HP-15c: Why so popular?

[Johnh]

As an aside, I had a TI58 that I bought in 1979. It was powerful and useful and the module idea was clever. When you engaged a module program, it activated a series of soft keys across the top to give very easy control of it. The free Master Library module had genuinely useful features including solving a function, Simpson's integration of a function, or of discreet inputs , triangles etc..here is the full list, all included in the one free module:

ML-01
Master Library Diagnostic
ML-02
Matrix Inversion, Determinants and Simultaneous Equations
ML-03
Matrix Addition and Multiplication
ML-04
Complex Arithmetic
ML-05
Complex Functions
ML-06
Complex Trigonometric Functions
ML-07
Polynomial Evaluation
ML-08
Zeros of Functions
ML-09
Simpson’s Approximation (Continuous)
ML-10
Simpson's Approximation (Discrete)
ML-11
Triangle Solution (1)
ML-12
Triangle Solution (2)
ML-13
Curve Solution
ML-14
Normal Distribution
ML-15
Random Number Generator
ML-16
Combinations, Permutations And Factorials
ML-17
Moving Averages
ML-18
Compound Interest
ML-19
Annuities
ML-20
Days Of The Week, Days Between Dates
ML-21
HI-LO Game
ML-22
Checking/Savings Account
ML-23
DMS Operations
ML-24
Unit Conversions (1)
ML-25
Unit Conversions (2)

I found it fun to write programs for. I remember writing one for tic-tac-toe (noughts and crosses) that could not be beaten.

I used this for all my university engineering and the first years of my career until 1986. In my final Structures exam at Cambridge, I used the 'Zeros of Functions' and the 'Simultaneous Equations' routines to complete my exam. I'm quite sure the professors in 1982 had no idea that a student might have access to such a useful device!

But, after 8 years, if was worn out and not reliable. It just asn't an HP!

The ti5x emulator on Android has all the features, including the Module. It's fun to remember it!