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Author Date Id Revision
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File size:
1.6 KB
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1 | ! |
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2 | ! $Header$ |
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3 | ! |
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4 | subroutine spline(x,y,n,yp1,ypn,y2) |
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5 | |
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6 | c |
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7 | |
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8 | c Routine to set up the interpolating function for a cubic spline |
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9 | |
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10 | c interpolation (see "Numerical Recipes" for details). |
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11 | |
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12 | c |
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13 | implicit real (a-h,o-z) |
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14 | implicit integer (i-n) |
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15 | |
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16 | parameter(nllm=4096) |
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17 | |
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18 | dimension x(n),y(n),y2(n),u(nllm) |
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19 | |
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20 | c |
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21 | c write(6,*)(x(i),i=1,n) |
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22 | c write(6,*)(y(i),i=1,n) |
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23 | |
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24 | if(yp1.gt.0.99E30) then |
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25 | c the lower boundary condition is set |
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26 | y2(1)=0. |
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27 | c either to be "natural" |
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28 | u(1)=0. |
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29 | |
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30 | else |
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31 | c or else to have a specified first |
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32 | y2(1)=-0.5 |
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33 | c derivative |
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34 | u(1)=(3./(x(2)-x(1)))*((y(2)-y(1))/(x(2)-x(1))-yp1) |
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35 | |
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36 | end if |
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37 | |
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38 | do 11 i=2,n-1 |
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39 | c decomposition loop of the tridiagonal |
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40 | sig=(x(i)-x(i-1))/(x(i+1)-x(i-1)) |
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41 | c algorithm. Y2 and U are used |
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42 | p=sig*y2(i-1)+2. |
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43 | c for temporary storage of the decompo- |
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44 | y2(i)=(sig-1.)/p |
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45 | c sed factors |
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46 | u(i)=(6.*((y(i+1)-y(i))/(x(i+1)-x(i))-(y(i)-y(i-1)) |
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47 | |
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48 | . /(x(i)-x(i-1)))/(x(i+1)-x(i-1))-sig*u(i-1))/p |
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49 | |
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50 | 11 continue |
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51 | |
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52 | if(ypn.gt.0.99E30) then |
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53 | c the upper boundary condition is set |
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54 | qn=0. |
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55 | c either to be "natural" |
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56 | un=0. |
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57 | |
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58 | else |
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59 | c or else to have a specified first |
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60 | qn=0.5 |
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61 | c derivative |
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62 | un=(3./(x(n)-x(n-1)))*(ypn-(y(n)-y(n-1))/(x(n)-x(n-1))) |
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63 | |
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64 | end if |
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65 | |
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66 | y2(n)=(un-qn*u(n-1))/(qn*y2(n-1)+1.) |
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67 | |
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68 | do 12 k=n-1,1,-1 |
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69 | c this is the backsubstitution loop of |
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70 | y2(k)=y2(k)*y2(k+1)+u(k) |
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71 | c the tridiagonal algorithm |
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72 | 12 continue |
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73 | |
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74 | c |
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75 | |
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76 | return |
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77 | |
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78 | end |
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79 | |
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