Apply function to combine columns

Question

I have a data frame with many columns and I would like to combine columns started from column 5. I try to use apply to do this.

Data Frame:

1   682333  191.858 191517119   C   A   C   A   A   A   C   A   A   A     A   A
2   1862626 71.9275 56032940    A   C   C   C   A   A   A   C   A   C   A   A
3   11957134    155.78  150230950   B   B   B   B   A   B   A   B   A   B   A   B
4   2516482 51.2692 31496569    B   A   A   A   A   A   A   A   A   A   A   A
5   9378200 51.2798 31572927    A   A   B   B   B   A   A   A   A   A   B   A
6   2071534 52.1573 32824318    A   B   A   B   A   B   A   B   B   B   A   B
7   2074633 33.068  19035920    A   A   B   A   A   A   B   A   B   A   B   A
8   7856856 121.811 117540910   A   A   A   A   A   A   A   A   B   A   B   A
9   3741206 2.18574 2169864 A   A   A   A   A   A   A   A   A   A   A   A
10  4411364 12.5959 24191374    C   C   A   C   A   C   C   C   A   C   A   C

Output:

1   682333  191.858 191517119   CA      CA      AA      CA      AA      AA
2   1862626 71.9275 56032940    AC      CC      AA      AC      AC      AA
3   11957134    155.78  150230950   BB      BB      AB      AB      AB      AB
4   2516482 51.2692 31496569    BA      AA      AA      AA      AA      AA
5   9378200 51.2798 31572927    AA      BB      BA      AA      AA      BA
6   2071534 52.1573 32824318    AB      AB      AB      AB      BB      AB
7   2074633 33.068  19035920    AA      BA      AA      BA      BA      BA
8   7856856 121.811 117540910   AA      AA      AA      AA      BA      BA
9   3741206 2.18574 2169864 AA      AA      AA      AA      AA      AA
10  4411364 12.5959 24191374    CC      AC      AC      CC      AC      AC

I tried like this:

col <- apply(df[,-1:-4], 2, function(x) {paste(x,x+1,sep="")}
df <- cbind(df[,1:4],col)

But there is an error:

Error in x + 1 : non-numeric argument to binary operator.

Answer 1

You can use Map to loop through odd position columns and even position columns in parallel and paste corresponding column together:

as.data.frame(c(df[1:4], Map(function(x, y) paste(x, y, sep = ""), 
                             df[-(1:4)][c(TRUE, FALSE)],      # use cycling rule to pick 
                                                              # odd position columns
                             df[-(1:4)][c(FALSE, TRUE)])))    # pick even position columns

#   V1       V2        V3        V4 V5 V7 V9 V11 V13 V15
#1   1   682333 191.85800 191517119 CA CA AA  CA  AA  AA
#2   2  1862626  71.92750  56032940 AC CC AA  AC  AC  AA
#3   3 11957134 155.78000 150230950 BB BB AB  AB  AB  AB
#4   4  2516482  51.26920  31496569 BA AA AA  AA  AA  AA
#5   5  9378200  51.27980  31572927 AA BB BA  AA  AA  BA
#6   6  2071534  52.15730  32824318 AB AB AB  AB  BB  AB
#7   7  2074633  33.06800  19035920 AA BA AA  BA  BA  BA
#8   8  7856856 121.81100 117540910 AA AA AA  AA  BA  BA
#9   9  3741206   2.18574   2169864 AA AA AA  AA  AA  AA
#10 10  4411364  12.59590  24191374 CC AC AC  CC  AC  AC