[R] Data frame referencing?

[Sander Oom]

Dear R users,

When you do:
> x <- rnorm(10)
> y <- rnorm(10)
> z <- rnorm(10)
> a <- data.frame(x,y,z)
> a$x
 [1]  1.37821893  0.21152756 -0.55453182 -2.10426048 -0.08967880  0.03712110
 [7] -0.80592149  0.07413450  0.15557671  1.22165341

Why does this not work:
> a[a$y>0.5,y] <-1
Error in "[<-.data.frame"(`*tmp*`, a$y > 0.5, y, value = 1) :
        only 0's may be mixed with negative subscripts

While this works:
> a[a$y>0.5,2] <-1

> a
             x          y          z
1   1.37821893 -1.0887363  1.7340522
2   0.21152756 -0.7256467 -1.3165373
3  -0.55453182  1.0000000 -2.1116072
4  -2.10426048 -0.4898596 -1.5863823
5  -0.08967880  1.0000000 -0.9139706
6   0.03712110  1.0000000 -1.3004970
7  -0.80592149 -0.7004193 -0.1958059
8   0.07413450  1.0000000 -1.3574303
9   0.15557671 -0.3335407 -2.1991236
10  1.22165341  1.0000000 -0.7576708

For a complex loop I would prefer to reference the right colomn by name,
not by number! Now, when the colomns change, I need to check my code to
make sure that the right colomns are referenced.

Suggestions much appreciated!

Thanks in advance,

Sander.