string type in vim script language

Hi, all

I'm new to vim script language and struggling to code a simple
function to switch between header and source.
Here are the

function! SwitchSourceHeader()
if (expand ("%:e") == "cpp" || expand ("%:e") == "c" || expand
("%:e") == "cc" )
find %:t:r.h
elseif ( expand ("%:e") == "h" )
let filename = ""
filename = expand(%:t:r)
filename = filename + ".cpp"
if filereadable( filename )
find %:t:r.cpp
else
echo " cannot find source file. "
endif
else
echo "file type unrecognized."
endif
endfunction
nmap ,s :call SwitchSourceHeader()<CR>

However, I get the following

Error detected while processing function SwitchSourceHeader:
line 5:
E492 Not an editor command: filename = expand(%:t:r)

I am wondering if anyone can point out a correct way for doing this.

best,
--
Shuda Li
--------------------------------
PhD  Candidate
Computer Vision Group
Room 1.15
Merchant Venturers Building
Woodland Road
the University of Bristol
Bristol BS8 1UB
United Kingdom
---------------------------------
Email:  csxsl@bristol.ac.uk
            csxsl@compsci.bristol.ac.uk
web:    http://www.cs.bris.ac.uk/~csxsl/
Office:    +44 (0)117 954 5629
Fax:       +44 (0)117 954 5208
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