extract rows for which first non-zero element is one - r

Question

I would like to extract every row from the data frame my.data for which the first non-zero element is a 1.
my.data <- read.table(text = '
x1 x2 x3 x4
0 0 1 1
0 0 0 1
0 2 1 1
2 1 2 1
1 1 1 2
0 0 0 0
0 1 0 0
', header = TRUE)
my.data
desired.result <- read.table(text = '
x1 x2 x3 x4
0 0 1 1
0 0 0 1
1 1 1 2
0 1 0 0
', header = TRUE)
desired.result
I am not even sure where to begin. Sorry if this is a duplicate. Thank you for any suggestions or advice.

Answer 1

Here's one approach:
# index of rows
idx <- apply(my.data, 1, function(x) any(x) && x[as.logical(x)][1] == 1)
# extract rows
desired.result <- my.data[idx, ]
The result:
x1 x2 x3 x4
1 0 0 1 1
2 0 0 0 1
5 1 1 1 2
7 0 1 0 0

Answer 2

Probably not the best answer, but:
rows.to.extract <- apply(my.data, 1, function(x) {
no.zeroes <- x[x!=0] # removing 0
to.return <- no.zeroes[1] == 1 # finding if first number is 0
# if a row is all 0, then to.return will be NA
# this fixes that problem
to.return[is.na(to.return)] <- FALSE # if row is all 0
to.return
})
my.data[rows.to.extract, ]
x1 x2 x3 x4
1 0 0 1 1
2 0 0 0 1
5 1 1 1 2
7 0 1 0 0

Answer 3

Use apply to iterate over all rows:
first.element.is.one <- apply(my.data, 1, function(x) x[x != 0][1] == 1)
The function passed to apply compares the first [1] non-zero [x != 0] element of x to == 1. It will be called once for each row, x will be a vector of four in your example.
Use which to extract the indices of the candidate rows (and remove NA values, too):
desired.rows <- which(first.element.is.one)
Select the rows of the matrix -- you probably know how to do this.
Bonus question: Where do the NA values mentioned in step 2 come from?