Checking for identical columns in a data frame in R

You could use all():

> data <- data.frame(A=c(1,2,3,4), B=c(1,2,3,4))
> all(data$A == data$B)
[1] TRUE

You could use identical

identical(DT[['A']],DT[['B']])

This might be overkill for your problem, but you may also want to look into compare() from the "compare" package. Consider the following examples:

> data <- data.frame(A = c(1, 2, 3, 4), B = c(1, 2, 3, 4))
> compare(data[1], data[2]) ## Should be false
FALSE [TRUE]
> compare(data[1], data[2], ignoreNames = TRUE) # Allow different names
TRUE
  dropped names
> data <- data.frame(A = c(1, 2, 3, 4), B = as.character(c(1, 2, 3, 4)))
> str(data) ## "B" is now a factor, so use `coerce` to test for equality
'data.frame':   4 obs. of  2 variables:
 $ A: num  1 2 3 4
 $ B: Factor w/ 4 levels "1","2","3","4": 1 2 3 4
> compare(data[1], data[2], ignoreNames = TRUE, coerce = TRUE)
TRUE
  [A] coerced from <factor> to <numeric>
  dropped names

There is a general logical argument, allowAll, that can be set to TRUE to allow the compare function to try different transformations to test for equality.