I switched to an iPhone a few years ago and I really miss the Android calculator app. It showed the entire expression typed so far and its current value! In desperation, I have resorted to SSHing to my desktop and using `python` as a calculator.
With the added benefit of having to press not just one, but two buttons in order to add a "+" sign. First press "123", then press "#+=" and now you can add your complex mathematical characters :)
If I have to use the default keyboard to enter arithmetic expressions, I'd rather SSH to my own server. I'm not exactly sure of the privacy implications of typing things into Siri.