matlab - 在 MATLAB 中经常附加文本的最佳方法

Question

我正在编写一个 matlab 脚本，它最终将数百行文本输出到一个文件中。现在我只是继续附加文本，如：

Output = [];
Output = [Output NewText];

但我认为这是低效的，因为它每次都必须创建一个新矩阵。有什么更好的方法。

在我准备好写入所有文本之前，我无法打开文件，所以我不能只在输出文件上继续使用 fprintf。

Answer 1

至少对我来说，对此没有明显的最佳答案。一些选项是：

1. 正是你在做什么，每次迭代都递增地附加到一个字符串
2. 智能地增加您的累积字符串以减少重新分配的数量（@macduff 答案的核心）
3. 使用字符串单元数组，并智能地重新分配它。（我很确定）这只会强制重新分配指针，而不是完全重新分配字符串内容。
4. 使用一些 Java 魔法来处理字符串累积。Java 库有很多有用的特性（例如 StringBuilder 类），但 Matlab-Java 接口速度很慢。
5. 直接增量写入文件（我知道您已将其从问题的考虑中删除，但它仍然是一个有用的基线。）

我的直觉表明表演顺序是：

• 最佳：（2 或 3）
• 中：（4或5）
• 最差： (1)

但这并不明显。

幸运的是，它很容易测试。所有 5 个选项（加上一些测试包装器）的实现都包含在下面的大测试块中。我的电脑上的结果（一台非常好的带有 SSD 的电脑，结果可能会有所不同）如下（在代码输出中添加了空格以进行格式化）：

-------------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

因此，如果您使用“100 行”，那可能并不重要；做任何工作。如果您知道性能很重要，那么我会使用“AutoAllocateCell”选项。这是非常简单的代码（见下文）。如果您没有足够的内存一次将整个文件存储在内存中，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

Answer 2

就像 Oli 在对这个问题的评论中所说的那样，一个字符是一个行向量，所以任何适用于行向量的技术都适用于字符串。首先分配您认为合理的数量，例如 1000 个字符，然后如果您超出限制，将大小加倍或选择您自己的算法。

这是一个老生常谈的例子：

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))