"function" as a keyword is only used for defining functions, and cannot be used inside a loop (or inside an "if" or "switch" or other control statement.)
The only kinds of functions that can be defined within loops are anonymous functions.
If you are trying to call a function that you have previously defined, do not include the "function" keyword.
For example,
function r = d(a,b)
r = sqrt(a.^2 + b.^2);
end
could be called in a loop like
dist = inf;
while dist > 1/10
p = rand();
q = rand();
dist = d(p,q);
end
