关于欧拉梯形法计算数值微分

Question

tixingfa[x_, y_] := Module[{h = 0.1, k, y0 = y, x0 = x, y1, y2, eps = 0.00001, a = 0},
  For[k = 0, k <= 9,
   x0 = a + h k;
   y1 = y0 + h f[x0, y0];
   y2 = y0 + h/2*(f[x0, y0] + f[x0 + h, y1]);
   While[Abs[y2 - y1] >= eps,
    y1 = y2; y2 = y0 + h/2*(f[x0, y0] + f[x0 + h, y1])]; Print["y2=    ", y2]; y0 = y2, 
   k = k + 1]; y2]
f[x_, y_] := y - 2 x/y

tixingfa[0, 1]

(*
y2=    1.07516
y2=    1.14107
y2=    1.19838
y2=    1.24719
y2=    1.2872
y2=    1.31772
y2=    1.33755
y2=    1.34494
y2=    1.33737
y2=    1.31112
1.31112
*)

下面是用VB编的：

Sub tixingfa ()                                *VB编程实现*
 Dim y1 As Double, y0 As Double, h As Double, x0 As Double, k As Integer, n As Integer, \
y As Double
h = 0.1 : a = 0 : b = 1 : eps = 0.000001 : y0 = 1 : x0 = 0
k = (b - a)/h
Text1.Text = "n" & "          " & "x0" & "   " & "yn" & vbCrLf
For n = 0 To k - 1
x0 = a + n*h
y1 = y0 + h*f (x0, y0)
y = y0 + h/2*(f (x0, y0) + f (x0 + h, y1))
Do While Abs (y - y1) >= eps
y1 = y
y = y0 + h/2*(f (x0, y0) + f (x0 + h, y1))
Loop
Text1.Text = 
 Text1.Text & n & "       " & Format (x0, "##0.000#########") & "   " & y & vbCrLf
y0 = y
Next
End Sub

但是mathematica实现后，结果不一样，VB的答案正确，想知道哪里错了

Answer 1

请仔细查看For的帮助文档里的语法使用。

For[start,test,incr,body]
executes start, then repeatedly evaluates body and incr until test fails to give True.

其中incr位于第三位置。

Comments

是我粗心，非常感谢，问题解决了。