Calculate daily values from cumulative values

Hello Matlab experts,

I'm trying to calculate daily values from given cumulative data. Cumulative data is saved in a .mat file. The data about COVID 19 pandemic.

It seems that this code is doing what it is supposed to, but I wonder if there is a better, more efficient way.

Thank you.

load ('cumul_variables.mat')

% Plot cumulative data for the state 
dtv = datetime(dates, 'InputFormat','MM/dd/yy');
yyaxis left
bar (dtv, state_cases);
hold on
yyaxis right
plot(dtv,state_deaths);
set(get(gca(),'YAxis'),'Exponent',0,'TickLabelFormat','%0.f')

% Calculate daily values from cumulative data
state_cases_d = zeros(1,length(dates));
state_deaths_d = zeros(1,length(dates));
state_cases_d(1) = state_cases(1);
state_deaths_d(1) = state_deaths(1);

for i = 2:length(state_cases)
    state_cases_d(i) = state_cases(i) - state_cases(i-1);
    state_deaths_d(i) = state_deaths(i) - state_deaths(i-1);
end

% Plot daily data
figure(2)
yyaxis left
bar (dtv, state_cases_d);
hold on
yyaxis right
plot(dtv,state_deaths_d);
set(get(gca(),'YAxis'),'Exponent',0,'TickLabelFormat','%0.f')

[Jeffrey Clark]

Replace the entire % Calculate daily values from cumulative data section with:

% Calculate daily values from cumulative data
state_cases_d = [state_cases(1) -diff(state_cases(1:length(state_cases)-1))];
state_deaths_d = [state_deaths(1) -diff(state_deaths(1:length(state_deaths)-1))];

[Askic V]

Hello Jeffrey and thank you for the reply.

The suggested code results in variables that have one less element than original ones, therefore the rest of the code (plot) will not work. And why minus exists before diff?

[Voss]

@Askic V I believe those expressions are in error. See my answer for a simpler (and correct) method.

load ('cumul_variables.mat')
state_cases_d_1 = [state_cases(1) -diff(state_cases(1:length(state_cases)-1))]; % erroneous expression
state_cases_d_2 = [state_cases(1) diff(state_cases)]; % intended expression (correct)
state_cases_d_3 = diff([0 state_cases]); % simpler expression (also correct)

isequal(state_cases_d_1,state_cases_d_2)

ans = logical
   0

isequal(state_cases_d_2,state_cases_d_3)

ans = logical
   1

Answer 1

[Voss]

You can simplify your calculations of state_cases_d and state_deaths_d by using the diff function:

load ('cumul_variables.mat')

% Plot cumulative data for the state 
dtv = datetime(dates, 'InputFormat','MM/dd/yy');
yyaxis left
bar (dtv, state_cases);
hold on
yyaxis right
plot(dtv,state_deaths);
set(get(gca(),'YAxis'),'Exponent',0,'TickLabelFormat','%.0f')

% Calculate daily values from cumulative data
state_cases_d = diff([0 state_cases]);
state_deaths_d = diff([0 state_deaths]);

% Plot daily data
figure(2)
yyaxis left
bar (dtv, state_cases_d);
hold on
yyaxis right
plot(dtv,state_deaths_d);
set(get(gca(),'YAxis'),'Exponent',0,'TickLabelFormat','%.0f')