Скачать Numerical Integration by the Simpson Method

program simp1;      { -> 273 }
{ integration by Simpson's method }
 
const   tol      = 1.0E-4;
var   sum,upper,lower   : real;
 
external procedure cls;
 
function fx(x: real): real;
{ find f(x)=1/x }
{ watch out for x=0 }
 
begin
  fx:=1.0/x
end;   { function fx }
 
procedure simps(function f(x: real): real;
      lower,upper,tol   : real;
      var sum      : real);
 
{ numerical integration by Simpson's rule }
{ function is f (as paramater), limits are lower and upper }
{ with number of regions equal to pieces }
{ partition is delta_x, answer is sum }
 
var   i      : integer;
   x,delta_x,even_sum,
   odd_sum,end_sum,
   sum1      : real;
   pieces      : integer;
begin
  pieces:=2;
  delta_x:=(upper-lower)/pieces;
  odd_sum:=f(lower+delta_x);
  even_sum:=0.0;
  end_sum:=f(lower)+f(upper);
  sum:=(end_sum+4.0*odd_sum)*delta_x/3.0;
  writeln(pieces:5,sum);
  repeat
    pieces:=pieces*2;
    sum1:=sum;
    delta_x:=(upper-lower)/pieces;
    even_sum:=even_sum+odd_sum;
    odd_sum:=0.0;
    for i:=1 to pieces div 2 do
      begin
   x:=lower+delta_x*(2.0*i-1.0);
   odd_sum:=odd_sum+f(x)
      end;
    sum:=(end_sum+4.0*odd_sum+2.0*even_sum)*delta_x/3.0;
    writeln(pieces:5,sum)
  until abs(sum-sum1)<=abs(tol*sum)
end;   { simps }
 
begin      { main program }
  cls;
  lower:=1.0;
  upper:=9.0;
  writeln;
  simps(fx,lower,upper,tol,sum);
  writeln;
  writeln(chr(7),'area= ',sum)
end.