Question

Suppose the user sets dim (1, …., n), and then one or more x_i variables values, with i = 1,…, n.

After some more computations I need to automatically return a dim-dimensional vector vec of the form: (0, 0, 0.2, 0, 0, …, 0.3), where in this specific case the user has set:

dim <- 10
x_3 <- 0.2
x_10 <- 0.3

Of course it is immediate to do:

vec <- rep(0, dim)
vec[3] <- x_3
vec[10] <- x_10

However, since I want to automatize as much operations as possible, I ask you how would you link the x_i variables together with the “respective” element of vec, taking into account that a priori it is not known which/how many variables will be set different from 0.

In other languages this could be done using a for-loop with macro variables… the syntax is wrong, but the idea is something like this:

vec <- rep(0, dim)

for (i in 1:dim) {
  if (as.integer(exists(x_i))==1) {
    vec[i] <- x_i
 } 
} 

what would you suggest? Thanks!

Answer 1

sapply(paste("x_",seq(dim),sep=""),function(x) if(exists(x)) get(x) else 0)
 x_1  x_2  x_3  x_4  x_5  x_6  x_7  x_8  x_9 x_10 
 0.0  0.0  0.2  0.0  0.0  0.0  0.0  0.0  0.0  0.3