R: Aggregate character strings
<p>I have a data frame <code>ModelDF</code> having columns with numeric as well as character values like:</p>

<pre><code>Quantity        Type        Mode        Company
   1            Shoe        hello        Nike
   1            Shoe        hello        Nike
   2            Jeans       hello        Levis
   3            Shoe        hello        Nike
   1            Jeans       hello        Levis
   1            Shoe        hello        Adidas
   2            Jeans       hello        Spykar
   1            Shoe        ahola        Nike
   1            Jeans       ahola        Levis
</code></pre>

<p>I have to aggregate it in this form</p>

<pre><code>Quantity        Type        Mode        Company
   5            Shoe        hello        Nike
   3            jeans       hello        Levis
   1            Shoe        hello        adidas
   2            jeans       hello        Spykar
   1            Shoe        ahola        Nike
   1            jeans       ahola        Levis
</code></pre>

<p>i.e. I have to aggregate and sum Quantity if all other columns are same.</p>

<p>I have tried it using <code>aggregate</code> but as it doesn't work on character values it is giving me wrong results.</p>

<p>What are my options?
Thanks</p>