# History and previous versions

## PCalc

Our first program (1993) replacing tables of probability distributions, was running on the command line under MS-DOS. It was written in Pascal by Sytse Knypstra and Arjen Merckens. In fact it was a combination of a calculator and a replacement of probability tables.

### Installation

Download the compressed file PCalc.zip (50 kB), unzip it and start PCalc.exe under the program DOSBox.

## PQRS 2

Around the year 2000 PQRS (Probabilities, Quantiles and Random Samples) was designed as a successor to PCalc with a graphical user interface. PQRS was written in Delphi and runs under Windows and - with the help of Wine - under Linux.

### Installation

Download the compressed file PQRS.zip (0.7 MB), unzip it and start PQRS.exe.

## PyQRS 3

In 2013 PQRS was rewritten in Python. In order to distinguish it from the previous PQRS version, and as a tribute to Python and its author, Guido van Rossem, the first letter in its name was replaced by 'Py'.

## PyQRS 4.0

PyQRS 4.0 (March 2016) is functionally equivalent to its predecessor PyQRS 3. It runs on Android (mobile) devices. The app is written in Python using the Kivy framework. For PyQRS 4.0 we could not use Scipy as we did for PyQRS 3. The probability (density) functions and the cumulative distribution functions were therefore programmed in Python. The code could easily be adapted from the Delphi code that we used earlier for PQRS 2.

## PyQRS 4.1

PyQRS 4.1 (June 2017) has an improved user interface over its predecessor PyQRS 4.0. It runs on Android (mobile) devices, and on the Linux (64 bits) and Windows (64 bits) platforms.
Screenshots can be found under Using PyQRS.