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??

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

Answer 1

[Dyuman Joshi]

%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]}

Answer 2

[Scott MacKenzie]

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

Answer 3

[Campion Loong]

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