title: Get all elements in a list where the value is equal to certain value
body: <p>I have a list which looks like this:</p>
  
  <pre><code>[[3, 4.6575, 7.3725], 
  [3, 3.91, 5.694],
  [2, 3.986666666666667, 6.6433333333333335],
  [1, 3.9542857142857137, 5.674285714285714],....]
  </code></pre>
  
  <p>I would like to sum (in fact take the mean ... but it is a detail) all the values of the rows together where the value of the first element are equal. This would mean that in the example above the first two rows would be summed together.</p>
  
  <pre><code>[[3, 8.5675, 13.0665],
  [2, 3.986666666666667, 6.6433333333333335],
  [1, 3.9542857142857137, 5.674285714285714],....]
  </code></pre>
  
  <p>This means the first values should be unique.</p>
  
  <p>I thought of doing this by finding all the "rows" where the first value is equal to for example to 1 and sum them together. My question is now, how can I find all the rows where the first value is equal to a certain value.</p>