Pad with leading zeros to common width [duplicate]

Question

I am handling a DB with hour format like:

HOUR ID 1  2 10 4 5  6 20 6 

I would like to place a zero in the value with 1 character and store them in a new column named NHOUR, like:

NHOUR HOUR ID 01 1  2 10 10 4 05 5 6 20 20 6 

Until now I am struggling with something like (I follow some suggestions already provided for ifelse in the forum) :

DB$NHOUR<-with(DB,ifelse(nchar(HOUR,type="chars")==1),sprintf("%02d",HOUR),as.numeric(HOUR)) 

but without any success! R always reports "yes" element is not specified, etc.

As always, any tips is appreciated!

Answer 1

Simply following the advise in @joran's comment,

DB <- data.frame( HOUR  = c(1, 10, 5, 20), ID  = c(2, 4, 6, 6))  NHOUR <- sprintf("%02d",DB$HOUR) # fix to 2 characters   cbind(NHOUR, DB) # combine old and newdata    NHOUR HOUR ID 1    01    1  2 2    10   10  4 3    05    5  6 4    20   20  6 

Update 2013-01-21 23:42:00Z Inspired by daroczig's performance test below, and because I wanted to try out the microbenchmark package, I've updated this question with a small performance test of my own comparing the three different solutions suggested in this thread.

# install.packages(c("microbenchmark", "stringr"), dependencies = TRUE) require(microbenchmark) require(stringr)  SPRINTF <- function(x) sprintf("%02d", x) FORMATC <- function(x) formatC(x, width = 2,flag = 0) STR_PAD <- function(x) str_pad(x, width=2, side="left", pad="0")  x <- round(runif(1e5)*10) res <- microbenchmark(SPRINTF(x), STR_PAD(x), FORMATC(x), times = 15)  ## Print results: print(res) Unit: milliseconds         expr       min        lq    median        uq      max 1 FORMATC(x) 623.53785 629.69005 638.78667 671.22769 679.8790 2 SPRINTF(x)  34.35783  34.81807  35.04618  35.53696  37.1622 3 STR_PAD(x) 116.54969 118.41944 118.97363 120.05729 163.9664  ### Plot results: boxplot(res) 

Answer 2

I like to use the stringr package:

DB$NHOUR <- str_pad(DB$HOUR, width=2, side="left", pad="0")