Lagrange

#include < stdio.h > 
#define FMT "%-10.5g"
#define N 3 
typedef float DBL[N];
float Lag(DBL x, DBL f, int n, float xx) {
    int k,
    j;
    float r,
    s = 0.0;
    for (k = 0; k <= n; k++) {
        r = 1.0;
        for (j = 0; j <= n; j++) if (j != k) r *= (xx - x[j]) / (x[k] - x[j]);
        s += f[k] * r;
    }
    return s;
}
void main() {
    int i;
    DBL x,
    f;
    float xx;
    printf("\n");
    for (i = 0; i < N; i++) {
        printf("x[%d]=", i);
        scanf("%f", x + i);
        printf("f(%d)=", i);
        scanf("%f", f + i);
    }
    printf("x=");
    scanf("%f", &xx);
    xx = Lag(x, f, N - 1, xx);
    printf("\nf(x)=");
    printf(FMT, xx);
}