title: Add value exists in the key - Flutter
body: <p>I have the following code and I would like to add to the value already exists in the key:</p>
  <pre><code> final dataMap = &lt;String, double&gt;{};
   json.forEach((myKey) =&gt; {
      dataMap.putIfAbsent(myKey[&quot;sigla&quot;], () =&gt; 1),
   });
  </code></pre>
  <p>In the example above, I am assigning 1, but if the key already exists and its value was 2, I would like to add 1 more, that is, the value would be 3.</p>
  <p>How to do this?</p>
  <p>Thanks.</p>