user-defined function and for loop

Question

I would like to create a user-defined function with a for-loop that adds to each column name the number of the column in order to add clarity to my data frame.

Here is the code (but doesn't work):

addncol = function(df)
   {
    for(i in 1:ncol(df))
       {
        names(df)[[i]] <- paste(names(df)[[i]], i, sep = '_')
       }
   }

The for-loop alone works well but in the function it doesn't work (I have no error message though).

Answer 1

The function doesn't return the data frame df . It will work if you add df as the last line of your function.

However, a simpler version without loops is a vectorized approach with setNames :

addncol <- function(df)
   setNames(df, paste(names(df), seq(df), sep = "_"))

Answer 2

You could use data.tables for this, which updates by reference using setnames(...) (note the different spelling - it's a different function).

library(data.table)
DT <- as.data.table(mtcars)   # example only
head(DT)
mpg cyl disp  hp drat    wt  qsec vs am gear carb
# 1: 21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
# 2: 21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
# 3: 22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
# 4: 21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
# 5: 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
# 6: 18.1   6  225 105 2.76 3.460 20.22  1  0    3    1
addncol <- function(DT) setnames(DT,paste(names(DT),seq(DT),sep="_"))
addncol(DT)
head(DT)
#    mpg_1 cyl_2 disp_3 hp_4 drat_5  wt_6 qsec_7 vs_8 am_9 gear_10 carb_11
# 1:  21.0     6    160  110   3.90 2.620  16.46    0    1       4       4
# 2:  21.0     6    160  110   3.90 2.875  17.02    0    1       4       4
# 3:  22.8     4    108   93   3.85 2.320  18.61    1    1       4       1
# 4:  21.4     6    258  110   3.08 3.215  19.44    1    0       3       1
# 5:  18.7     8    360  175   3.15 3.440  17.02    0    0       3       2
# 6:  18.1     6    225  105   2.76 3.460  20.22    1    0       3       1