How to get a value from the output of a operator in RapidMiner?

You can use macro's to achieve this. You may need to add some extra operators but this would be one possible way to achieve this : 

After your aggregate add an additional filter operator, and filter on 'class=outlier', you can keep the rest of the process as is, just use the original output of the filter operator to continue your current process.

On the filter output you now add an extract macro operator, select data value as type and set example index to 1. Under attribute name you set 'class' and name the macro like outlier. This basically stores the value of the filtered class (in this case your outlier) and keeps it in memory.

Now you can change your expression to if(id<%{outlier},"outlier","normal") and the value will automatically change without further human impact needed.

Note that it is important that your macro process runs before your expression is called.

Sorry, I wrongly click one button.

Click the icon with the blue box and transparant boxes behind them. These show your order. Now you can select your operators and define the order (use right click for contextual info). It's getting a bit used to but once you get the trick it's pretty easy to set your order.

@kayman @liming,
alternatively, I love to add "Delay" operators with a Delay of 0. It has port extenders on both sides and you can thus ensure that an operator needs to be executed before. Kind of handy trick I learned from @Edin_Klapic.
Best,
Martin

@mschmitz Thanks for the praise
In case you have an Operator which has no input port (like the Retrieve Operator) you can put it into a subprocess which basically has the same functionality. Delay has the advantage of being able to stay in the same "layer".