Substitute symbols with values and simplify equations in R

Question

I would like to substitute symbols in equation as well as simplify equations in R.

data:

num_names <- c("num_a","num_aa","num_aaa","num_b")
num_values <- c(1,2,3,4)

df <- data.frame(id=c(1:3),
                 equation=c("2*x_a*num_a","num_a*(num_aa+1)^2","num_aaa+num_b*x_b"),
                 stringsAsFactors = F)

df

  id           equation
1  1        2*x_a*num_a
2  2 num_a*(num_aa+1)^2
3  3  num_aaa+num_b*x_b

expected output:

  id equation
1  1    2*x_a
2  2        9
3  3  3+4*x_b

Answer 1

Try this:

df$eqn2 <- Reduce(function(prev, this) gsub(paste0("\\b", num_names[this], "\\b"), num_values[this], prev),
                  seq_along(num_names), init = df$equation)
df$eqn2 <- sapply(df$eqn2, function(eq) if (grepl("[A-Za-z_]", eq)) eq else eval(parse(text = eq)))
df$eqn2 <- gsub("(\\b1\\*|\\*1\\b)", "", df$eqn2)
df
#   id           equation    eqn2
# 1  1        2*x_a*num_a   2*x_a
# 2  2 num_a*(num_aa+1)^2       9
# 3  3  num_aaa+num_b*x_b 3+4*x_b

Not the most elegant, but it works well-enough here.

One problem with doing this perfectly symbolically is that some objects exist (as referenced by the num_* variables, not the preferred format for true symbolic lookup) and some do not. I don't know of a way to evaluate only part of the equations without running into "not found" errors.