MATLAB: How to take a difference within the data of the same city but not across the city, Part 3

Question

I have a table

City Date Cumulative_Visitors
-----------------------
NY January 1 100
NY January 2  150
NY January 3  200
SF January 1 70
SF January 2 200
SF January 3 400

I explained the problem of calculating the new_visitors.

If I just use "diff", then, it will create a problem at the first entry of each city.

Then the idea is

run diff
then replace the first entry of each city with the cumulative_visitor of that day

How can one write "replace the first entry of each city with the cumulative_visitor of that day"?

Answer 1

Same answer as this: https://www.mathworks.com/matlabcentral/answers/685423-how-to-take-a-difference-within-the-data-of-the-same-city-but-not-across-the-city