Runge-Kutta method: Difference between revisions

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 =={{header|FutureBasic}}==
-<lang futurebasic>
+<lang futurebasic>window 1
-include "ConsoleWindow"
+def fn dydx( x as double, y as double ) as double = x * sqr(y)
-def tab 9
+def fn exactY( x as long ) as double = ( x ^2 + 4 ) ^2 / 16
+long i
-local fn dydx( x as double, y as double ) as double
+double h, k1, k2, k3, k4, x, y, result
-end fn = x * sqr(y)
-local fn exactY( x as long ) as double
-end fn = ( x ^2 + 4 ) ^2 / 16
-dim as long i
-dim as double h, k1, k2, k3, k4, x, y, result
 h = 0.1
 y = 1
 for i = 0 to 100
-x = i * h
+  x = i * h
-if x == int(x)
+  if x == int(x)
-result = fn exactY( x )
+    result = fn exactY( x )
-print "y("; mid$( str$(x), 2, len(str$(x) )); ") = "; y, "Error = "; result - y
+    print "y("; mid$( str$(x), 2, len$(str$(x) )); ") = "; y, "Error = "; result - y
-end if
+  end if
-k1 = h * fn dydx( x, y )
+  k1 = h * fn dydx( x, y )
-k2 = h * fn dydx( x + h / 2, y + k1 / 2 )
+  k2 = h * fn dydx( x + h / 2, y + k1 / 2 )
-k3 = h * fn dydx( x + h / 2, y + k2 / 2 )
+  k3 = h * fn dydx( x + h / 2, y + k2 / 2 )
-k4 = h * fn dydx( x + h,     y + k3     )
+  k4 = h * fn dydx( x + h,     y + k3     )
+  y = y + 1 / 6 * ( k1 + 2 * k2 + 2 * k3 + k4 )
+next
+HandleEvents</lang>
-y = y + 1 / 6 * ( k1 + 2 * k2 + 2 * k3 + k4 )
-next
-</lang>
 Output:
 <pre>