我正在编写一个matlab脚本，最终将文本行的hundreads输出到文件中。现在我只是继续添加如下文字：
Output = [];
Output = [Output NewText];
但我认为这是低效的，因为它必须每次都创建一个新的矩阵。什么是更好的方式。
在我准备好编写所有文本之前，我无法打开文件，所以我不能继续在输出文件上使用fprintf。
答案 0 :(得分：5)
至少对我来说，没有一个明显的最佳答案。一些选项是：
我的直觉表明性能顺序是：
但这并不明显。
幸运的是，它很容易测试。所有5个选项（以及一些测试包装器）的实现包含在下面的大型测试块中。我的计算机上的结果（一台带有SSD的计算机，结果可能会有所不同）低于（格式化代码输出中添加的空格）：
-------------Start of file write speed tests. (nLines = 1)------------
Time for BaseLine operation: 0.001540 sec
Time for AutoAllocate operation: 0.001264 sec
Time for AutoAllocateCell operation: 0.003492 sec
Time for JavaStringBuilder operation: 0.001395 sec
Time for IncrementalWriteToFile operation: 0.001057 sec
-------------Start of file write speed tests. (nLines = 100)------------
Time for BaseLine operation: 0.011909 sec
Time for AutoAllocate operation: 0.014067 sec
Time for AutoAllocateCell operation: 0.011517 sec
Time for JavaStringBuilder operation: 0.021291 sec
Time for IncrementalWriteToFile operation: 0.016213 sec
-------------Start of file write speed tests. (nLines = 10000)------------
Time for BaseLine operation: 3.778957 sec
Time for AutoAllocate operation: 1.048480 sec
Time for AutoAllocateCell operation: 0.856269 sec
Time for JavaStringBuilder operation: 1.657038 sec
Time for IncrementalWriteToFile operation: 1.254080 sec
-------------Start of file write speed tests. (nLines = 100000)------------
Time for BaseLine operation: 358.312820 sec
Time for AutoAllocate operation: 10.349529 sec
Time for AutoAllocateCell operation: 8.539117 sec
Time for JavaStringBuilder operation: 16.520797 sec
Time for IncrementalWriteToFile operation: 12.259307 sec
所以，如果你使用“100”的线条，它可能并不重要;做任何事都行。如果你知道性能问题，那么我会使用“AutoAllocateCell”选项。这是非常简单的代码（见下文）。如果你没有足够的内存来将整个文件一次存储在内存中，我会使用“AutoAllocateCell”选项并定期刷新文件。
测试代码：
%Setup
cd(tempdir);
createLineLine = @(n, s) sprintf('[%04d] %s\n', n, s);
createRandomLine = @(n) createLineLine(n, char(randi([65 122],[1, round(rand*100)])));
for nLines = [1 100 10000 100000]
fprintf(1, ['-------------Start of file write speed tests. (nLines = ' num2str(nLines) ')------------\n']);
%% Baseline -----------------------------
strName = 'BaseLine';
rng(28375213)
tic;
str = [];
for ix = 1:nLines;
str = [str createRandomLine(ix)];
end
fid = fopen(['WriteTest_' strName],'w');
fprintf(fid, '%s', str);
fclose(fid);
fprintf(1, 'Time for %s operation: %f sec\n', strName, toc);
%% AutoAllocated string -----------------------------
strName = 'AutoAllocate';
rng(28375213)
tic;
str = blanks(256);
ixLastValid = 0;
for ix = 1:nLines;
strNewLine = createRandomLine(ix);
while (ixLastValid+length(strNewLine)) > length(str)
str(end*2) = ' '; %Doubles length of string
end
str(ixLastValid + (1:length(strNewLine))) = strNewLine;
ixLastValid = ixLastValid+length(strNewLine);
end
fid = fopen(['WriteTest_' strName],'w');
fprintf(fid, '%s', str(1:ixLastValid));
fclose(fid);
fprintf(1, 'Time for %s operation: %f sec\n', strName, toc);
%% AutoAllocated cell array -----------------------------
strName = 'AutoAllocateCell';
rng(28375213)
tic;
strs = cell(256,1);
ixLastValid = 0;
for ix = 1:nLines;
if ix>length(strs);
strs{end*2} = {}; %Doubles cell array size;
end
strs{ix} = createRandomLine(ix);
ixLastValid = ixLastValid + 1;
end
fid = fopen(['WriteTest_' strName],'w');
fprintf(fid, '%s', strs{1:ixLastValid});
fclose(fid);
fprintf(1, 'Time for %s operation: %f sec\n', strName, toc);
%% Java string builder -----------------------------
strName = 'JavaStringBuilder';
rng(28375213)
tic;
sBuilder = java.lang.StringBuilder;
for ix = 1:nLines;
sBuilder.append(createRandomLine(ix));
end
fid = fopen(['WriteTest_' strName],'w');
fprintf(fid, '%s', char(sBuilder.toString()));
fclose(fid);
fprintf(1, 'Time for %s operation: %f sec\n', strName, toc);
%% Incremental write to file -----------------------------
strName = 'IncrementalWriteToFile';
rng(28375213)
tic;
fid = fopen(['WriteTest_' strName],'w');
for ix = 1:nLines;
fprintf(fid, '%s', createRandomLine(ix));
end
fclose(fid);
fprintf(1, 'Time for %s operation: %f sec\n', strName, toc);
end
答案 1 :(得分：1)
就像Oli在this question的评论中所说，一个字符是一个行向量，因此任何适用于行向量的技术都将用于字符串。首先分配你认为合理的，如1000个字符，然后如果你超出限制，加倍大小或选择你自己的算法。
这是一个陈腐的例子：
testStrings = {['Is this a dagger which I see before me,' sprintf('\n') ],...
['The handle toward my hand? Come, let me clutch thee.' sprintf('\n') ],...
['I have thee not, and yet I see thee still.' sprintf('\n') ],...
['Art thou not, fatal vision, sensible' sprintf('\n') ],...
['To feeling as to sight? or art thou but' sprintf('\n') ],...
['A dagger of the mind, a false creation,' sprintf('\n') ],...
['Proceeding from the heat-oppressed brain?' sprintf('\n') ],...
['I see thee yet, in form as palpable' sprintf('\n') ],...
['As this which now I draw.' sprintf('\n') ],...
['Thou marshall''st me the way that I was going;' sprintf('\n') ],...
['And such an instrument I was to use.' sprintf('\n') ],...
['Mine eyes are made the fools o'' the other senses,' sprintf('\n') ],...
['Or else worth all the rest; I see thee still,' sprintf('\n') ],...
['And on thy blade and dudgeon gouts of blood,' sprintf('\n') ],...
['Which was not so before. There''s no such thing:' sprintf('\n') ],...
['It is the bloody business which informs' sprintf('\n') ],...
['Thus to mine eyes. Now o''er the one halfworld' sprintf('\n') ],...
['Nature seems dead, and wicked dreams abuse' sprintf('\n') ],...
['The curtain''d sleep; witchcraft celebrates' sprintf('\n') ],...
['Pale Hecate''s offerings, and wither''d murder,' sprintf('\n') ],...
['Alarum''d by his sentinel, the wolf,' sprintf('\n') ],...
['Whose howl''s his watch, thus with his stealthy pace.' sprintf('\n') ],...
['With Tarquin''s ravishing strides, towards his design' sprintf('\n') ],...
['Moves like a ghost. Thou sure and firm-set earth,' sprintf('\n') ],...
['Hear not my steps, which way they walk, for fear' sprintf('\n') ],...
'Thy very stones prate of my whereabout,'};
A = zeros(1,1000);
idx = 1;
for ii=1:length(testStrings)
str = testStrings{ii};
N = length(str);
eIdx = idx+N-1;
if( eIdx > length(A) )
A = [ A zeros(1,length(A)*2) ];
end
A( idx:(idx+N-1) ) = str;
idx = idx + N;
end
fprintf('%s',char(A))