How to conditionally drop colum from R data frame

Question

What is a good way to conditionally drop a data frame column based on the sum of its values?

For example, in the following data frame, I want to drop all columns where the sum of the values are zero.

df = data.frame(Dum1=c(0,0,0,1,0,0,0,0,0,0),
Dum2=c(0,0,0,0,0,0,0,0,0,0),
Dum3=c(0,0,0,1,0,1,0,0,0,0),
Dum4=c(0,0,0,0,0,0,0,0,0,0))

colSums(as.matrix(df))

Dum1 Dum2 Dum3 Dum4 
1    0    2    0 

Dum2 and Dum4 are all zeros, so I would like to drop them. Unfortunately, in my application I will not know in advance which columns sum to zero or I could drop them using something like this:

df$Dum2 <- NULL
df$Dum4 <- NULL

str(df)

'data.frame':   10 obs. of  2 variables:
 $ Dum1: num  0 0 0 1 0 0 0 0 0 0
 $ Dum3: num  0 0 0 1 0 0 0 0 0 0

Any assistance would be greatly appreciated

Answer 1

df0 <- df[, colSums(df) != 0]

Answer 2

df[,which(colSums(df) == 0)] <- list(NULL)