Question

I have a data frame where columns are constantly being added to it. I also have a total column that I would like to stay at the end. I think I must have skipped over some really basic command somewhere but cannot seem to find the answer anywhere. Anyway, here is some sample data:

x=1:10
y=21:30
z=data.frame(x,y)
z$total=z$x+z$y
z$w=11:20
z$total=z$x+z$y+z$w

When I type z I get this:

    x  y total  w
1   1 21    33 11
2   2 22    36 12
3   3 23    39 13
4   4 24    42 14
5   5 25    45 15
6   6 26    48 16
7   7 27    51 17
8   8 28    54 18
9   9 29    57 19
10 10 30    60 20

Note how the total column comes before the w, and obviously any subsequent columns. Is there a way I can force it to be the last column? I am guessing that I would have to use ncol(z) somehow. Or maybe not.

Answer 1

You can reorder your columns as follows:

z <- z[,c('x','y','w','total')]

To do this programmatically, after you’re done adding your columns, you can retrieve their names like so:

nms <- colnames(z)

Then you can grab the ones that aren’t ‘total’ like so:

nms[nms!='total']

Combined with the above:

z <- z[, c(nms[nms!='total'],'total')]