Get the previous item in Smarty

In a big shortcut, my code looks like this:

{foreach from=$raport key=thekey item=i name=itemnumber}

    <p>{$i->publicate_date|date_format:"%H:%M"}</p>

{/foreach}  

Now I would like to compare publicate dates - the current one with the previous one.

I understand that I should somehow use "-1", but no matter what I do, there's an error.

Can anyone give me a hint how to achieve it?

You could use the following code:

{assign var="prev" value=false}

{foreach from=$raport key=thekey item=i name=itemnumber}

    <p>{$i->publicate_date|date_format:"%H:%M"}</p>

    {if $prev neq false}
      {$prev->publicate_date} {$i->publicate_date} <-- here you can compare
    {/if} 

    {assign var="prev" value=$i}
{/foreach}