One reason why you shouldn't use a calculator on a mobile phone

[Garth Wilson]

+1 for the slide-rule days. While in the process of getting the detailed result on the slide rule, you had to get a rough idea in your head so you at least got the decimal point in the right place. No room for GIGO. Not being able to do that is a common problem with kids trained on calculators before they can do the calculation by hand or be able to get a close approximation in their heads. One of my beefs with using a phone as a calculator though is that the phone only lasts a couple of years, whereas I've had a few of my most-often-used programs in my HP-41 for 25+ years, continuously, without ever re-loading. And since programming is an important part of my calculator usage, it is important to have the same programming language and environment for many years to really get good at it and not have to spend time over and over to learn new ones.