VERSION 5.00
Begin VB.Form Form1 
   Caption         =   "Form1"
   ClientHeight    =   5430
   ClientLeft      =   60
   ClientTop       =   345
   ClientWidth     =   4425
   LinkTopic       =   "Form1"
   ScaleHeight     =   5430
   ScaleWidth      =   4425
   StartUpPosition =   3  'Windows Default
   Begin VB.CommandButton Command1 
      Caption         =   "Command1"
      Height          =   375
      Left            =   2520
      TabIndex        =   0
      Top             =   4560
      Width           =   1335
   End
End
Attribute VB_Name = "Form1"
Attribute VB_GlobalNameSpace = False
Attribute VB_Creatable = False
Attribute VB_PredeclaredId = True
Attribute VB_Exposed = False
Private Sub Command1_Click()
    'PROGRAM D4R23
    'Driver for routine BESSI1
    Open "D:\VB常用數(shù)值算法集\DATA\FNCVAL.DAT" For Input As #1
    Do
      Line Input #1, Text$
    Loop While Text$ <> "Modified Bessel Function I1"
    Line Input #1, NVAL$
    NVAL = Val(NVAL$)
    Print Text$
    Print
    Print "    X         Actual         BESSI1(X)"
    For I = 1 To NVAL
        Line Input #1, DUM$
        X = Val(Mid(DUM$, 1, 6))
        Value = Val(Mid(DUM$, 7))
        Print Tab(4); Format$(X, "#0.##");
        Print Tab(12); Format$(Value, ".0000000E+00");
        Print Tab(28); Format$(BESSI1(X), ".0000000E+00")
    Next I
    Close #1
End Sub
Function BESSI1(X)
    P1 = 0.5:              P2 = 0.87890594
    P3 = 0.51498869:       P4 = 0.15084934
    P5 = 0.02658733:       P6 = 0.00301532
    P7 = 0.00032411
    Q1 = 0.39894228:       Q2 = -0.03988024
    Q3 = -0.00362018:      Q4 = 0.00163801
    Q5 = -0.01031555:      Q6 = 0.02282967
    Q7 = -0.02895312:      Q8 = 0.01787654
    Q9 = -0.00420059
    If Abs(X) < 3.75 Then
        Y = (X / 3.75) ^ 2
        AAA = Y * (P4 + Y * (P5 + Y * (P6 + Y * P7)))
        BESSI1 = X * (P1 + Y * (P2 + Y * (P3 + AAA)))
    Else
        AX = Abs(X)
        Y = 3.75 / AX
        AAA = Exp(AX) / Sqr(AX)
        BBB = Y * (Q5 + Y * (Q6 + Y * (Q7 + Y * (Q8 + Y * Q9))))
        BESSI1 = AAA * (Q1 + Y * (Q2 + Y * (Q3 + Y * (Q4 + BBB))))
    End If
End Function