gusucode.com > bigdata 工具箱 matlab源码程序 > bigdata/@tall/head.m
function out = head(in, k) %HEAD Get first few rows of tall array. % TY = HEAD(TX) returns the first few rows of tall array TX. The result % is an unevaluated tall array TY. % % TY = HEAD(TX,K) returns up to K rows from the beginning of tall array % TX. If TX contains fewer than K rows, then the entire array is % returned. % % Example: % % Create a datastore. % varnames = {'ArrDelay', 'DepDelay', 'Origin', 'Dest'}; % ds = datastore('airlinesmall.csv', 'TreatAsMissing', 'NA', ... % 'SelectedVariableNames', varnames) % % % Create a tall table from the datastore. % tt = tall(ds); % % % Extract the first 10 rows of the variable ArrDelay. % f10 = head(tt.ArrDelay,10) % % % Collect the results into memory. % first10 = gather(f10) % % See also: TALL, TALL/TAIL, TALL/GATHER, TALL/TOPKROWS. % Copyright 2016 The MathWorks, Inc. if nargin<2 k = matlab.bigdata.internal.util.defaultHeadTailRows(); else % Check that k is a non-negative integer-valued scalar validateattributes(k, ... {'numeric'}, {'real','scalar','nonnegative','integer'}, ... 'head', 'k') end try outPA = matlab.bigdata.internal.lazyeval.extractHead(in.ValueImpl, k); outAdapt = resetTallSize(in.Adaptor); catch err err = matlab.bigdata.internal.lazyeval.parseExecutionError(err); matlab.bigdata.BigDataException.hThrowAsCallerWithSubmissionStack(err); end out = tall(outPA, outAdapt); % Try to cache the result so that we don't have to revisit the original % data again in future. markforreuse(out); end