How to check whether a column contains only identical elements in R?

Question

Example data:

x <- matrix(c("Stack","Stack","Stack",
              "Overflow","Overflow","wolfrevO"),
            nrow=3,ncol=2)

How can I check whether x[,1] contains entirely identical elements?

If x contains NAs, does this method still apply?

Thanks

Answer 1

You count the unique elements of the column:

length(unique(x[,1]))==1

works even if there are NA's in your data.

For checking every column use:

apply(x, 2, function(a) length(unique(a))==1)

Answer 2

You can compare the vector's first value to the rest of the vector.

all(x[-1, 1] == x[1, 1])
# [1] TRUE

If NA values are present, then this exact method does not still apply. However, it can be easily rectified by using na.omit(). For example -

## create a vector with an NA value
x2 <- c(x[, 1], NA)

## standard check returns NA
all(x2 == x2[1])
# [1] NA

## call na.omit() to remove, then compare
all(na.omit(x2) == x2[1])
# [1] TRUE

So then, with your matrix x, this last line would become

all(na.omit(x[-1, 1]) == x[1, 1])