【求助】想实现非线性数据的插值

redelf

x=[1 2 3 4 5]
y=[10 20 30 20 10 ]
假如想得到在y=25处的两个x值，应该怎么做
interp1是只能实现线性插值，高手指点

redelf

找到一个解决办法
table=[a; b].'
xi=mminterp(table, 2, 25)

函数定义如下
function y=mminterp(tab, col, val)
%MMINTERP 1-D Table Search by Linear Interpolation.

%Y=MMINTERP(TAB,COL,VAL) linearly interpolates the table

%TAB searching for the scalar value VAL in the column COL.

%All crossings are found and TAB(:,COL) need not be monotonic.

%Each crossing is returned as a separate row in Y and Y has as

%many columns as TAB.Naturally,the column COL of Y contains

%the value VAL. If VAL is not found in the table,Y=.

%Copyright (c) 1996 by Prentice-Hall,Inc.

[rt, ct]=size(tab);

if length(val) > 1,error(' VAL must be a scalar. '),end

if col>ct|col < 1,error(' Chosen column outside table width. '),end

if rt < 2,error(' Table too small or not oriented in columns. '),end

above=tab(: , col) > val;%True where > VAL

below=tab(: , col) < val;%True where < VAL

equal=tab(: , col) == val;%True where = VAL

if all(above == 0) | all(below == 0),%handle simplest case

y=tab(find(equal), : );return

end

pslope=find(below(1:rt-1)&above(2:rt));%indices where slope is +

nslope=find(below(2:rt)&above(1:rt-1));%indices where slope is -

ib=sort([pslope; nslope+1]);%put indices below in order

ia=sort([nslope; pslope+1]);%put indices above in order

ie=find(equal);%indices where equal to val

[tmp,ix]=sort( [ib, ie] );%find where equals fit in result

ieq=ix > length(ib);%True where equals values fit

ry=length(tmp);%# of rows in result y

y=zeros(ry, ct);%poke data into a zero matrix

alpha=(val-tab(ib,col))./(tab(ia,col)-tab(ib,col));

alpha=alpha(: , ones(1, ct));%duplicate for all columns

y(~ieq, : )=alpha.*tab(ia, : )+(1-alpha).*tab(ib, : );%interpolated values

y(ieq, : )=tab(ie, : );%equal values

y( : , col)=val*ones(ry, 1);%remove roundoff error

lcl5878280

强，试一下。

bainhome

有这么复杂吗？

1. xx=linspace(1,5,150);
   
2. yy=spline(x,y,xx);
   
3. f=xx(yy>=24.9&yy<=25.001)

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result return:

1. 
   
2. f =
   
3.     2.3423    3.6577

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