Unable to properly use struct pointers in C - c

Question

I am a beginner with C and currently struggling with using structs in functions. Even tho I give my functions the pointer to my struct and use that to change their values it seems like my functions are unable to change the value of the pointer themselves
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <assert.h>
typedef struct Vector
{
int n;
double *entry;
}Vector;
//defining the struct vector with n for its length and entry as a pointer which will become an array for its values
Vector *newVector(int n)
{
int i = 0;
Vector *X = NULL;
assert(n > 0);
X = malloc(sizeof(Vector));
assert(X != NULL);
X->n = n;
X->entry = malloc(n+1*sizeof(double));
assert(X->entry != NULL);
X->entry[0] = 0;
for (i=1; i<n+1; ++i) {
scanf("%lf",&X->entry[i]);
}
return X;
}
//making a new vector struct from the size given and returning its pointer (the array is made so it goes from 1-n instead of 0-(n-1)
void delVector(Vector *X)
{
assert(X != NULL);
assert(X->entry != NULL);
free(X->entry);
free(X);
X = NULL;
}
int getVectorLength (Vector *X)
{
assert(X != NULL);
return X->n;
}
void setVectorLength(Vector *X, int k)
{
assert(X != NULL);
assert(k>0);
delVector(X);
printf("so far.\n");
X = newVector(k);
} //deleting the vector and then replacing it with the new sized vector
double getVectorEntry (Vector *X, int h)
{
return X->entry[h];
}
void setVectorEntry (Vector *X, int h)
{
printf("Value %d.\n",h);
scanf("%lf",&X->entry[h]);
}
main()
{
Vector *h = NULL;
h = newVector(3);
printf("%f\n",h->entry[1]);
printf("%f\n",getVectorEntry(h,1));
setVectorEntry(h,1);
printf("%f\n",getVectorEntry(h,1));
printf("%d\n",getVectorLength(h));
setVectorLength(h,6);
printf("%f\n",getVectorEntry(h,6));
setVectorEntry(h,6);
printf("so far.\n");
printf("%f\n",getVectorEntry(h,6));
}
Running the code would make it crash once it reaches delVector. If I were to comment the delVector out it would crash at X = newVector(k); for a reason I also cannot find out (it starts the function newVector, but crashes before I can input). So what is causing the errors?
Thanks a lot in advance!

Answer 1

Let's just focus on the setVectorLength function
void setVectorLength(Vector *X, int k)
{
assert(X != NULL);
assert(k>0);
delVector(X);
printf("so far.\n");
X = newVector(k);
}
Here, Vector *X is a local variable that only exists in this function. It points to some location in memory, but using it you can't modify the original pointer. So there are two ways around it.
1. You don't delete and recreate the vector. Instead do something like this
void setVectorLength(Vector *X, int k)
{
X->entry = realloc(X->entry, k*sizeof(double)); // frees X->entry and allocates it to a new heap array
X->k = k;
}
2. Use a pointer pointer. Do note, that this will be slower and is unnecessary. Go for the 1 solution.
void setVectorLength(Vector **X, int k)
{
assert(*X != NULL);
assert(k>0);
delVector(*X);
printf("so far.\n");
*X = newVector(k);
}
The variable Vector **X points to the original variable, so you can modify it.

Answer 2

Problem seem to be in function
Vector *newVector(int n)
{
int i = 0;
Vector *X = NULL;
assert(n > 0);
X = malloc(sizeof(Vector));
assert(X != NULL);
X->n = n;
X->entry = malloc(n+1*sizeof(double));
assert(X->entry != NULL);
X->entry[0] = 0;
for (i=1; i<n+1; ++i) {
scanf("%lf",&X->entry[i]);
}
return X;
}
In above statement, you are allocating X->entry with n + sizeof(double) but as per your requirement you should allocated (n + 1) * sizeof(double)
You should allocate memory for X->entry as
X->entry = malloc((n + 1) * sizeof(double));

Answer 3

Your allocation is wrong, you have to group the n and the 1 like you do it on paper so. Not that the expression you have n + 1 * sizeof(double) means the same as n + sizeof(double).
X->entry = malloc((n + 1) * sizeof(*X->entry));
would be the right way!
As it was, your code invokes undefined behavior and once possible consequence is for the program to crash. Most likely because trying to access memory out of it's memory space so a segmentation fault occurs.

Answer 4

void setVectorLength(Vector *X, int k)
{
assert(X != NULL);
assert(k>0);
delVector(X);
printf("so far.\n");
X = newVector(k);
}
In this function, X is a copy of the pointer variable the caller passes in. It contains a copy of the address. Modifying a function parameter doesn't change the caller's value. Just as doing k = 0 would have no effect on the caller, reassigning X is a local change that is invisible to the caller.
If you want the caller to see the result of newVector(k), you have a couple options. You could change setVectorLength() to work like newVector() and return the new pointer.
Vector *setVectorLength(Vector *X, int k) {
...
printf("so far.\n");
return newVector(k);
}
int main() {
...
h = setVectorLength(h, 6);
}
Or you could add another layer of indirection—make X a pointer to a pointer. That would require the caller to pass the address of the variable they want changed. Instead of passing h you'd pass &h.
void setVectorLength(Vector *X, int k) {
...
printf("so far.\n");
*X = newVector(k);
}
int main() {
...
setVectorLength(&h, 6);
}
By the way, the way you've written setVectorLength(), it will destroy any existing data in the vector. You may want to copy the data from the old vector to the new one. (Or, as #pi_pi3 suggests, modify the vector in place instead of destroying it and creating a new one.)