Truncate decimal to specified places

Question

This seems like it should be a fairly easy problem to solve but I am having some trouble locating an answer.

I have a vector which contains long decimals and I want to truncate it to a specific number of decimals. I do not wish to round it, but rather just remove the values beyond my desired number of decimals.

For example I would like 0.123456789 to return 0.1234 if I desired 4 decimal digits. This is not an issue of printing a specific number of digits but rather returning the original value truncated to a given number.

Thanks.

Answer 1

trunc(x*10^4)/10^4

yields 0.1234 like expected.

More generally,

trunc <- function(x, ..., prec = 0) base::trunc(x * 10^prec, ...) / 10^prec;
print(trunc(0.123456789, prec = 4) # 0.1234
print(trunc(14035, prec = -2), # 14000

Answer 2

I used the technics above for a long time. One day I had some issues when I was copying the results to a text file and I solved my problem in this way:

trunc_number_n_decimals <- function(numberToTrunc, nDecimals){
numberToTrunc <- numberToTrunc + (10^-(nDecimals+5))
splitNumber <- strsplit(x=format(numberToTrunc, digits=20, format=f), split="\\.")[[1]]
  decimalPartTrunc <- substr(x=splitNumber[2], start=1, stop=nDecimals)
  truncatedNumber <- as.numeric(paste0(splitNumber[1], ".", decimalPartTrunc))
  return(truncatedNumber)
}
print(trunc_number_n_decimals(9.1762034354551236, 6), digits=14)
[1] 9.176203
print(trunc_number_n_decimals(9.1762034354551236, 7), digits=14)
[1] 9.1762034
print(trunc_number_n_decimals(9.1762034354551236, 8), digits=14)
[1] 9.17620343
print(trunc_number_n_decimals(9.1762034354551236, 9), digits=14)
[1] 9.176203435

This solution is very handy in cases when its necessary to write to a file the number with many decimals, such as 16. Just remember to convert the number to string before writing to the file, using format()

numberToWrite <- format(trunc_number_n_decimals(9.1762034354551236, 9), digits=20)