Matlab interp1函数的C实现(线性插值)

Question

你知道Matlab interp1函数的任何C实现(只是'线性'函数)吗？我认识一个Java.

Answer 1

我已将Luis的代码移植到c ++中.它似乎工作但我没有检查过很多,所以要注意并重新检查你的结果.

#include <vector>
#include <cfloat>
#include <math.h>

vector< float > interp1( vector< float > &x, vector< float > &y, vector< float > &x_new )
{
    vector< float > y_new;
    y_new.reserve( x_new.size() );

    std::vector< float > dx, dy, slope, intercept;
    dx.reserve( x.size() );
    dy.reserve( x.size() );
    slope.reserve( x.size() );
    intercept.reserve( x.size() );
    for( int i = 0; i < x.size(); ++i ){
        if( i < x.size()-1 )
        {
            dx.push_back( x[i+1] - x[i] );
            dy.push_back( y[i+1] - y[i] );
            slope.push_back( dy[i] / dx[i] );
            intercept.push_back( y[i] - x[i] * slope[i] );
        }
        else
        {
            dx.push_back( dx[i-1] );
            dy.push_back( dy[i-1] );
            slope.push_back( slope[i-1] );
            intercept.push_back( intercept[i-1] );
        }
    }

    for ( int i = 0; i < x_new.size(); ++i ) 
    {
        int idx = findNearestNeighbourIndex( x_new[i], x );
        y_new.push_back( slope[idx] * x_new[i] + intercept[idx] );

    }

}

int findNearestNeighbourIndex( float value, vector< float > &x )
{
    float dist = FLT_MAX;
    int idx = -1;
    for ( int i = 0; i < x.size(); ++i ) {
        float newDist = value - x[i];
        if ( newDist > 0 && newDist < dist ) {
            dist = newDist;
            idx = i;
        }
    }

    return idx;
}

Answer 2

我自己实现了这种线性插值(其中一些是用西班牙语写的,对不起).名为encuentraValorMasProximo的函数只是在数组(xD)中找到最近的值(elementoMasProximo)和索引(indiceEnVector)到另一个值(xx [i]).

void interp1(int *x, int x_tam, double *y, int *xx, int xx_tam, double *yy)
{
double *dx, *dy, *slope, *intercept, *elementoMasProximo, *xD;
int i, *indiceEnVector;

dx=(double *)calloc(x_tam-1,sizeof(double));
dy=(double *)calloc(x_tam-1,sizeof(double));
slope=(double *)calloc(x_tam-1,sizeof(double));
intercept=(double *)calloc(x_tam-1,sizeof(double));
indiceEnVector=(int *) malloc(sizeof(int));
elementoMasProximo=(double *) malloc(sizeof(double));
xD=(double *)calloc(x_tam,sizeof(double));

for(i=0;i<x_tam;i++){
    xD[i]=x[i];
}

for(i = 0; i < x_tam; i++){
    if(i<x_tam-1){
        dx[i] = x[i + 1] - x[i];
        dy[i] = y[i + 1] - y[i];
        slope[i] = dy[i] / dx[i];
        intercept[i] = y[i] - x[i] * slope[i];
    }else{
        dx[i]=dx[i-1];
        dy[i]=dy[i-1];
        slope[i]=slope[i-1];
        intercept[i]=intercept[i-1];
    }
}

for (i = 0; i < xx_tam; i++) {
    encuentraValorMasProximo(xx[i], xD, x_tam, x_tam, elementoMasProximo, indiceEnVector);
    yy[i]=slope[*indiceEnVector] * xx[i] + intercept[*indiceEnVector];
}
}

该测试功能可以是:

void main(){

int x_tam, xx_tam, i;
double *yy;
int x[]={3,6,9};
double y[]={6,12,18};
int xx[]={1,2,3,4,5,6,7,8,9,10};
x_tam=3;
xx_tam=10;
yy=(double *) calloc(xx_tam,sizeof(double));

interp1(x, x_tam, y, xx, xx_tam, yy);

for(i=0;i<xx_tam;i++){
    printf("%d\t%f\n",xx[i],yy[i]);
}

}

而其结果:

1 2.000000

2 4.000000

3 6.000000

4 8.000000

5 10.000000

6 12.000000

7 14.000000

8 16.000000

9 18.000000

10 20.000000

Answer 3

常用函数的优秀实现可以在C语言中的Numerical Recipes一书中找到,该书可以在线免费查看.第3.1.2章有一个线性插值配方,本章的其余部分涵盖了更高级的配方.

我强烈推荐这本书,它编写得很好,涵盖了大量的算法,这些算法也以非常有效且仍然可以理解的方式实现.

Answer 4

Luis提交的C-Code存在问题.首先,encuentraValorMasProximo丢失了.其次,阵列预留是一个数字短.我也清理了这个功能.下面是带有encuentraValorMasProximo函数的功能性C代码(重命名为findNearestNeighbourIndex).

#include <float.h> 

int findNearestNeighbourIndex( double value, double *x, int len )
{
    double dist;
    int idx;
    int i;

    idx = -1;
    dist = DBL_MAX;
    for ( i = 0; i < len; i++ ) {
        double newDist = value - x[i];
        if ( newDist > 0 && newDist < dist ) {
            dist = newDist;
            idx = i;
        }
    }

    return idx;
}

void interp1(double *x, int x_tam, double *y, double *xx, int xx_tam, double *yy)
{
    double dx, dy, *slope, *intercept;
    int i, indiceEnVector;

    slope=(double *)calloc(x_tam,sizeof(double));
    intercept=(double *)calloc(x_tam,sizeof(double));

    for(i = 0; i < x_tam; i++){
        if(i<x_tam-1){
            dx = x[i + 1] - x[i];
            dy = y[i + 1] - y[i];
            slope[i] = dy / dx;
            intercept[i] = y[i] - x[i] * slope[i];
        }else{
            slope[i]=slope[i-1];
            intercept[i]=intercept[i-1];
        }
    }

    for (i = 0; i < xx_tam; i++) {
        indiceEnVector = findNearestNeighbourIndex( xx[i], x, x_tam);
        if (indiceEnVector != -1)
            yy[i]=slope[indiceEnVector] * xx[i] + intercept[indiceEnVector];
        else
            yy[i]=DBL_MAX;
    }
    free(slope);
    free(intercept);
}