how to map keys for popup menu in vim

function! OmniPopup(action)
    if pumvisible()
        if a:action == 'j'
            return "\<C-N>"
        elseif a:action == 'k'
            return "\<C-P>"
        endif
    endif
    return a:action
endfunction

inoremap <silent>j <C-R>=OmniPopup('j')<CR>
inoremap <silent>k <C-R>=OmniPopup('k')<CR>

I prefer using the tab key for completion (I am not sure where I got this from):

"tab complete
function! InsertTabWrapper(direction)
    let col = col('.') - 1
    if !col || getline('.')[col - 1] !~ '\k'
        return "\<tab>"
    elseif "backward" == a:direction
        return "\<c-p>"
    else
        return "\<c-n>"
    endif
endfunction
inoremap <tab> <c-r>=InsertTabWrapper ("forward")<cr>
inoremap <s-tab> <c-r>=InsertTabWrapper ("backward")<cr>