Question

I’m rather new to coding and have been building a code to interpolate a cubic spline, but I am stuck on the last equation do the following error: _”invalid operands to binary ^ (have ‘double’ and ‘double’)|”

The problem is in the last bit of the code with “population =” for the two first lines. I would really appreciate it if someone could point me in the right direction.

#include <stdio.h>


main () {

    int x;
    int y;
    double stats[10][2];
    double gpp[8][9] = {0};
    double gppr[10] = {0};
    double year;
    double population;
    int xi = 0;
    year=1950;
    population=0;
    x=0;

    stats[0][0] = 1930; stats[1][0] = 1940; stats[2][0] = 1949;
    stats[3][0] = 1955; stats[4][0] = 1960; stats[5][0] = 1970;
    stats[6][0] = 1980; stats[7][0] = 1990; stats[8][0] = 2000;
    stats[9][0] = 2005;

    stats[0][1] = 21.058; stats[1][1] = 23.547; stats[2][1] = 20.167;
    stats[3][1] = 21.502; stats[4][1] = 24.989; stats[5][1] = 30.852; 
    stats[6][1] = 37.407; stats[7][1] = 43.390; stats[8][1] = 45.985;
    stats[9][1] = 47.041;

    //Initiate  g'' system of equation
    for (x=0;x<8;x++) {
        gpp[x][x] = ((stats[x+1][0]-stats[x][0])+(stats[x+2][0]-stats[x+1][0]))/3;
        if (x<7) {
            gpp[x][x+1] = (stats[x+2][0]-stats[x+1][0])/6;
        }
        if (x>0) {
            gpp[x][x-1] = (stats[x+2][0]-stats[x+1][0])/6;
        }
        gpp[x][8] = ((stats[x+2][1]-stats[x+1][1])/(stats[x+2][0]-stats[x+1][0]))-((stats[x+1][1]-stats[x][1])/(stats[x+1][0]-stats[x][0]));
    }

    //Forward sweep
    for (x=0;x<7;x++) {
        gpp[x+1][x] = 0;
        gpp[x+1][x+1] = gpp[x+1][x+1] - (gpp[x][x+1]/gpp[x][x])*gpp[x+1][x];
        gpp[x+1][8] = gpp[x+1][8] - (gpp[x][x+1]/gpp[x][x])*gpp[x][8];
    }
    //Backward sweep
    gppr[9] = 0;gppr[0] = 0;
    gppr[8] = gpp[7][8]/gpp[7][7];
    for (x=7;x > 0;x=x-1) {
        gppr[x] = (gpp[x][8]-(gppr[x+1]*gpp[x][x+1]))/gpp[x][x];
    }

    //check where is xi
    for (x=0;x<10;x++) {
        if (stats[x][0] > year) {
            xi = x;
            break;
        }
    }

//Calculate population at x
population = (gppr[xi]/6)*((((stats[xi+1][0]-year)^3.0)/(stats[xi+1][0]-stats[xi][0]))-(stats[xi+1][0]-stats[xi][0])*((stats[xi+1][0]-year)))
       + (gppr[xi+1]/6)*((((year-stats[xi][0])^3.0)/(stats[xi+1][0]-stats[xi][0]))-(stats[xi+1][0]-stats[xi][0])*((year-stats[xi+1][0])))
       + (stats[xi][1])*((stats[xi+1][0]-year)/(stats[xi+1][0]-stats[xi][0]))
       + (stats[xi+1][1])*((year-stats[xi][0])/(stats[xi+1][0]-stats[xi][0]));

}

I look forward to learning a little more about C!

Hugo

Answer 1

^ is the exclusive-OR operator in C and is unsuitable for doubles. It’s a bitwise operator and you can find more details on it (and the other bitwise operators) here.

If you want to raise one number to the power of another, you need the pow() function.

So something like:

((stats[xi+1][0]-year)^3.0)

should actually be written:

pow (stats[xi+1][0] - year, 3.0)

Section 7.12.7.4 The pow functions of the latest (C11) standard states:

Synopsis:
#include <math.h>
double pow(double x, double y);
float powf(float x, float y);
long double powl(long double x, long double y);

Description:
The pow functions compute x raised to the power y. A domain error occurs if x is finite
and negative and y is finite and not an integer value. A range error may occur. A domain
error may occur if x is zero and y is zero. A domain error or pole error may occur if x is
zero and y is less than zero.

The pow functions return x^y.