Return Error

[NICE8x]

I just can't get this function to work. Right now it won't compile. I just cannot figure out why it says the variable row is uninitialized. Am I doing the pointer comparison wrong?


By the way the code below compiled but did not return the correct value.

Thank you for your help!!

[keskiverto]

it says the variable row is uninitialized

Consider the case, where the if condition is never true. On such call the function is effectively this:

int Matrix_row( const Matrix*, const int* ) {
int row;
// waste some cpu cycles
return row ;
}

What was the value that the function returns? Uninitialized. Undefined.


You have type class Matrix. Why do you have to do index math? Why the Matrix does not have a function for it?


It does seem that a Matrix has a one-dimensional array, but represents a 2-D array.
If the ptr is a pointer to element k in mat->data, then ptr == mat->data + k
In other words, auto k = ptr - mat->data; and you can calculate the row from k.

mat->data + mat->width * r + c == mat->data + k
=>
mat->width * r + c == k
=>
mat->width * r == k - c
=>
r = k / mat->width - c / mat->width
=>
r = k / mat->width

the code below compiled

Really? Is this legal, and if yes, what does it return?

int foo()
{
  for ( int bar = 0; bar < 7; ++bar ) {
  }

  return bar - 1;
}

https://msdn.microsoft.com/en-us/library/84wcsx8x.aspx writes:

Standard behavior is to let a for loop's initializer [bar] go out of scope after the for loop [on line 4].

The line 6 should thus be an error.

// REQUIRES: mat points to a valid Matrix

If the mat were a reference and not a pointer, then it could never be an invalid pointer.

[NICE8x]

This makes so much sense but it doesn't work either. I'm still not getting the right answer.

[NICE8x]

Any idea why this wouldn't work???

// REQUIRES: mat points to a valid Matrix
//           ptr points to an element in the Matrix
// EFFECTS:  Returns the row of the element pointed to by ptr.
int Matrix_row(const Matrix* mat, const int* ptr) {

	int k = ptr - (mat->data);
	int row = k / (mat->width);
	return row;
}