The Trabb Pardo-Knuth algorithm is a program introduced by Donald Knuth and Luis Trabb Pardo to illustrate the evolution of computer programming languages.

In their 1980 work "The Early Development of Programming Languages", Trabb Pardo and Knuth introduced a trivial program which involved arrays, indexing, mathematical functions, subroutines, I/O, conditionals and iteration. They then wrote implementations of the algorithm in several early programming languages to show how such concepts were expressed.

The simpler Hello world program has been used for much the same purpose.

The algorithm

In ALGOL:

   begin integer i; real y; real array a[0:10];
   real procedure f(t); real t; value t;
     f := sqrt(abs(t))+5*t^3;
   for i := 0 step 1 until 10 do read(a[i]);
   for i := 10 step -1 until 0 do
     begin y := f(a[i]);
           if y > 400 then write(i, "TOO LARGE")
           else write(i,y);
     end
   end

The algorithm reads eleven numbers from an input device, stores them in an array, and then processes them in reverse order, applying a user-defined function to each value and reporting either the value of the function or a message to the effect that the value has exceeded some threshold.

Perl golf

The following Perl implementation is 147 bytes. It is shown here on several lines due to page width considerations, but it could be run as a one-liner.

print map$_.$/,reverse map"@$_",
map[$$_[0],$$_[1]>400?'TOO LARGE':$$_[1]],
map[$$_[0],sub{5*$_[0]**3+sqrt abs$_[0]}->($$_[1])],
map[$_+0,$_=<>],0..10

References

• "The Early Development of Programming Languages" in A History of Computing in the Twentieth Century, New York, Academic Press, 1980. ISBN 0124916503

External link

• Implementations in several modern languages