MATLAB向量/一维数组的索引约定

Question

考虑以下两个向量的预分配：

vecCol = NaN( 3, 1 );
vecRow = NaN( 1, 3 );

现在的目标是给这些向量赋值（例如，在循环中，如果无法进行矢量化）。关于索引，是否有惯例或最佳实践？以下方法是否推荐？

for k = 1:3
    vecCol( k, 1 ) = 1; % Row, Column
    vecRow( 1, k ) = 2; % Row, Column
end

还是按照以下方式编码更好吗？

for k = 1:3
    vecCol(k) = 1; % Element
    vecRow(k) = 2; % Element
end

- Discbrake

Answer 1

从功能上来说并没有什么区别。如果上下文表示向量始终是1D(您在此示例中的命名约定有所帮助)，那么您可以仅使用vecCol(i)来简洁灵活地使用它。但是，使用vecCol(i,1)语法也有一些优点：

• It's explicitly clear which type of vector you're using. This is good if it matters, e.g. when using linear algebra, but might be irrelevant if direction is arbitrary.
• If you forget to initialise (bad but it happens) then it will ensure the direction is as expected
• It's a good habit to get into so you don't forget when using 2D arrays

• It appears to be slightly quicker. This will be negligible on small arrays but see the below benchmark for vectors with 10^8 elements, and a speed improvement of >10%.

  function benchie()
  % Benchmark. Set up large row/column vectors, time value assignment using timeit.
      n = 1e8;
      vecCol = NaN(n, 1); vecRow = NaN(1, n);
      f = @()fullidx(vecCol, vecRow, n);
      s = @()singleidx(vecCol, vecRow, n);
      timeit(f)
      timeit(s)
  end
  function fullidx(vecCol, vecRow, n)
  % 2D indexing, copied from the example in question
      for k = 1:n
          vecCol(k, 1) = 1; % Row, Column
          vecRow(1, k) = 2; % Row, Column
      end
  end
  function singleidx(vecCol, vecRow, n)
  % Element indexing, copied from the example in question
      for k = 1:n
          vecCol(k) = 1; % Element
          vecRow(k) = 2; % Element
      end
  end
  

  Output (tested on Windows 64-bit R2015b, your mileage may vary!)

  % f (full indexing):    2.4874 secs
  % s (element indexing): 2.8456 secs
  

  Iterating this benchmark over increasing n, we can produce the following plot for reference.

  

- Wolfie

Answer 2

在编程中，有一个普遍的经验法则是“显式优于隐式”。由于这两种方法之间没有功能上的区别，因此我认为它取决于上下文哪一种更清晰/更好：

• 如果上下文使用了大量矩阵代数，并且行向量和列向量之间的区别很重要，则使用两个参数的索引来减少错误并方便阅读。

• 如果上下文不太区分两者，而您只是将向量用作简单数组，则使用一个参数的索引会更加清晰。

- Rody Oldenhuis