if filetype == tex

You have to manually detect the filetype first, because vim will detect it automatically only after sourcing your vimrc.

Just add this:

`filetype detect`

before your code.

For those who want to check the current filetype and do something while editing, this should work:

if (&ft=='c' || &ft=='cpp')
    :!g++ %
endif

You can use auto commands to achieve what you want:

autocmd BufNewFile,BufRead *.tex set spell

Another way is to use index():

let fts = ['c', 'cpp']
if index(fts, &filetype) == -1
    " do stuff
endif

index() searches for the first index of an element in a string, and returns -1 if not found. This way you can just append more filetype values to your list, and not add more conditions for each file type.