PROGRAM d2r4(input,output,dfile);
(* driver for routine TRIDAG *)
LABEL 10,99;
CONST
   np=20;
TYPE
   glnarray = ARRAY [1..np] OF real;
VAR
   k,n: integer;
   diag,superd,subd,rhs,u: glnarray;
   dfile : text;

(*$I MODFILE.PAS *)
(*$I TRIDAG.PAS *)

BEGIN
   glopen(dfile,'matrx2.dat');
10:   readln(dfile);
   readln(dfile);
   readln(dfile,n);
   readln(dfile);
   FOR k := 1 to n-1 DO read(dfile,diag[k]);
   readln(dfile,diag[n]);
   readln(dfile);
   FOR k := 1 to n-2 DO read(dfile,superd[k]);
   readln(dfile,superd[n-1]);
   readln(dfile);
   FOR k := 2 to n-1 DO read(dfile,subd[k]);
   readln(dfile,subd[n]);
   readln(dfile);
   FOR k := 1 to n-1 DO read(dfile,rhs[k]);
   readln(dfile,rhs[n]);
(* carry out solution *)
   tridag(subd,diag,superd,rhs,u,n);
   writeln ('the solution vector is:');
   FOR k := 1 to n-1 DO write(u[k]:12:6);
   writeln(u[n]:12:6);
(* test solution *)
   writeln ('(matrix)*(sol''n vector) should be:');
   FOR k := 1 to n-1 DO write(rhs[k]:12:6);
   writeln(rhs[n]:12:6);
   writeln ('actual result is:');
   FOR k := 1 to n DO BEGIN
      IF (k = 1) THEN BEGIN
         rhs[k] := diag[1]*u[1] + superd[1]*u[2]
      END ELSE IF (k = n) THEN BEGIN
         rhs[k] := subd[n]*u[n-1] + diag[n]*u[n]
      END ELSE BEGIN
         rhs[k] := subd[k]*u[k-1] + diag[k]*u[k]
         + superd[k]*u[k+1]
      END
   END;
   FOR k := 1 to n-1 DO write(rhs[k]:12:6);
   writeln(rhs[n]:12:6);
   writeln ('***********************************');
   IF eof(dfile) THEN GOTO 99;
   writeln ('press return for next problem:');
   readln;
   GOTO 10;
99:   close(dfile)
END.