title: Perform subtraction of the index in a *ngFor
body: <p>How can I make a calculation using the index? I want to apply the class danger when the condition is true, but I need to calculate i-1 within *ngFor loop.</p>
  
  <pre><code>&lt;tr *ngFor="#org of data; #i = index"&gt;
      &lt;td [ngClass]="{danger: org.price &lt; data.stock[i-1].val }"&gt;
  &lt;/tr&gt;
  </code></pre>