Converting factor variables to numeric ones by replacing blanks cells to NA or changing them to "." [duplicate]

1. I want to convert some factor variables to numeric variables by this code:

    df$col <- as.numeric(df$col)

2. The missing values in my dataset are not represented by a dot (i.e., "."). Instead, they are blank cells.

3. Therefore, the above code allocates a number (i.e., "1") to each blank cells in my dataset.

4. My question is how I can convert factor variables to numeric ones by replacing blanks cells to NA or changing them to ".".

Thank you so much.