在MATLAB中经常附加文本的最佳方法

我正在编写一个matlab脚本，最终将成行的文本输出到一个文件中。现在，我只是不断添加文本，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的矩阵。还有什么更好的办法

---

在准备好写入所有文本之前，我无法打开该文件，因此我不能在输出文件上继续使用fprintf。

正如Oli在对的评论中所说，字符是行向量，因此任何适用于行向量的技术都适用于字符串。从分配你认为合理的数量开始，比如1000个字符，然后如果你超出了限制，就将大小加倍或者选择你自己的算法

下面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))

---

至少对我来说，没有一个明显的最佳答案。有些选择是：

确切地说，您正在做什么，每次迭代都递增地附加到字符串

智能地增加累积字符串以减少重新分配的数量（@macduff答案的核心）

使用字符串的单元格数组，并智能地重新分配该字符串。（我很确定）这只是强制重新分配指针，而不是完全重新分配字符串内容

使用一些Java魔术来处理字符串累积。Java库有许多有用的特性（如StringBuilder类），但MatlabJava接口速度较慢

以增量方式直接写入文件（我知道您在问题中已将其从考虑范围中删除，但它仍然是一个有用的基线。）

我的直觉表明，绩效顺序是：

• 最佳：（2或3）
• 中期：（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）的实现都包含在下面的大型测试块中。我的计算机（一台带SSD的非常好的计算机，结果可能会有所不同）上的结果如下（在代码输出中添加空格以进行格式化）：

所以，如果你用的是“100”行，可能没什么关系；做任何有用的事。如果您知道性能问题，那么我将使用“AutoAllocateCell”选项。这是一个非常简单的代码（见下文）。如果您没有足够的内存将整个文件一次存储在内存中，我将使用“AutoAllocateCell”选项定期刷新文件

---

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

---

您是否预先知道将要追加的“NewText”条目的数量界限？数百行的可能重复项并不多。坚持使用普通的附加，直到您证明这是一个需要担心的性能瓶颈。当涉及I/O时，不太可能。

%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