Question

Do you know any C implementation of the Matlab interp1 function (just ‘linear’ one)? I know one for Java.

Answer 1

I have implemented this linear interpolation myself (some of it is written in Spanish, sorry). The function called encuentraValorMasProximo just finds the nearest value (elementoMasProximo) and index (indiceEnVector) to another one (xx[i]), in an array (xD).

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];
}
}

The test function could be:

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]);
}

}

And its outcome:

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