Replacing characters in an array with numbers

user_18363843 0
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I have read in data from an excel file, and made it into a vector. I then made it into a 3d array.

The vector that forms the array, and also inside the array now has in it characters, like this:

D <- c('g', 't', NA, 'd')
nPeriods = 3
column.names = c('aaa', 'bbb')
row.names = c('jjj', 'hhh')
threeD.names = c(1:nPeriods)
E = array(c(D), dim=c(2, 2, nPeriods),
          dimnames = list(row.names, column.names, threeD.names))

However, now I want to assign 'g', 't', etc as variables with starting values, e.g.

g = 5
t = 2
d = 7

But the only way I know how to do this is by manually figuring out which element of the array it is and assigning it like that, e.g.

E[1,1,]=5

When it's a large matrix that's going to be difficult and annoying to find the corresponding position every time I refer to an element. I know that each element of the vector (that is not NA) is unique, so I wondered, is there a shortcut for referring to each element of the array? (Maybe something in the apply family? but there are so many of them).

I will also need to refer later to them to loop over nPeriods. Yesterday someone showed me I can do this with:

for (i in 2:nPeriods){
  C[1,1,i]=C[1,1,i-1]*2
}

But in real life my matrix might be quite large so I'd rather just be able to refer with d, t, etc.

apply r for-loop arrays variables
2 Answers

If you store your replacement values in a named vector, you can then use apply to replace elements by name. apply handles the looping automatically, and even converts the result to a numeric array if all non-NA elements are numeric. Note that the 1:3 in the apply call refers to the three dimensions of your array, not the length of the third dimension, so this should work no matter how large the array is.

values <- c(g = 5,
            t = 2,
            d = 7)

num_array <- apply(E, 1:3, function(x) values[x])

num_array

, , 1

    aaa bbb
jjj   5  NA
hhh   2   7

, , 2

    aaa bbb
jjj   5  NA
hhh   2   7

, , 3

    aaa bbb
jjj   5  NA
hhh   2   7

Your second question is unclear, but you can which to get the elements efficiently. You only need to loop over the slices:

result <- num_array
for (i in 2:dim(num_array)[3]) {
  idx <- which(E[, , 1] == 'g', arr.ind = T)
  row <- idx[1, 'row']
  col <- idx[1, 'col']
  result[row, col, i] <- result[row, col, i-1] * 2
}

, , 1

    aaa bbb
jjj   5  NA
hhh   2   7

, , 2

    aaa bbb
jjj  10  NA
hhh   2   7

, , 3

    aaa bbb
jjj  20  NA
hhh   2   7

If you wanted to store operations per each character (relatively simple operations), you could take advantage of some of R's meta-programming features:

funcs <- c(g = '*', t = '+', d = '-')
modifiers <- c(g = 2, t = 3, d = 4)

num_array <- apply(E, 1:3, function(x) values[x])

result <- num_array
for (i in 2:dim(num_array)[3]) {
  for (j in names(values)) {
    idx <- which(E[, , 1] == j, arr.ind = T)
    row <- idx[1, 'row']
    col <- idx[1, 'col']
    result[row, col, i] <- do.call(funcs[j], args = list(result[row, col, i-1], modifiers[j]))
  }
}

, , 1

    aaa bbb
jjj   5  NA
hhh   2   7

, , 2

    aaa bbb
jjj  10  NA
hhh   5   3

, , 3

    aaa bbb
jjj  20  NA
hhh   8  -1
user_64365450

This task is more manageable in a list. You can use the following:

# Example data
D <- c('g', 't', NA, 'd')
nPeriods = 3
column.names = c('aaa', 'bbb')
row.names = c('jjj', 'hhh')
threeD.names = c(1:nPeriods)
E = array(c(D), dim=c(2, 2, nPeriods),
          dimnames = list(row.names, column.names, threeD.names))

# Convert array to list
array_list <- lapply(seq(dim(E)[3]), function(x) E[,,x])

# Re-assign values
array_converted <- lapply(array_list, function(x){
  x <- ifelse(x == "g", 5, x) # your conversion values
  x <- ifelse(x == "t", 2, x)
  x <- ifelse(x == "d", 7, x)
  x <- apply(x, 2, as.numeric) # Ensures values are numeric
  return(x)
})

# For final format as array (if you want)
final_array <- simplify2array(array_converted)
user_93073350
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