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First of all, y is not defined. And what is the size of mat.pointer? Set a breakpoint on the line (I know you know how to do this because you've been around here long enough), and do this:
If it's not a 1 row by 4 column character array, then the size of mat.pointer and 'none' don't match. And, needless to say, because you're using "end" mat.pointer must already exist, which it might because I can tell you've left out some code.