What should I return from a function?

Question

I have a function in C language which takes 3 matrices as arguments; It performs matrix multiplication of first 2 matrices and stores the result in the 3rd one.

My question is what should I return from the function. Should I return the resultant matrix it self (rather the pointer to it) so that function calls can be chained togather like so :

display_matrix(multiply_matrix(A, B, C));

Or should I return 0 or 1 based on whether the operation was successful or not (incorrect dimensions might return failure)? In this case the line shown above would look something like this.

multiply_matrix(A, B, C);
display_matrix(C);

So, what would be the better way to do it? Or is it just a matter of choice?

Answer 1

Since your operation can fail, you need some way to represent this. The chaining design is nice-to-have, but is possible only if you don't need to return anything else instead.

The options are:

• return success or failure codes

  • con: in this case you obviously can't use the function chaining style
  • con: you also need to pepper your code with tests and error handling
  • con: any operations whose result isn't checked may leave the out-param in an unexpected state
  • pro: if some operations does fail, you know exactly where it happened (assuming you don't copy-paste your error handling boilerplate incorrectly...)

  Your sample code should actually look something like:

  if ((rc=multiply_matrix(A, B, C)) != MAT_OK) {
    display_matrix_error(rc, A, B);
  } else {
    display_matrix(C);
  }
  

• use some matrix equivalent of NaN to represent failed computations, and just pass them through your function chain silently.

  • pro: in this case you get the nice function chaining style
  • pro: you don't need explicit error-checking everywhere
  • con: you can't tell which step failed if a long chain of computations results in an invalid matrix

  Your sample code really will look like you posted:

  display_matrix(multiply_matrix(A, B, C));
  

  where the display function needs to know how to handle an invalid input.

Answer 2

Number of issues here. Let me tease them apart.

First, don't give back your result with both an out parameter and a return. That's just confusing. Pick one.

Second, c has no exception handling. Instead c functions either return errors or they handle/avoid them so that no error escapes. If you choose to return your result then you need a robust way of always returning something meaningful.

Third, if you return your error status and return the result in an out parameter please document this clearly.

Answer 3

In C it's not uncommon to return pointers. Returning a null pointer could signify failure. Plus returning a pointer avoids a potentially large amount of data copying.

So you can have both error returns and (maybe somewhat limited, depending on other choices you make) function chaining. (Note your matrix functions would need to accept input parameters as pointers as well).

Function chaining would have to deal with getting a null pointer on input. But this could be treated analogously to NAN values for floating point arithmetic. If a null ptr is passed to display_matrix() then a "Not a Matrix" message would be displayed. If multiply_matrix() received a null, if would return a null.

Finally if you need more diagnostic info than just "it failed", you could use an approach like C's errno which gives a failure reason.