gather

Create a datastore for theairlinesmall.csvdata set. Select a subset of variables to work with, and treat'NA'values as missing data so thattabularTextDatastorereplaces them withNaNvalues. Convert the datastore into a tall table.

varnames = {“年”,'ArrDelay','UniqueCarrier'}; ds = tabularTextDatastore('airlinesmall.csv','TreatAsMissing','NA',...'SelectedVariableNames',varnames); T = tall(ds)

T = Mx3 tall table Year ArrDelay UniqueCarrier ____ ________ _____________ 1987 8 {'PS'} 1987 8 {'PS'} 1987 21 {'PS'} 1987 13 {'PS'} 1987 4 {'PS'} 1987 59 {'PS'} 1987 3 {'PS'} 1987 11 {'PS'} : : : : : :

Calculate the size of the tall table.

sz = size(T)

sz = 1x2 tall double row vector ? ?

MATLAB® does not immediately evaluate most operations on tall arrays. Instead, MATLAB remembers the operations you perform as you enter them and optimizes the calculations in the background.

When you usegather在未鉴定的高数组,MATLAB执行of the queued operations using the minimum number of passes through the data. This optimization greatly reduces the execution time of large calculations. For this reason, you should usegatheronly when you need to see a result.

Usegatherto execute the calculation and collect the result into memory.

S = gather(sz)

Evaluating tall expression using the Local MATLAB Session: - Pass 1 of 1: Completed in 0.97 sec Evaluation completed in 1.2 sec

S =1×2123523 3

Usegatherwith several inputs to simultaneously evaluate several tall arrays.

Create a tall array from an in-memory array of random integers between 1 and 1000. Calculate the maximum and minimum values in each column.

A = tall(randi(1000,100,7))

A = 100x7 tall double matrix 815 163 645 60 423 583 851 906 795 379 682 95 541 561 127 312 812 43 599 870 930 914 529 533 72 471 265 697 633 166 351 522 696 319 583 98 602 940 97 700 120 816 279 263 876 819 639 940 880 547 655 551 818 34 646 989 : : : : : : : : : : : : : :

b = min(A); c = max(A);

Use the results to determine the overall minimum and maximum values in the array. Collect the final result into memory.

[mnA,mxA] = gather(min(b),max(c));

Evaluating tall expression using the Local MATLAB Session: - Pass 1 of 1: Completed in 0.3 sec Evaluation completed in 0.71 sec

valRange = [mnA mxA]

valRange =1×21 1000