I can usually appreciate Apple's design homages. The original iOS Calculator is inspired by the Braun ET, and it makes some sense to provide a familiar design for basic use.
But it inherits baggage from the limitations of the handheld calculators of the 1970s. Why can't I use the - button to write a negative sign? What does "AC" mean? The scientific calculator is an even worse design. There's a ton of invisible state, like the value stored in memory, or whether you're inside parentheses. The user has to hold the whole sequence of operations in their head, without clicking a single wrong button. Want to repeat a calculation with a different operand? Tough.
Graphing calculators like the TI-84 that let you see and edit your input are so much more usable. Even better are notebook-style interfaces like Mathematica. It's a shame Apple won't pay homage to those designs.
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