Replace variable name in string with variable value [R]

Question

I have a character in R, say "\\frac{A}{B}". And I have values for A and B, say 5 and 10. Is there a way that I can replace the A and B with the 5 and 10?

I tried the following.

words <- "\\frac{A}{B}"
numbers <- list(A=5, B=10)
output <- do.call("substitute", list(parse(text=words)[[1]], numbers))

But I get an error on the \. Is there a way that I can do this? I an trying to create equations with the actual variable values.

Your general approach is not going to work for you. If you try your code with words <- "frac{A}{B}" you get Error in parse(text = words) : <text>:1:5: unexpected '{' 1: frac{ — Shawn Mehan
– Shawn Mehan, Commented Oct 8, 2015 at 19:50
@ShawnMehan -- Yeah, I tried that, too. It seems that there should be some general way to do this, like sprintf or something, but I can't figure it out. Thanks. — Richard Herron
– Richard Herron, Commented Oct 8, 2015 at 19:52

Rich Scriven · Accepted Answer · 2015-10-08 21:31:16Z

3

You could use the stringi function stri_replace_all_fixed()

stringi::stri_replace_all_fixed(
    words, names(numbers), numbers, vectorize_all = FALSE
)
# [1] "\\frac{5}{10}"

edited Oct 8, 2015 at 21:31

answered Oct 8, 2015 at 21:16

Rich Scriven

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Rich Scriven · Accepted Answer · 2015-10-08 21:10:56Z

2

Try this:

sprintf(gsub('\\{\\w\\}','\\{%d}',words),5,10)

edited Oct 8, 2015 at 21:10

Rich Scriven

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answered Oct 8, 2015 at 20:15

Shenglin Chen

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Lizzie Silver · Accepted Answer · 2015-10-08 20:04:25Z

1

I'm more familiar with gsub than substitute. The following works:

words <- "\\frac{A}{B}"
numbers <- list(A=5, B=10)
arglist = mapply(list, as.list(names(numbers)), numbers, SIMPLIFY=F)
for (i in 1:length(arglist)){
    arglist[[i]]["x"] <- words
    words <- do.call("gsub", arglist[[i]])
}

But of course this is unsafe because you're iterating over the substitutions. If, say, the first variable has value "B" and the second variable has name "B", you'll have problems. There's probably a cleaner way.

edited Oct 8, 2015 at 20:04

answered Oct 8, 2015 at 19:56

Lizzie Silver

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