title: Using predicate to find if string value exists
body: <p>I have a NSMutableArray of objects with tag attributes assigned to them.  I am trying to find if any of these objects have a tag assigned to them of the string value "a".  </p>
  
  <p>my code so far:</p>
  
  <pre><code>for (Object *object in self.array)
      {
          NSDictionary *attrs = [object propertyValue:@"attrs"];
          NSString *tag = attrs[@"tag"];
      }
  </code></pre>
  
  <p>Can I do something like: </p>
  
  <pre><code>NSPredicate *predicate = [NSPredicate predicateWithFormat:@"a"];
  NSArray *results = [self.array filteredArrayUsingPredicate:predicate];
  </code></pre>
  
  <p>not sure how it works?</p>