I want to save this so I can reference how to use code tags

[Ganado]

You're passing 'mat' (a single double, despite its name), instead of 'arr' (a 2D array of doubles).

When posting code, please use code formatting with [code] [/code] tags, and include the stuff before main, like #include statements and using statements.

[deleted account xyzzy]

Here's some reading for how to use code tags as well as other tags you can use:

http://www.cplusplus.com/articles/jEywvCM9/
http://www.cplusplus.com/articles/z13hAqkS/

HINT: you can edit your post and add code tags.

Using formatting and indentation wouldn't be a bad thing either.