[R] Conditional looping over a set of variables in R

[William Dunlap]

You were a bit vague about the format of your data.
I'm assuming all columns were numeric and the entries
are one of 0, 1, and NA (missing value).  I made a
little function to generate random data of that format
for testing purposes:

makeData <- function (nrow = 1500, ncol = 140, pMissing = 0.1) 
{
    # pMissing if proportion of missing values
    m <- matrix(sample(c(1, 0), size = nrow * ncol, replace = TRUE), 
        nrow, ncol)
    m[runif(nrow * ncol) < pMissing] <- NA
    data.frame(m)
}

E.g.,

  > set.seed(168)
  > d <- makeData(15,3)
  > d
      X1 X2 X3
   1   1  1  1
   2   0  0 NA
   3   0  1  0
   4   0  0 NA
   5   0  1  1
   6   0  0 NA
   7   1  0  0
   8   0  1  1
   9   0  0  1
  10   1  1 NA
  11   0  0  1
  12   0  0  0
  13  NA NA NA
  14   0  0  0
  15   1  0  0

I think the following function does what you want.
The algorithm is pretty similar to what you showed.

  columnOfFirstOne <- function(data) {
      # col will be return value, one entry per row of data.
      # Fill it with NA's: NA in output will mean there were no 1's in
row
      col <- rep(as.integer(NA), nrow(data))
      for (j in seq_len(ncol(data))) { # loop over columns
          # For each entry in 'col', if it has not been set yet
          # and this entry the j'th column of data is 1 (and not
missing)
          # then set to the column number.
          col[is.na(col) & !is.na(data[, j]) & data[, j] == 1] <- j
      }
      col # return this from function
  }

With the above data we get
  > columnOfFirstOne(d)
   [1]  1 NA  2 NA  2 NA  1  2  3  1  3 NA NA NA  1

It seems quick enough for a dataset of your size
  > dd <- makeData(nrow=1500, ncol=140)
  > system.time(columnOfFirstOne(dd)) # time in seconds
     user  system elapsed 
     0.08    0.00    0.08
 
Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com  

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of David Herzberg
> Sent: Friday, October 22, 2010 8:34 AM
> To: r-help at r-project.org
> Subject: [R] Conditional looping over a set of variables in R
> 
> Here's the problem I'm trying to solve in R: I have a data 
> frame that consists of about 1500 cases (rows) of data from 
> kids who took a test of listening comprehension. The columns 
> are their scores (1 = correct, 0 = incorrect,  . = missing) 
> on 140 test items. The items are numbered sequentially and 
> are ordered by increasing difficulty as you go from left to 
> right across the columns. I want R to go through the data and 
> find the first correct response for each case. Because of 
> basal and ceiling rules, many cases have missing data on many 
> items before the first correct response appears.
> 
> For each case, I want R to evaluate the item responses 
> sequentially starting with item 1. If the score is 0 or 
> missing, proceed to the next item and evaluate it. If the 
> score is 1, stop the operation for that case, record the item 
> number of that first correct response in a new variable, 
> proceed to the next case, and restart the operation.
> 
> In SPSS, this operation would be carried out with LOOP, 
> VECTOR, and DO IF, as follows (assuming the data set is 
> already loaded):
> 
> * DECLARE A NEW VARIABLE TO HOLD THE ITEM NUMBER OF THE FIRST 
> CORRECT RESPONSE, SET IT EQUAL TO 0.
> numeric LCfirst1.
> comp LCfirst1 = 0
> 
> * DECLARE A VECTOR TO HOLD THE 140 ITEM RESPONSE VARIABLES.
> vector x=LC1a_score to LC140a_score.
> 
> * SET UP A LOOP THAT WILL RUN FROM 1 TO 140, AS LONG AS 
> LCfirst1 = 0. "#i" IS AN INDEX VARIABLE THAT INCREASES BY 1 
> EACH TIME THE LOOP RUNS.
> loop #i=1 to 140 if (LCfirst1 = 0).
> 
> * SET UP A CONDITIONAL TRANSFORMATION THAT IS EVALUATED FOR 
> EACH ELEMENT OF THE VECTOR.  THUS, WHEN #i = 1, THE 
> EXPRESSION EVALUATES THE FIRST ELEMENT OF THE VECTOR (THAT 
> IS, THE FIRST OF THE 140 ITEM RESPONSES). AS THE LOOP RUNS 
> AND #i INCREASES, SUBSEQUENT VECTOR ELELMENTS ARE EVALUATED. 
> THE do if STATEMENT RETAINS CONTROL AND KEEPS LOOPING THROUGH 
> THE VECTOR UNTIL A '1' IS ENCOUNTERED.
> + do if x(#i) = 1.
> 
> * WHEN A '1' IS ENCOUNTERED, CONTROL PASSES TO THE NEXT 
> STATEMENT, WHICH RECODES THE VALUE OF THAT VECTOR ELEMENT TO '99'.
> + comp x(#i) = 99.
> 
> * AND THEN CONTROL PASSES TO THE NEXT STATEMENT, WHICH 
> RECODES THE VALUE OF LCfirst1 TO THE CURRENT INDEX VALUE, 
> THUS CAPTURING THE ITEM NUMBER OF THE FIRST CORRECT RESPONSE 
> FOR THAT CASE. CHANGING THE VALUE OF LCfirst1 ALSO CAUSE S 
> THE LOOP TO STOP EXECUTING FOR THAT CASE, AND THE PROGRAM 
> MOVES TO THE NEXT CASE AND RESTARTS THE LOOP.
> + comp LCfirst1 = #i.
> + end if.
> end loop.
> exe.
> 
> After several hours of trying to translate this procedure to 
> R, I'm stumped. I played around with creating a list to hold 
> the item responses variables (analogous to 'vector' in SPSS), 
> but when I tried to use the list in an R procedure, I kept 
> getting a warning along the lines of  'the list contains > 1 
> element, only the first element will be used'. So perhaps a 
> list is not the appropriate class to 'hold' these variables?
> 
> It seems that some nested arrangement of 'for' 'while' and/or 
> 'lapply' will allow me to recreate the operation described 
> above? How do I set up the indexing operation analogous to 
> 'loop #i' in SPSS?
> 
> Any help is appreciated, and I'm happy to provide more 
> information if needed.
> 
> David S. Herzberg, Ph.D.
> Vice President, Research and Development
> Western Psychological Services
> 12031 Wilshire Blvd.
> Los Angeles, CA 90025-1251
> Phone: (310)478-2061 x144
> FAX: (310)478-7838
> email: davidh at wpspublish.com
> 
> 
> 
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> 
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