Add characters to a numeric column in dataframe

Question

I have a dataframe like this:

  V1      V2      V3 
1  1 3423086 3423685 
2  1 3467184 3467723 
3  1 4115236 4115672 
4  1 5202437 5203057 
5  2 7132558 7133089 
6  2 7448688 7449283 

I want to change the V1 column and add chr before the number. Just like this:

  V1      V2      V3 
1  chr1 3423086 3423685 
2  chr1 3467184 3467723 
3  chr1 4115236 4115672 
4  chr1 5202437 5203057 
5  chr2 7132558 7133089 
6  chr2 7448688 7449283 

Is there a way to do this in R?

Answer 1

The regex pattern "^" (outside any character-class brackets) represents the point just before the first character of a "character"-class item (aka "string" in other computer languages). This just replaces the beginning of each "character" element in vector with a stem of "chr". It implicitly coerces a "numeric" input value to "character" so alters the mode of the result.

> dat$V1 <- sub("^", "chr", dat$V1 )
> dat
    V1      V2      V3
1 chr1 3423086 3423685
2 chr1 3467184 3467723
3 chr1 4115236 4115672
4 chr1 5202437 5203057
5 chr2 7132558 7133089
6 chr2 7448688 7449283

Could, of course, have used paste("chr", dat$V1, sep=""), but I thought a regex solution might be neater.

Answer 2

sprintf is a lot more powerful than plain concatenation.

dat$V1 <- sprintf('chr%i', dat$V1)

Answer 3

We can also use interaction:

df$V1 <- interaction( "chr", df$V1, sep = "")
df

Or using sqldf:

library(sqldf)    
df$V1 <- as.character(df$V1)
df$V1 <- sqldf("select 'chr'|| V1 as V1 from df")