I have written the sentence
One difficulty when performing a numerical integral arises when one does not know the exact function being used (for example, in signal processing), but is instead cumulatively calculating data points that are then fed into an integral function.
"Is" sounds incorrect here; what should the verb be? I tried "are", but that doesn't sound right either. Would it be better to split up the sentence?
In short, how can I formulate this so that it is grammatically correct?