Finding the sum of every nth row ( n is not fixed)

Hello,

Consider a nx2 matrix M with

M= [
0 4
1 10
1 11
1 20
2 4
2 9
2 8
2 7
.
23 8
23 6
0 8
0 7
1 2
2 5
.
.
];

The 1st row refers to hours of the day, i.e. 0,1,...,23

The values of hours per sequence varies, for example the 1st sequence (1st day) has

1 "0"
3 "1" etc

while the 2nd sequence (2nd day) has

2 "0"
1 "1"

etc.

I need to find the sum of the 2nd column per hour and per day.

For example, for the 1st day

4 (for "0")

41 (for "1")

etc.

and for the 2nd day

15 (for "0")

2 (for "1")

etc.

Thank you very much

Best,

Pavlos

Answer 1

[dpb]

You'll have to introduce a day variable...by locating the positions where the hour returns to 0 if is always a 0-hour reading or when the diff() is <0 if not; then fill between those with either an arbitrary day (but sequential) number or an actual date if have it.

Once that's done,

[g,da,hr]=findgroups(M(:),M(:));  % grouping variable by day, hour
tot=splitapply(@sum,M(:,3),g);    % sum by group
res=table(da,hr,tot);             % assemble results in table

the above assumes the day number is prepended to M as the first column.

[pavlos]

Thank you for your response.

I get the following error

"Index in position 2 exceeds array bounds (must not exceed 2)."

when applying the tot=splitapply(@sum,M(:,3),g);

[dpb]

As noted, I presumed you introduced the new and needed day variable as column one into your array M so it subsequently has three, not two columns.