Question

For some reason I was getting a dramatically incorrect answer for a problem, so I put in this printf to try and debug.

for (s = 0; s <= 100; s++) {
    for (t = 0; t <= 100; t++) {
        printf("At (%f,%f), spl = %f\n", s, t, spl(loc_data, s, t)); */
            if (spl(loc_data, s, t) > 80) { 
                     p++;
            }
        }  
}

If I omit the printf line, I get an incorrect answer. I think that it has something to do with memory allocation or uninitialised variables, but this is beyond my abilities. Any help would be appreciated.

Whole code:

#include <stdio.h>
#include <math.h>

typedef struct {
double x;
double y;
double W;
} data_t;

double spl(data_t *, double, double);

int main(int argc, char **argv) {

data_t loc_data[1000];

double spl0, p = 0, pp;

int i = 0, j = 0;

double s, t;

while (scanf("%lf %lf %lf", &loc_data[i].x, &loc_data[i].y, &loc_data[i].W) == 3) {
    i++;
}

printf("\nStage 1\n=======\n");
printf("Number of sound sources: %d\n", i);
for (j = 0; j < i; j++) {
    printf("%.1f meters east, %.1f meters north, power %1.5f Watts\n", 
    loc_data[j].x, loc_data[j].y, loc_data[j].W);
}






printf("\nStage 2\n=======\n");
spl0 = spl(loc_data, 0, 0);
printf("SPL at (0.0,0.0): %.1f dB\n", spl0);



printf("\nStage 3\n=======\n");


for (s = 0; s <= 100; s++) {
    for (t = 0; t <= 100; t++) {
        printf("At (%f,%f), spl = %f\n", s, t, spl(loc_data, s, t)); 
        if (spl(loc_data, s, t) > 80) { 
            p++;
        }
    }
}
pp = p / 102.01;

printf("Points sampled: 10201\nAbove 80.0 dB: %.1f%%\n", pp);

return 0;
}

double spl(data_t *loc_data, double pointx, double pointy) {
int i = 0;
double r_sq, powi, spli, spl;
while (loc_data[i].W != 0) {
    r_sq = pow(loc_data[i].x - pointx,2) + pow(loc_data[i].y - pointy,2);
    powi = 10*log10(loc_data[i].W / pow(10,-12));
    spli = powi + 10*log10((2 / (4 * M_PI * r_sq)) + (4 / (2.5 * M_PI * r_sq)));
    spl = 10*log10(pow(10, spl/10) + pow(10, spli/10));
    i++;
}
return spl;
}

Apologies for the poor formatting.

Answer 1

You”re using a variable (spl) before it’s initialized inthe function spl():

double spl(data_t *loc_data, double pointx, double pointy) {
int i = 0;
double r_sq, powi, spli, spl;
while (loc_data[i].W != 0) {
    r_sq = pow(loc_data[i].x - pointx,2) + pow(loc_data[i].y - pointy,2);
    powi = 10*log10(loc_data[i].W / pow(10,-12));
    spli = powi + 10*log10((2 / (4 * M_PI * r_sq)) + (4 / (2.5 * M_PI * r_sq)));
    spl = 10*log10(pow(10, spl/10) + pow(10, spli/10));
    //                     ^^^
    i++;
}
return spl;
}

Calling printf() is probably influencing the value the variable happens to have.

Also, you read in a number of entries into the loc_data array, but don’t pass that information to the spl() function. In spl() you treat the array entry with field W as the ‘sentinel’ – the end of the array. Is it a given that the last entry in the input data will have a zero value? If so, you should probably let us know, and probably check for that when done reading the input.