I need help with pointers and arrays

[proper ivy]

Create a C function named square_reverse with three parameters. The two first
parameters are 64-bit floating-point pointers x and y, and the third parameter is an integer
parameter called len. Pointer x points to an array of length len of floating-point values.
The function reads out each element of the array, computes the square value of the element
(x2) and then writes back the result into the array y in reverse order. That is, the output
array y has also the length len. The function also returns the sum of all input values
(those in x)

So far this is what I've done

void square_reverse(double *x, double *y, int len){

    for (int i = 0; i<len; i++){
        
    }
}

[dry shoal]

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[proper ivy]

*x points at the an array right? So how can I access the array?

[static vigil]

Use the array access syntax as normal.

[proper ivy]

wdym?

[static vigil]

x[index].

[proper ivy]

I wanted to iterate over the array but can I just use "x" as the array and do like

[static vigil]

You can.

[proper ivy]

why does that work?

isn't x just a pointer

[static vigil]

Because the syntax is really working with pointers.

An array is implicitly converted into a pointer.

Then x[i] is just syntactic sugar for *(x + i).

[proper ivy]

I kinda get that but what I am struggling to understand is, we have a pointer that is pointing at the array right? The only thing stored in the pointer is the adress so how come you can get the first adress by just doing x[i] when x is not even an array to begin with.

[static vigil]

This.

When x is a pointer to an element of an array, x + i is the pointer to the element i positions after that.

[proper ivy]

If I understand it correctly, we are talking about a pointer that points at the first element of an array? But what if we want to have a pointer that points at the entire array.

[static vigil]

You will need to dereference the pointer to get the array first.

x there is pointer to an element, not the whole array.

[proper ivy]

okay would you say this correct ```void square_reverse(double *x, double *y, int len)
{
int j = len;

for (int i = 0; i < len; i++)
{
    y[j] = x[i] * x[i];
    j--;
}

}```

I forgot doing the sum

[static vigil]

The syntax is correct but the value of j is not correct.

[proper ivy]

j-1?

[static vigil]

Yes.

Or you could just initilize j using len - 1.

[proper ivy]

sorry that is what I meant.

Here is the most recent ```int square_reverse(double *x, double *y, int len)
{
int j = len - 1;
double sum = 0;

for (int i = 0; i < len; i++)
{
    sum += x[i];
    y[j] = x[i] * x[i];
    j--;
}
return sum;

}```

It also says "Note that your function should also declare parameters as const when appro-
priate." What do they mean? A constant pointer is a pointer that can't change the adress right? So how do I know which one should be constant or if they both should be?

[static vigil]

If you never modify the values they can be const.

constness for a pointer can apply to two points: the element the pointer points to or the pointer variable itself.