Calculate row-wise proportions

following should do the trick

cbind(id = x[, 1], x[, -1]/rowSums(x[, -1]))
##   id       val0      val1      val2
## 1  a 0.08333333 0.3333333 0.5833333
## 2  b 0.13333333 0.3333333 0.5333333
## 3  c 0.16666667 0.3333333 0.5000000

And another alternative (though this is mostly a pretty version of sweep)... prop.table:

> cbind(x[1], prop.table(as.matrix(x[-1]), margin = 1))
  id       val0      val1      val2
1  a 0.08333333 0.3333333 0.5833333
2  b 0.13333333 0.3333333 0.5333333
3  c 0.16666667 0.3333333 0.5000000

From the "description" section of the help file at ?prop.table:

This is really sweep(x, margin, margin.table(x, margin), "/") for newbies, except that if margin has length zero, then one gets x/sum(x).

So, you can see that underneath, this is really quite similar to @Jilber's solution.

And... it's nice for the R developers to be considerate of us newbies, isn't it? :)

Another alternative using sweep

sweep(x[,-1], 1, rowSums(x[,-1]), FUN="/")
        val0      val1      val2
1 0.08333333 0.3333333 0.5833333
2 0.13333333 0.3333333 0.5333333
3 0.16666667 0.3333333 0.5000000