Formatting numbers for printing in R: rounding and trailing zeroes

In journal articles, descriptive statistics results are often presented as a table of means, with standard deviation/standard error given in parentheses following the mean. Here is how you can prepare the print formatting when working in R: it’s rather trivial, but I wasted a sufficient amount of time on it that I thought it was worth mentioning. (I follow this by using latex() from the Hmisc package to generate LaTeX output of such a table.)

Suppose I have a vector of means for percent correct from 5 different experimental conditions, and a vector of the corresponding standard errors, and both are sorted in the same order:

all.res.corr$per.correct # Vector of mean percent correct, comes from a data frame called all.res.corr
#[1] 52.53561 60.51282 64.13105 66.38177 67.46439

all.res.corr$se.per.corr # Vector of standard errors, from same data frame
#[1] 2.409955 2.763525 2.831450 2.909737 2.902924

1. First, use round() to round the numbers to two decimal places.

   round(all.res.corr$per.correct,2)
   #[1] 52.54 60.51 64.13 66.38 67.46
   round(all.res.corr$se.per.corr,2)
   #[1] 2.41 2.76 2.83 2.91 2.90
   

2. Then use formatC() to access C-like formatting options to format the two vectors to print up to 2 digits after the decimal point. The argument specification format = "f" allows you to set the number of digits after the decimal point to print. This is a crucial step! If you don’t do it, you won’t get trailing zeroes to print, even though it looks like in the preceding code block, for instance, that round() preserves the trailing zero. It gets lost in a numeric to character data type conversion. See this post for an alternative using sprintf() (which may be familiar from many other programming languages).

   formatC(round(all.res.corr$per.correct,2), 2, format = "f"
   #[1] "52.54" "60.51" "64.13" "66.38" "67.46" # note conversion to character type
   formatC(round(all.res.corr$se.per.corr,2), 2, format = "f")
   [1] "2.41" "2.76" "2.83" "2.91" "2.90"
   

3. Finally, use paste() to concatenate the vectors for publication, together with parentheses in the appropriate place. The code below concatenates the arguments given: the formatted mean vector all.res.corr$per.correct, a space and opening parentheses (, the formatted SE vector all.res.corr$se.per.corr, and a closing parentheses ). The separator sep is specified to be an empty string, for there to be no separator between the arguments concatenated.

(per.all.res.corr.print <- paste(formatC(round(all.res.corr$per.correct,2), 2, format = "f"), " (", formatC(round(all.res.corr$se.per.corr,2), 2, format = "f"), ")", sep="") # n.b. Parentheses around an assignment command print the assigned value
#[1] "52.54 (2.41)" "60.51 (2.76)" "64.13 (2.83)" "66.38 (2.91)" "67.46 (2.90)"