How to resolve the algorithm History variables step by step in the ALGOL W programming language

Published on 12 May 2024 09:40 PM
#Algol w

How to resolve the algorithm History variables step by step in the ALGOL W programming language

Table of Contents

Problem Statement

Storing the history of objects in a program is a common task. Maintaining the history of an object in a program has traditionally required programmers either to write specific code for handling the historical data, or to use a library which supports history logging. History variables are variables in a programming language which store not only their current value, but also the values they have contained in the past. Some existing languages do provide support for history variables. However these languages typically have many limits and restrictions on use of history variables.

[http://www.bod.com/index.php?id=3435&objk_id=148050 "History Variables: The Semantics, Formal Correctness, and Implementation of History Variables in an Imperative Programming Language" by Mallon and Takaoka] Concept also discussed on LtU and Patents.com. Demonstrate History variable support: For extra points, if the language of choice does not support history variables, demonstrate how this might be implemented.

Let's start with the solution:

Step by Step solution about How to resolve the algorithm History variables step by step in the ALGOL W programming language

Source code in the algol programming language

begin
    % implements integer history variables                                 %
    % similar history types could be defined for other types of variable   %
    record HInteger ( integer iValue; reference(HInteger) iPrev );
    % sets the latest value of hv to n                                     %
    procedure setIhv ( reference(HInteger) value result hv; integer value n ) ; hv := HInteger( n, hv );
    % declare an integer history variable                                  %
    reference(HInteger) hv;
    % initialise the history to a null value                               %
    hv := null;
    % assign three values                                                  %
    setIhv( hv, 1 );
    setIhv( hv, 2 );
    setIhv( hv, 3 );
    % show the history of hv                                               %
    begin
        reference(HInteger) h;
        write( "hv history: " );
        h := hv;
        while h not = null do begin
            writeon( i_w := 3, s_w := 0, iValue(h), " " );
            h := iPrev(h)
        end while_h_ne_null
    end;
    % remove the values from hv, summing them as in the Ada sample         %
    begin
        integer s;
        s := 0;
        while hv not = null do begin
            s  := s + iValue(hv);
            hv := iPrev(hv)
        end while_hv_ne_null ;
        write( "Sum of the historic values: ", s )
    end
end.

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