Separating table data by year

From a given table of data from 1900 to 2017 of low temperatures i want to separate by year then run functions to find the number of days per year where temperatures were lower than the average over the period of 1900-2017.

below is what was provided as the example, but im having trouble separating out specified years to work on

gDays = findgroups(day(BostonTemps.Date, 'dayofyear'));
avgTmin = splitapply(@mean, BostonTemps.Tmin, gDays);
stdTmin = splitapply(@std, BostonTemps.Tmin, gDays);
[gYears,years] = findgroups(year(BostonTemps.Date));

Answer 1

[Scott MacKenzie]

Here's what I put together using random temperatures between 30 and 100 over the period of interest. Since the temperatures are random, about half of the 365 or 366 days each year are lower than the period average.

% test data (random hourly temperatures from 1900 to 2017)
dt1 = datetime('1900-01-01', 'InputFormat','yyyy-MM-dd');
dt2 = datetime('2017-12-31', 'InputFormat','yyyy-MM-dd');
dt = (dt1:hours(1):dt2)';
tmp = randi([30 100], length(dt),1); % random temperatures between 30 and 100 

% organize in timetable
TT = timetable(dt, tmp);

% retime to get daily mean temperature
TT1 = retime(TT, 'daily', 'mean');

% average daily temperature for the period 1900 to 2017
epochAverage = mean(TT1.tmp);

% add a column flagging each day where the average temperature < epochAverage
TT1.LowerThanAverage = TT1.tmp < epochAverage;

% retime to get the number of days each year with temperature < epochAverage
TT2 = retime(TT1, 'yearly', 'sum');

epochAverage % average temperature over the period of interest

epochAverage = 64.9968

TT2([1:5 (end-4):end], 2) % results for first 5 years and last 5 years

ans = 10×1 timetable
        dt         LowerThanAverage
    ___________    ________________

    01-Jan-1900          189       
    01-Jan-1901          191       
    01-Jan-1902          180       
    01-Jan-1903          189       
    01-Jan-1904          172       
    01-Jan-2013          164       
    01-Jan-2014          174       
    01-Jan-2015          174       
    01-Jan-2016          181       
    01-Jan-2017          184