adding two polynomials using dynamic arrays - c

Question

I have the following code written. After i run the code it works fine until it tries to excecute the function add2Poly. Thereafter i get a segementation fault. I am new to programming so i cant understand what is causing it.
#include <stdio.h>
#include <stdlib.h>
struct poly{
double power[5];
double* coeff;
};
struct poly add2Poly(struct poly add1, struct poly add2)
{
struct poly p3;
p3.coeff = malloc(5 * sizeof * p3.coeff);
printf("We will add the two polynomials and put the result in a third polynomial p3.");
int size1 = sizeof(add1.coeff);
int size2 = sizeof(add2.coeff);
for(int i = 0; i < size1; i++)
{
int power = add1.power[i];
for(int j= 0; j < size2; j++)
{
if( power == add2.power[j])
{
p3.coeff[i] = add1.power[i] + add2.power[j];
p3.power[i] = add2.power[j];
}
}
}
return p3;
}
int main(){
struct poly p1;
p1.coeff = malloc(5 * sizeof * p1.coeff);
double a;
printf("Please enter a double value for poly1 coeff:");
scanf("%lf", &a);
double b;
printf("Please enter a double value for poly1 coeff:");
scanf("%lf", &b);
p1.coeff[1] = a;
p1.coeff[2] = b;
printf("Power of poly1: ");
scanf("%lf", &p1.power[1]);
printf("Power poly2: ");
scanf("%lf", &p1.power[2]);
printf("p1: %.1fx^%.1f", p1.coeff[1], p1.power[1]);
printf("+ %.1fx^%.1f\n", p1.coeff[2], p1.power[2]);
struct poly p2;
p2.coeff = malloc(5 * sizeof * p2.coeff);
double c;
printf("Please enter a double value for poly2 coeff:");
scanf("%lf", &c);
double d;
printf("Please enter a double value for poly2 coeff:");
scanf("%lf", &d);
p2.coeff[1] = c;
p2.coeff[2] = d;
printf("Power of poly1: ");
scanf("%lf", &p2.power[1]);
printf("Power poly2: ");
scanf("%lf", &p2.power[2]);
printf("p2: %.1fx^%.1f", p2.coeff[1], p2.power[1]);
printf("+ %.1fx^%.1f\n", p2.coeff[2], p2.power[2]);
struct poly p3;
p3.coeff = malloc(5 * sizeof * p3.coeff);
p3 = add2Poly(p1,p2);
printf("p3: %.1fx^%.1f", p3.coeff[1], p3.power[1]);
printf("+ %.1fx^%.1f\n", p3.coeff[2], p3.power[2]);
}
to be precise it gives me the following error:
segmentation fault(core dumped). I assume it has something to do with the p3 struct but i am not sure.

Answer 1

You want to initialize power by add1.power[i](line 18) when you have not assigned a value of add1.power[i], because at the first iteration i = 0
Then you have not assigned a value of add1.power[0], but only add1.power[1] and add1.power[2] (add1 = p1) and you want use it,
which means that conditional jump depends on uninitialised value : if( power == add2.power[j]) line 22, power is NULL also add2.power[j]
Delete lines 14 and 15 it does not serve you because
sizeof(add1.coeff) return the size in bytes of a pointer to double on your system : the result = 8 here.
You'll have an invalid read, your array poly.power and poly.coeff have a size of 5 while you want to read up to 8 values.
that's why you have this error segmentation fault(core dumped), Because you want to access a memory location that was not allocated to it.
Starting with your idea, if you want your code to work change the loop interval to 1 and 3: because only poly.power[1], poly.power[2], poly.coeff[1] and poly.power[2] are initialized.
Also in line 24 & 25 Do the sum of coefficients and not the sum of power to get the coefficients of the 3rd polynomial.
The way you define the structure of your polynomial can be changed, knowing only the degree of the polynomial. You can avoid the nested loop in your add2Poly function too.
I hope that will heplp you !
Look at the following code:
struct poly add2Poly(struct poly add1, struct poly add2)
{
struct poly p3;
p3.coeff = malloc(5 * sizeof *p3.coeff);
printf("We will add the two polynomials and put the result in a third polynomial p3.\n");
int size1 = sizeof(add1.coeff);// to delate
int size2 = sizeof(add2.coeff);// to delate
for (int i = 1; i < 3; i++)
{
int power = add1.power[i];
for (int j = 1; j < 3; j++)
{
if (power == add2.power[j])
{
p3.coeff[i] = add1.coeff[i] + add2.coeff[i];// do the sum of coefficients and not the sum of power
p3.power[i] = add2.power[i];
}
}
}
return p3;
}
here is an optimized proposal of your problem
#include <stdio.h>
#include <stdlib.h>
struct poly
{
int degree;
double *coeff; // the number of coefficient is equal to degree +1
};
struct poly add2Poly(struct poly p1, struct poly p2)
{
struct poly p3;
struct poly p_min_degree;
if (p1.degree > p2.degree)
{// the degree of the polynomial p3 is the one which has the greatest degree between p1 and p2
p3 = p1;
p_min_degree = p2;
}
else
{
p3 = p2;
p_min_degree = p1;
}
for (int i = 0; i <= p_min_degree.degree; i++)
{
p3.coeff[i] += p_min_degree.coeff[i];
}
return p3;
}
struct poly create_poly()
{// Ask the user to enter the degree of the polynomial and (degree +1) coefficients
struct poly p;
printf("Power of the polynomial : ");
scanf("%d", &p.degree);
p.coeff = malloc((p.degree + 1) * sizeof *p.coeff);
for (int i = 0; i <= p.degree; i++)
{// Read a double as coeff
printf("Enter the coeff x^%d : ", i);
scanf("%lf", &p.coeff[i]);
}
return p;
}
void display_poly(struct poly p)
{// Displays a polynomial p passed as a parameter
//Display example: p degree 2
//p = + 3.00x^2 + 4.00x^1 + 2.00x^0
printf(" p = ");
for (int i = p.degree; i >= 0; i--)
{
printf("+ %.2lfx^%d ", p.coeff[i], i);
}
printf("\n");
}
int main()
{
struct poly p1 = create_poly();
display_poly(p1);
struct poly p2 = create_poly();
display_poly(p2);
struct poly p3 = add2Poly(p1, p2);
printf("The sum of p1+p2 is :");
display_poly(p3);
}