Mutate data conditionally in dplyr
<p>I am trying to mutate data by dividing based on an identifier.
For example, I would like to mutate <code>mass</code> in the following data.  If it is depth 10, I would like to divide by 2.  If it is depth 20, I would like to divide by 3.</p>

<pre><code>day    year    depth    mass 
1      2008    10       13
2      2008    10       15
1      2008    20       14
2      2008    20       12
1      2009    10       14
2      2009    10       16
1      2009    20       12
2      2009    20       18   
</code></pre>

<p>Division leads to:</p>

<pre><code>day    year    depth    mass 
1      2008    10      6.5
1      2008    10      6.5
2      2008    10      7.5
2      2008    10      7.5
1      2008    20      4.6
1      2008    20      4.6
2      2008    20      4
2      2008    20      4
1      2009    10      7
1      2009    10      7
2      2009    10      8
2      2009    10      8   
1      2009    20      4
1      2009    20      4  
2      2009    20      6 
2      2009    20      6  
</code></pre>

<p>I am trying <code>ifelse</code> in the following way, but get an error "unused arguments(c(13, 15, 14. . .)"</p>

<pre><code>df%&gt;% 
  group_by(day, year, depth) %&gt;% 
  bind_rows(., .) %&gt;%
  mutate(mass = ifelse(depth == 10), mass/2,
         ifelse(depth == 20), mass/3)%&gt;%
  arrange(day, year, depth, mass)
</code></pre>