In tMap, you can use the column expression like this: Var.a<Var.b?myValue1:myValue2
Use a routine and code your while into, then call this routine from tMap.
Here are online documentations about TalendHelpCenter:tMap scenarios
What's your job requirement about while in tMap? Does the If / else statement work for you?
Introduction to Talend Open Studio for Data Integration.
Practical steps to developing your data integration strategy.
Create systems and workflow to manage clean data ingestion and data transformation.