'   program DIFF-LI2.bas
 'nomainwin

 DIM C(102,102)
 OPEN "c:\kpdat" FOR OUTPUT AS #1
 WindowWidth = 590 : WindowHeight = 500'470 270
 '                               left top length hight
 StaticText #w.1,            "A:"             ,140, 10,  45, 20
 StaticText #w.2,            "B:"             ,200, 10,  45, 20
 StaticText #w.3,            "C:"             ,260, 10,  45, 20
 StaticText #w.4,            "D:"             ,320, 10,  45, 20
 STATICTEXT #w.5,            "E:"             ,380, 10,  45, 20
 staticText #w.6,       "Xstart:"             ,380, 85,  40, 20
 StaticText #w.7,       "Ystart:"             ,380, 115 ,40, 20
 StaticText #w.8,        "dY/dX:"             ,380, 145 ,40, 20
 StaticText #w.9,        "Xend:"              ,470, 85,  30, 20
 StaticText #w.10,        "Yend:"             ,470, 115, 30, 20
 StaticText #w.11,        "dY/dX:"            ,465, 145 ,35, 20
 StaticText #w.12,  " Finite Step:"           ,160, 120, 50, 40

 StaticText #w.status1, "",          60,  50 ,  350,  20'diffeq
 StaticText #w.status3, "",          10,  30,   40,  40'START HERE
 StaticText #w.status5, "",          400, 175, 200,  20'recnum
 StaticText #w.status6, "",            5, 175,  70,  20'recnum
 StaticText #w.status9,  "",         10,  70,   100,  30

 TextBox #w.eq,                      100, 80,  270,  20
 TextBox #w.n,                       120, 30,   50,  20
 TextBox #w.n1,                      180, 30,   50,  20
 TextBox #w.t,                       240, 30,   50,  20
 TextBox #w.xx,                      300, 30,   50,  20
 TextBox #w.comp1,                   360, 30,   50,  20

' TextBox #w.mol1,                     460, 65,   30,  20
 TextBox #w.comp2,                    420, 80,    40,  20'xin
 TextBox #w.mol2,                     420, 110,   40,  20'yin
 TextBox #w.comp3,                    420, 140,   40,  20'dy/dx in
 TextBox #w.mol3,                     500,  80,   40,  20'xout
 TextBox #w.mol4,                     200, 120,   40,  20'runge step
 TextBox #w.comp4,                    500, 110,   40,  20'yout
 TextBox #w.comp5,                    500, 140,   40,  20'dy/dx out
' TextBox #w.mol5,                     620, 65,   30,  20
' TextBox #w.comp6,                    660, 35,   30,  20
' TextBox #w.mol6,                     660, 65,   30,  20


 Button #w.c1, "Reset", [reset],                  UL, 30,  10, 50, 30
' Button #w.c1, "Continue1", [continue1],          UL, 355, 30, 60, 30
' Button #w.c2, "Continue2", [continue2],          UL, 695, 30, 60, 30
 Button #w.c3, "Calc",      [calc],               UL, 280,120, 60, 30
 Button #w.exit, "Close",   [close.w],            UL, 500,20, 40, 30
 Button #w.add, "Add New at end",   [add.rec],    UL,  80,170, 90, 25
 Button #w.upd, "Update present",   [update.rec], UL, 180,170, 90, 25
 Button #w.prv, "Previous",         [prev.rec],   UL, 280,170, 50, 25
 Button #w.next, "Next",            [next.rec],   UL, 340,170, 50, 25

 TEXTEDITOR #w.res,                                  10,200, 550,250'HEIGHT
' TEXTeditor #w.conf,                                 400,90, 280, 70'HEIGHT
' TEXTeditor #w.form,                                 10, 80, 370, 80'HEIGHT

    open "DIFFERENTIAL EQUATION,  TWO POINT VALUE, FINITE ELEMENT" for dialog as #w
    print #w, "trapclose [close.w]"
    print #w.n, "!setfocus"
    'print #w.status3,"START HERE"

    '              **** Adress file ****
    axx=6
   'prompt " Choose Indata file(kplib) number (1-10)";axx
    open "DIFFEQ";axx;".dat" for random as #vcl len=128
    print #w.status9, "Indata file: DIFFEQ";axx;".dat "

    print #w.status1,"D2Y/DX2 +   A*DY/DX  +    B*Y    +              C*Y*X^D      +          E   =  0"

    Field #vcl, _
    60 as EQ$, _
    5 as A11, _
    5 as B11, _
    5 as C11, _
    5 as D11, _
    5 as mol1,_
    6 as YS,_
    5 as XE,_
    5 as N1,_
    5 as YE,_
    5 as DERE,_
    5 as E11,_
    5 as XS,_
    6 as DER,_
    1 as RES,



rec= lof(#vcl) / 128
    print #w.status6, "Next add at ";rec+1
    for I=1 to rec
      get #vcl,I
      print #w.res, " ";EQ$ 
    next I
print #w.eq,"!font Times_New_Roman 10 bold"
print #w.n,"!font Times_New_Roman 10 bold"
print #w.n1,"!font Times_New_Roman 10 bold"
print #w.t,"!font Times_New_Roman 10 bold"
print #w.xx,"!font Times_New_Roman 10 bold"
print #w.comp1,"!font Times_New_Roman 10 bold"
print #w.comp2,"!font Times_New_Roman 10 bold"
print #w.comp3,"!font Times_New_Roman 10 bold"
print #w.comp4,"!font Times_New_Roman 10 bold"
print #w.comp5,"!font Times_New_Roman 10 bold"
print #w.mol2,"!font Times_New_Roman 10 bold"
print #w.mol3,"!font Times_New_Roman 10 bold"
'print #w.res,"!font Courier_New 8 bold "
print #w.res,"!font Times_New_Roman 12  bold"
    goto [reset]
    goto [loop]
    PRINT" "

[validate]'subroutine
   valid = 1
    print #w.eq, "!contents?"
    input #w.eq, EQ$
    if len(EQ$)>60 then
    notice "Error!" +chr$(13)+"Allowed string=60,  Actual=";len(EQ$)
    valid=0
    return
    end if

    print #w.n, "!contents?"
    input #w.n, A11

    print #w.n1, "!contents?"
    input #w.n1, B11

    print #w.t, "!contents?"
    input #w.t, C11

    print #w.xx, "!contents?"
    input #w.xx, D11

    print #w.comp1, "!contents?"
    input #w.comp1,E11

    print #w.comp2, "!contents?"
    input #w.comp2,XS

    print #w.comp3, "!contents?"
    input #w.comp3,DER

    print #w.comp4, "!contents?"
    input #w.comp4,YE

    print #w.comp5, "!contents?"
    input #w.comp5,DERE

    'print #w.comp6, "!contents?"
    'input #w.comp6,comp6

    'print #w.mol1, "!contents?"
    'input #w.mol1,mol1

    print #w.mol2, "!contents?"
    input #w.mol2,YS

    print #w.mol3, "!contents?"
    input #w.mol3,XE

    print #w.mol4, "!contents?"
    input #w.mol4,N1

    'print #w.mol5, "!contents?"
    'input #w.mol5,mol5

    'print #w.mol6, "!contents?"
    'input #w.mol6,mol6
  return

[add.rec]
    gosub [validate]
    if valid = 0 then [loop]
    recNum = lof(#vcl) / 128 + 1 ' calc location of next record
    put #vcl, recNum
    print #w.res, "!cls"
    print #w.status5, " recNum:"; recNum;"  has been added at ";recNum
    rec= lof(#vcl) / 128
    print #w.status6, "Next add at ";rec+1
    goto [loop]

[update.rec]
    gosub [validate]
    if valid = 0 then [loop]
    put #vcl, recNum
    'print #w.res, "!cls"
    print #w.status5, " recNum: ";recNum;"  has been updated at ";recNum
    goto [loop]

[display.rec]'subroutine
     print #w.eq,EQ$
     print #w.n, A11
     print #w.n1,B11
     print #w.t, C11
     print #w.xx,D11
     'print #w.mol1,mol1
     print #w.mol2,YS
     print #w.mol3,XE
     print #w.mol4,N1
     'print #w.mol5,mol5
     'print #w.mol6,mol6
     print #w.comp1,E11
     print #w.comp2,XS
     print #w.comp3,DER
     print #w.comp4,YE
     print #w.comp5,DERE
     'print #w.comp6,comp6
    return

[prev.rec]
    if recNum > 1 then
    recNum = recNum - 1
    get #vcl, recNum
    yy1=lof(#vcl) / 128
    print #w.status5, "record-Nr=";recNum;":   End of file at ";yy1
    else
    print #w.status5, " Start of file:    End of file at ";yy1
    end if
    gosub [display.rec]
    goto [loop]

[next.rec]
    if recNum < lof(#vcl) / 128 then
    recNum = recNum + 1
    get #vcl, recNum
    yy=lof(#vcl) / 128
    print #w.status5, "record-Nr=";recNum;":   End of file at ";yy
    else
    print #w.status5, "End of file:   Next add at ";recNum+1
    end if
    gosub [display.rec]
    'print #w.conf, "press next again or cont1"
    goto [loop]

[reset]
    print #w.n, "0";
    print #w.n1, "0" ;
    print #w.t, "0" ;
    print #w.xx, "0" ;
    print #w.comp1, "0" ;
    'print #w.mol1,  "0" ;
    print #w.comp2, "0" ;
    print #w.mol2,  "0" ;
    print #w.comp3, "0" ;
    print #w.mol3,  "0" ;
    print #w.comp4, "0" ;
    print #w.mol4,  "0" ;
    print #w.comp5, "0" ;
    'print #w.mol5,  "0" ;
    'print #w.comp6, "0" ;
    'print #w.mol6,  "0" ;
    'print #w.conf, "!cls" ;
   ' print #w.form, "!cls" ;
    print #w.eq, " " ;
    print #w.res," Start: Press next to get saved data"
     print #w.res," Change or insert new data, followed by UPDATE or ADD"
     print #w.res," ADD places the data at the end of the file"
     print #w.res," Play with the buttons so you learn how to preserve your data"
    goto [loop]

[loop]
    input var$
    goto [loop]

[calc]

    '*************    INPUT DATA INFORMATION    *******
    'A11=A(INPUT WINDOW)  B11=B   C11=C   D11=D  E11=E
    'XS=XSTART(INPUT WINDOW) XE=XEND   YS= YSTART
    'DY/DX(START)=DER   DY/DX(END)=DERE
    'N1=FINITE STEP
    '**************************************************
'   INDATA
    M=N1-1'  M=NUMBER OF EQUATIONS
    A=.25
    B=8
    xin  =XS
    xend =XE
    yin  =YS
    yend =YE
    uin =DER
    uend=DERE
    pp=xend-xin
    DX=pp/(M+1)
    CC=E11*(-1)*DX^2
   'END INDATA
  print XS;"    ";YS
'  SETUP OF COEFF. MATRIX
                    '  C(i,j)
                    '  j=    1   2   3   4   5       6
                    '
                    'i=1     A   1              Y1 = CC
                    '  2     1   A   1          Y2
                    '  3         1   A   1      Y3
                    '  4             1   A   1  Y4
                    '  5                 1   A  Y5


    n1=M
    'FIRST EQUATION AT n1=0

    for i=1 to n1
      C(i,n1+1)=CC  'CONSTANT TERM
    next i
     C(1,n1+1)=CC-YS
     C(n1,n1+1)=CC-YE
    for i=1 to n1-1
      C(i,i+1)=1         'UPPER 1
    next i

    for i=1 to n1
      C(i,i)=B11*DX^2+ A11*DX + C11*DX^(2)*(DX*i)^D-2   'DIAGONAL
    next i

    for i=2 to n1
      C(i,i-1)=1-A11*DX   'LOWER 1
    next i

     'LAST EQUATION  AT  n1 giving DY/DX
            print #w.res,""

    CONFIRM" Shooting at DY/DX=yes at Y=no";answer$
        IF answer$="yes" THEN
         C(n1,n1+1)=DERE*DX*2
         C(n1,n1)=3
         C(n1,n1-1)=4*(-1)
         C(n1,n1-2)=1
         PRINt #w.res,"Shooting towards dy/dx at end";"  steps=";N1
         else
          PRINt #w.res,"Shooting towards Yend ";"  steps= ";N1
        END IF
   print DERE
'   END OF SETUP


    print #w.res,EQ$
    print #w.res,"XSTART=";XS;"   XEND=";XE;"   YSTART=";YS;"   YEND=";YE
    PRINT #w.res,"  DY/DX(END)=";DERE;"  DY/DX(START)=";DER
    print #w.res,"  X                 Y                    DY/DX(appr)"
    print 0 ;"       "; yin
    'print #w.res,"  "; 0 ;"            "; yin
    gosub [gauss]
    print n1+1;"   ";yend
    'print #w.res,"  "; pp/(n1+1)*(n1+1)+XS;"        ";yend;"    ";(C(n1+1,N)-C(n1,N))/DX
    print "analytical solution at x=5 ,71.9429"
    print #w.res,"Problem with repeated calculations; clear memory by closing and start new"
    print #w.res, "Use < 50 steps as a rule in the iteration. "
    print  #w.res,"The values obtained are not very accurate."
    print #w.res,"Increasing the number of steps creates rounding off error"
    GOTO [loop]

[close.w]
    close #w :CLOSE #1:close #vcl
    print" press ENTER to close"
    input r$
    print"     *** end ***"
    end

 [gauss]
     N=M+1
  FOR J=1 TO M
       FOR I=J TO M
         IF C(I,J)<>0 then
         FOR K=1 TO N
         X=C(J,K)
         C(J,K)=C(I,K)
         C(I,K)=X
       NEXT K
       end if
     NEXT I

     C(J,J)=C(J,J)
     Y=1/C(J,J)
     FOR K=1 TO N
       C(J,K)=Y*C(J,K)
     NEXT K

     FOR I=1 TO M
       IF I=J GOTO 41
       Y=C(I,J)*(-1)
         FOR K=1 TO N
           C(I,K)=C(I,K)+Y*C(J,K)
         NEXT K
41   NEXT I
  NEXT J

     FOR J=1 TO M
   if J<>1 then PRINT #w.res,using("##.##",J/(n1+1)*pp+XS);"     ";C(J,N);"       ";(C(J,N)-C(J-1,N))/DX
   if J=1 then print #w.res,using("##.##",J/(n1+1)*pp+XS);"     ";C(J,N)
   PRINT using("##.##",J/(n1+1)*pp+XS);"     ";C(J,N);"       ";(C(J,N)-C(J-1,N))/DX
   REM
 NEXT J
return