Write a function that takes a vector as input and returns the average of all the positive numbers that are multiples of 7

My current code is

function avg = avgpositive7multiples_for(x);

positive = x(x>=0);

y = sum(positive(find(mod(positive, 7)==0)));

a = numel(find((mod(positive, 7)==0)));

avg = y/a;

but i have to use a for loop for this problem and I am unsure of how to convert it