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Ancient history

A long time ago, many years ago, in a second-hand bookstore, I bought a book by P. Grogono "Programming in Pascal" . I have a mathematical education, and for teaching programming (which was not the main subject) we used the ALGOL-68 language, so Pascal, being an Algol-like language, was relatively simple and understandable for me. I happened to use this book in practice at one of the first places of work, when I needed to create a system for approximating one-dimensional curves using the Least Squares Method.(OLS). Among others, there was a need for the possibility of representing the approximating curves in the form of a linear combination of arbitrary basis functions. One of the examples in Grogono's book was a program that implements a simple calculator, the input of which was a string containing an expression of numbers, operation signs, and parentheses, and the output was the numerical value of this expression:

Based on this program, I wrote an expression (formula) interpreter that allowed using simple variables such as X or Y in an expression, as well as elementary functions such as SIN (X). Having set the values of the variables and having a syntactically correct formula in the form of a string, it was possible to calculate the numerical value of this formula. The system itself made it possible to set a set of basis functions such as sin (x), cos (x), sin (2x), cos (2x), ... sin (10x), cos (10x), ... or x, x ^ 3, x ^ 5, x ^ 7, ... , construct an OLS approximation based on it, and then display the results (points of the original curve and approximation) in the form of graphs. All this was written in Pascal for DCK computers.

The next event was the purchase of the book "Solving Ordinary Differential Equations: Nonrigid Problems", authors: Hairer E., Nørsett S., Wanner G., (). FORTRAN, . , , . : , , ? ( PHASER), , . , , («») . , , . , ( ) . PC AT 286 ( FORTRANA’a) , .