How to Define a Default Input Argument for a Function?

How to define a default input argument for a function? I want to set a default input argument which will be replaced later by new values entered by user. How?

Answer 1

[Adam]

function someFunc( someInput )

narginchk( 0, 1 )

if ~exist( 'someInput', 'var' )
   someInput = 7;
end

...

end

Generally I add an extra line something like the following, but I'm fussy:

if exist( 'someInput', 'var' )
   validateattributes( someInput, { 'double' }, { 'scalar', 'nonnegative' } )
else
   someInput = 7;
end