merging 2 columns followed by making a new column in a data.frame in R
<p>I want to ask, how can I make a new column in a data.frame  in R by merging 2 columns (with categorical values) with all rows in the file. Following example is showing alternate categorical conditions in <code>column_1</code> and <code>column_2</code> how could I make <code>column_3</code> by merging <code>columns_1</code> and <code>column_2</code>? Moreover, in 4th row both columns have values, so in this case what would be in <code>column_3</code>. (<code>NA</code> are blanks).</p>

<pre><code>Column_1 Column_2  Column_3
1          NA
NA          2
2           NA
NA          1
1           2
</code></pre>