经常在MATLAB中附加文本的最佳方法

Question

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,所以我不能只在输出文件上继续使用fprintf.

Answer 1

对此我没有明显的最佳答案,至少对我而言.一些选项是:

1. 正是你在做什么,逐渐附加到每个迭代的字符串
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. 使用字符串的单元格数组,并智能地重新分配.(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库有许多有用的功能(例如StringBuilder类),但Matlab-Java接口很慢.
5. 增量直接写入文件(我知道你已经在问题中考虑了这一点,但它仍然是一个有用的基线.)

我的直觉表明性能顺序是:

• 最佳:(2或3)
• 中:(4或5)
• 最糟糕的:(1)

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的相当好的计算机,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"选项和定期刷新文件.

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