switch from cpp to hpp and back

function! SwitchToAlternate()
     let l:Filename = expand('%:t:r')
     let l:Extension = expand('%:t:e')
     if l:Extension == "hpp"
         let l:Alternate = l:Filename.".cpp"
     elseif l:Extension == "cpp"
         let l:Alternate = l:Filename.".hpp"
     endif
     echom "switching to ".l:Alternate
     exec "e ".l:Alternate
endfunction

nnoremap <localleader>a :call SwitchToAlternate()<cr>