title: Calculate the dependency standard deviation in R
body: <p>I want to calculate the standard deviation in R. But the standard function "sd(x)" is not the function that I need. I'm looking for a function to calculate the sd(x, in dependency of another variable in my dataframe). So that I can add a new column with the sd by the dependency variable (image).
  Like this:</p>
  
  <pre><code>image   answer    sd
  a       1         0,70
  a       2         0,70
  b       2         2,12
  b       5         2,12
  </code></pre>