strncmp: bug comparing two character arrays

[Thorsten Meyer]

Hi,

John W. Eaton wrote:
> On  1-Jan-2009, Thorsten Meyer wrote:
>
> | Hi,
> | 
> | while patching the bug just reported I found another one in octave mercurial:
> | 
> | octave:56> strncmp(["ab";"cd"], ["ab";"ce"], 2)
> | ans =  1
> | octave:57> strncmp(["ab";"cd"], ["ab";"ce"], 3)
> | ans =  1
> | octave:58> strncmp(["ab";"cd"], ["ab";"ce"], 4)
> | ans = 0
> | 
> | The relevant part of the source code in src/strfns.cc is here:
> | 
> | 	      for (int i = 0; i < n; i++)
> | 		{
> | 		  if (s1(i) != s2(i))
> | 		    {
> | 		      retval = false;
> | 		      return retval;
> | 		    }
> | 		}
> | 
> | 	      retval = true;
> | 
> | So strncmp compares the first n elements of the complete character array and not the first n
> | elements of each  string in the character array.
>
> | I can probably provide a patch for the 2d-case. But what should the function do in case of two n
> | dimensional character arrays with n>2 (and how do I index into an n-d array?)?
>
> Do the N-d arrays have the same dimensions?  Or at least the same
> number of elements?  Probably all you need to do is iterate over all
> the elements.  Then you can use a single index.
>   
As Ben pointed out, compatible behaviour is to compare the first n chars
of the array (in the internal representation). The octave function
strncmp does this already even for the n-d case.

regards

Thorsten