Formatting Decimals in Texts with R

Yanping Chen raised a question in the Chinese COS forum on the output of Eviews: how to (re)format the decimal coefficients in equations as text output? For example, we want to round the numbers in CC = 16.5547557654 + 0.0173022117998*PP + 0.216234040485 * PP(-1) + 0.810182697599 * (WP + WG) to the 3rd decimal places. This can be simply done by regular expressions, as decimals always begin with a period (.). The basic steps are:

1. find out where are the decimals in the character string;
2. format them;
3. replace the original decimals with formatted values;

Given a character vector, we can format the decimals with the code below:

# x: equations; FUN: formatting function; ...: passed to FUN
coefFormat = function(x, FUN, ...) {
  sapply(x, function(s) {
    dig = sapply(gregexpr("\\.[0-9]+", s), function(m) {
      sapply(seq(along = m), function(i) {
        substr(s, m[i], m[i] + attr(m, "match.length")[i] - 1)
      })
    })
    for (j in {
      if (is.null(dim(dig))) NULL else 1:dim(dig)[1]
    }) {
      s = sub(dig[j, 1], substring(FUN(as.numeric(dig[j, 1]), ...),
        2), s, fixed = TRUE)
    }
    s
  })
}

I used sapply() for 3 times to avoid explicit loops but consequently the code might be difficult to read. The critical part is the regular expression \\.[0-9]+ which means one of more (controlled by + after [0-9]) digits ([0-9] or [:digit:]) after a decimal point .. As . is a metacharacter in regular expressions, we need to use a backslash before it, and again, \ is a special character in R, so we need another backslash to denote a backslash. o:-)

x = readLines(zz <- textConnection(
"CC = 16.5547557654 + 0.0173022117998 * PP + 0.216234040485 * PP(-1) + 0.810182697599 * (WP + WG)

II = 20.2782089394 + 0.150221823899 * PP + 0.61594357734 * PP(-1) - 0.157787636546 * KK

WP = 1.50029688603 + C(10) * XX + 0.146673821502 * XX(-1) + 0.130395687204 * AA
"))
close(zz)

writeLines(coefFormat(x, round, digits = 3))
#  CC = 16.555 + 0.017 * PP + 0.216 * PP(-1) + 0.81 * (WP + WG)
#
#  II = 20.278 + 0.15 * PP + 0.616 * PP(-1) - 0.158 * KK
#
#  WP = 1.5 + C(10) * XX + 0.147 * XX(-1) + 0.13 * AA
#
writeLines(coefFormat(x, formatC, digits = 3, format = "f"))
#  CC = 16.555 + 0.017 * PP + 0.216 * PP(-1) + 0.810 * (WP + WG)
#  
#  II = 20.278 + 0.150 * PP + 0.616 * PP(-1) - 0.158 * KK
#  
#  WP = 1.500 + C(10) * XX + 0.147 * XX(-1) + 0.130 * AA
#