double datatype behaving in an undefined manner - turbo-c

Question

Even though I know turbo c is completely obsolete now, my instructor has put a condition to code in it.
I am having an issue that when I am trying to pass a double value to a function it is not behaving properly. I am getting fluctuating output several of the time, sometimes even weird. Let's see my code first:
#include<stdio.h>
#include<conio.h>
double func(double input) {
return input * input;
}
double simpson1By3(double initial, double final, double parts) {
double sum = 0;
double h = (final - initial) / parts;
double oddSum = 0, evenSum = 0;
int i;
printf("%f %f %f\n", initial, final, parts);
printf("%f\n", h);
printf("%f %f\n", evenSum, oddSum);
sum += func(initial) + func(final);
printf("%f %f\n", evenSum, oddSum);
for (i = 1; i < parts; i = i + 2) {
oddSum += func(initial + (i * h));
}
for (i = 2; i < parts; i = i + 2) {
evenSum += func(initial + (i * h));
}
oddSum *= 4;
evenSum *= 2;
printf("%f %f\n", evenSum, oddSum);
sum += evenSum + oddSum;
sum *= h / 3;
return sum;
}
int main() {
clrscr();
printf ("%f", simpson1By3(0, 6, 6));
getch();
return 0;
}
sample output:
What am I doing wrong there? Why are the arguments passed printed erroneously there along with other variables there and why is that -0 printing? Please help. I have tried finding something similar to it in forums but completely failed. Please help.