer -- "DT.RS"
• @eddi's answer -- "Plyr.eddi"
• @GeekTrader's answer -- DT.GT
• GeekTrader's' answer with some modifications -- DT.GT_Mod

Here is the setup:

library(data.table); library(plyr); library(microbenchmark)

# data.table method - RS
usingDT.RS <- quote({DT <- copy(masterDT);
                    DT2 <- setkey(DT[, list(val=unlist(strsplit(messy_string, "\\$"))), by=list(ID, messy_string)], "val"); DT2[, c(elements_list) := 0L]
                    for (el in elements_list) DT2[el, c(el) := 1L]; DT2[, lapply(.SD, sum), by=list(ID, messy_string), .SDcols=elements_list]})

# data.table method - GeekTrader
usingDT.GT <- quote({dt <- copy(masterDT); myFunc()})

# data.table method - GeekTrader, modified by RS
usingDT.GT_Mod <- quote({dt <- copy(masterDT); myFunc.modified()})

# ply method from below
usingPlyr.eddi <- quote({dt <- copy(masterDT); indicators = do.call(rbind.fill, sapply(1:dim(dt)[1], function(i) dt[i, data.frame(t(as.matrix(table(strsplit(messy_string, split = "\\$"))))) ])); 
                    dt = cbind(dt, indicators); dt[is.na(dt)] = 0; dt })

Here are the benchmark results:

microbenchmark( usingDT.RS=eval(usingDT.RS), usingDT.GT=eval(usingDT.GT), usingDT.GT_Mod=eval(usingDT.GT_Mod), usingPlyr.eddi=eval(usingPlyr.eddi), times=5L)


  On smaller data: 

  N = 600
  Unit: milliseconds
              expr       min        lq    median        uq       max
  1     usingDT.GT 1189.7549 1198.1481 1200.6731 1202.0972 1203.3683
  2 usingDT.GT_Mod  581.7003  591.5219  625.7251  630.8144  650.6701
  3     usingDT.RS 2586.0074 2602.7917 2637.5281 2819.9589 3517.4654
  4 usingPlyr.eddi 2072.4093 2127.4891 2225.5588 2242.8481 2349.6086


  N = 1,000 
  Unit: seconds
       expr      min       lq   median       uq      max
  1 usingDT.GT 1.941012 2.053190 2.196100 2.472543 3.096096
  2 usingDT.RS 3.107938 3.344764 3.903529 4.010292 4.724700
  3  usingPlyr 3.297803 3.435105 3.625319 3.812862 4.118307

  N = 2,500
  Unit: seconds
              expr      min       lq   median       uq       max
  1     usingDT.GT 4.711010 5.210061 5.291999 5.307689  7.118794
  2 usingDT.GT_Mod 2.037558 2.092953 2.608662 2.638984  3.616596
  3     usingDT.RS 5.253509 5.334890 6.474915 6.740323  7.275444
  4 usingPlyr.eddi 7.842623 8.612201 9.142636 9.420615 11.102888

  N = 5,000
              expr       min        lq    median        uq       max
  1     usingDT.GT  8.900226  9.058337  9.233387  9.622531 10.839409
  2 usingDT.GT_Mod  4.112934  4.293426  4.460745  4.584133  6.128176
  3     usingDT.RS  8.076821  8.097081  8.404799  8.800878  9.580892
  4 usingPlyr.eddi 13.260828 14.297614 14.523016 14.657193 16.698229

  # dropping the slower two from the tests:
  microbenchmark( usingDT.RS=eval(usingDT.RS), usingDT.GT=eval(usingDT.GT), usingDT.GT_Mod=eval(usingDT.GT_Mod), times=6L)

  N = 10,000
  Unit: seconds
              expr       min        lq    median        uq       max
  1 usingDT.GT_Mod  8.426744  8.739659  8.750604  9.118382  9.848153
  2     usingDT.RS 15.260702 15.564495 15.742855 16.024293 16.249556

  N = 25,000
  ... (still running)

-----------------

Functions Used in benchmarking:

  # original random string function
  random_string <- function(min_length, max_length, separator) {  
      selection <- paste(sample(elements_list, ceiling(runif(1, min_length, max_length))), collapse = separator)  
      return(selection)  
  }  

  # GeekTrader's function
  myFunc <- function() {
    ll <- strsplit(dt[,messy_string], split="\\$")


    COLS <- do.call(rbind, 
                    lapply(1:length(ll), 
                           function(i) {
                             data.frame(
                               ID= rep(i, length(ll[[i]])),
                               COL = ll[[i]], 
                               VAL= rep(1, length(ll[[i]]))
                               )
                             }
                           )
                    )

    res <- as.data.table(tapply(COLS$VAL, list(COLS$ID, COLS$COL), FUN = length ))
    dt <- cbind(dt, res)
    for (j in names(dt))
      set(dt,which(is.na(dt[[j]])),j,0)
    return(dt)
  }


  # Improvements to @GeekTrader's `myFunc` -RS  '
  myFunc.modified <- function() {
    ll <- strsplit(dt[,messy_string], split="\\$")

    ## MODIFICATIONS: 
    # using `rbindlist` instead of `do.call(rbind.. )`
    COLS <- rbindlist( lapply(1:length(ll), 
                           function(i) {
                             data.frame(
                               ID= rep(i, length(ll[[i]])),
                               COL = ll[[i]], 
                               VAL= rep(1, length(ll[[i]])), 
  # MODICIATION:  Not coercing to factors                             
                               stringsAsFactors = FALSE
                               )
                             }
                           )
                    )

  # MODIFICATION: Preserve as matrix, the output of tapply
    res2 <- tapply(COLS$VAL, list(COLS$ID, COLS$COL), FUN = length )

  # FLATTEN into a data.table
    resdt <- data.table(r=c(res2))

  # FIND & REPLACE NA's of single column
    resdt[is.na(r), r:=0L]

  # cbind with dt, a matrix, with the same attributes as `res2`  
    cbind(dt, 
          matrix(resdt[[1]], ncol=ncol(res2), byrow=FALSE, dimnames=dimnames(res2)))
  }


  ### Benchmarks comparing the two versions of GeekTrader's function: 
  orig = quote({dt <- copy(masterDT); myFunc()})
  modified = quote({dt <- copy(masterDT); myFunc.modified()})
  microbenchmark(Modified = eval(modified), Orig = eval(orig), times=20L)

  #  Unit: milliseconds
  #        expr      min        lq   median       uq      max
  #  1 Modified  895.025  971.0117 1011.216 1189.599 2476.972
  #  2     Orig 1953.638 2009.1838 2106.412 2230.326 2356.802