[R] Recoding a multiple response question into a series of 1, 0 variables

[Jonathan Baron]

On 06/07/04 21:28, Greg Blevins wrote:
>Hello R folks.
>
>1) The question that generated the data, which I call Qx:
>Which of the following 5 items have you performed in the past month?  (multipe
>response)
>
>2) How the data is coded in my current dataframe:
>The first item that a person selected is coded under a field called Qxfirst; the
>second selected under Qxsecond, etc.  For the first Person, the NAs mean that that
>person only selected two of the five items.
>
>Hypothetical data is shown
>
>                    Qxfirst    Qxsecond    Qxthird    Qxfourth    Qxfifth
>Person1        4            2                NA            NA            NA
>Person2        1            3                4               5               NA
>Person3        3            2                NA            NA            NA
>
>3) How I want the data to be be coded:
>
>I want each field to be one of the five items and I want each field to contain a 1 or
>0 code--1 if they mentioned the item, 0 otherwise.
>
>Given the above data, the new fields would look as follows:
>
>                    Item1    Item2        Item3            Item4        Item5
>Person1        0            1               0                1               0
>Person2        1            0               1                1               1
>Person3        0            1               1                0               0

Here is an idea:
X <- c(4,5,NA,NA,NA) # one row
Y <- rep(NA,5) # an empty row
Y[X] <- 1

Y is now
NA NA NA 1 1
which is what you want.

So you need to do this on each row and then convert the NAs to
0s.  So first create an empty data frame, the same size as your
original one X, like my Y.  Callit Y.  Then a loop?  (I can't
think of a better way just now, like with mapply.)

for (i in [whatever]) Y[i][X[i]] <- 1

(Not tested.)  Jon
-- 
Jonathan Baron, Professor of Psychology, University of Pennsylvania
Home page:            http://www.sas.upenn.edu/~baron
R search page:        http://finzi.psych.upenn.edu/