I want to find all the names of columns with `NA` or missing data and store these column names in a vector. 

```
# create matrix
a <- c(1,2,3,4,5,NA,7,8,9,10,NA,12,13,14,NA,16,17,18,19,20)
cnames <- c("aa", "bb", "cc", "dd", "ee")
mymatrix <- matrix(a, nrow = 4, ncol = 5, byrow = TRUE)
colnames(mymatrix) <- cnames
mymatrix
#      aa bb cc dd ee
# [1,]  1  2  3  4  5
# [2,] NA  7  8  9 10
# [3,] NA 12 13 14 NA
# [4,] 16 17 18 19 20

```

The desired result: columns `"aa"` and `"ee"`.

My attempt:

```
bad <- character()
for (j in 1:4){     
  tmp <- which(colnames(mymatrix[j, ]) %in% c("", "NA"))
  bad <- tmp
}

```

However, I keep getting `integer(0)` as my output. Any help is appreciated.