I have 2 columns in a matrix. 1st column has the dates. 2nd column has the values. How do I grab all values from 2nd column associated with its date from the 1st column??

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I have a much larger data set, but this is just a smaller sample size example of what I would like to do.
I want to grab all values from 4/1/18: 6
I want to grab all values from 4/2/18: 22, 4
I want to grab all values from 4/2/18: 2, 12, 8
Matrix Example:
Column 1 Column 2
4/1/18 6
4/2/18 22
4/2/18 4
4/3/18 2
4/3/18 12
4/3/18 8

Answers (3)

Dyuman Joshi
Dyuman Joshi on 16 Apr 2022
%example, not sure which date format and data type you are using
y={datetime('2018-01-04') 6;datetime('2018-02-04') 22;datetime('2018-02-04') 4;datetime('2018-03-04') 2;datetime('2018-03-04') 12;datetime('2018-03-04') 8}
y = 6×2 cell array
{[04-Jan-2018]} {[ 6]} {[04-Feb-2018]} {[22]} {[04-Feb-2018]} {[ 4]} {[04-Mar-2018]} {[ 2]} {[04-Mar-2018]} {[12]} {[04-Mar-2018]} {[ 8]}
z=unique(cellfun(@(x) x, y(:,1)))
z = 3×1 datetime array
04-Jan-2018 04-Feb-2018 04-Mar-2018
for i=1:numel(z)
y(find(cellfun(@(x) x, y(:,1))==z(i)),2)'
%you can use curly parenthesis as well
end
ans = 1×1 cell array
{[6]}
ans = 1×2 cell array
{[22]} {[4]}
ans = 1×3 cell array
{[2]} {[12]} {[8]}

Scott MacKenzie
Scott MacKenzie on 16 Apr 2022
Something like this will work:
% test data
d = { '4/1/18', 6;
'4/2/18', 22;
'4/2/18', 4;
'4/3/18', 2;
'4/3/18', 12;
'4/3/18', 8 };
dt = datetime(d(:,1), 'InputFormat','MM/dd/yy');
value = cell2mat(d(:,2));
query1 = dt == datetime('4/1/18', 'InputFormat','MM/dd/yy');
v1 = value(query1)
v1 = 6
query2 = dt == datetime('4/2/18', 'InputFormat','MM/dd/yy');
v2 = value(query2)
v2 = 2×1
22 4
query3 = dt == datetime('4/3/18', 'InputFormat','MM/dd/yy');
v3 = value(query3)
v3 = 3×1
2 12 8

Campion Loong
Campion Loong on 18 May 2022
This kind of tasks is what timetable is built for:
% Using your small example
tt = timetable(datetime(2018,4,[1;2;2;3;3;3]), [6;22;4;2;12;8])
tt = 6×1 timetable
Time Var1 ___________ ____ 01-Apr-2018 6 02-Apr-2018 22 02-Apr-2018 4 03-Apr-2018 2 03-Apr-2018 12 03-Apr-2018 8
% Simply subscript to the data at the timestamp you are looking for,
% e.g. 4/2/2018
tt(datetime(2018,4,2),:)
ans = 2×1 timetable
Time Var1 ___________ ____ 02-Apr-2018 22 02-Apr-2018 4
Bonus: use timerange if you want to get at all data within a range of time
% Between 4/2/2018 and 4/3/2018 (closed interval)
tt(timerange(datetime(2018,4,2), datetime(2018,4,3), 'closed'), :)
ans = 5×1 timetable
Time Var1 ___________ ____ 02-Apr-2018 22 02-Apr-2018 4 03-Apr-2018 2 03-Apr-2018 12 03-Apr-2018 8

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