一千萬個為什麽

搜索

MATLAB中的單元格和數組的連接和索引有何不同?

我對MATLAB中的單元格和數組的使用有點困惑,並希望對幾點進行一些澄清。以下是我的觀察:

  1. An array can dynamically adjust its own memory to allow for a dynamic number of elements, while cells seem to not act in the same way:

    a=[]; a=[a 1]; b={}; b={b 1};
    
  2. Several elements can be retrieved from cells, but it doesn't seem like they can be from arrays:

    a={'1' '2'}; figure; plot(...); hold on; plot(...); legend(a{1:2});   
    b=['1' '2']; figure; plot(...); hold on; plot(...); legend(b(1:2));
    %# b(1:2) is an array, not its elements, so it is wrong with legend.
    

這些是正確的嗎?單元格和數組之間有什麽其他不同的用法?

最佳答案

Cell arrays can be a little tricky since you can use the [], (), and {} syntaxes in various ways for creating, concatenating, and indexing them, although they each do different things. Addressing your two points:

  1. To grow a cell array, you can use one of the following syntaxes:

    b = [b {1}];     % Make a cell with 1 in it, and append it to the existing
                     %   cell array b using []
    b = {b{:} 1};    % Get the contents of the cell array as a comma-separated
                     %   list, then regroup them into a cell array along with a
                     %   new value 1
    b{end+1} = 1;    % Append a new cell to the end of b using {}
    b(end+1) = {1};  % Append a new cell to the end of b using ()
    
  2. When you index a cell array with (), it returns a subset of cells in a cell array. When you index a cell array with {}, it returns a comma-separated list of the cell contents. For example:

    b = {1 2 3 4 5};  % A 1-by-5 cell array
    c = b(2:4);       % A 1-by-3 cell array, equivalent to {2 3 4}
    d = [b{2:4}];     % A 1-by-3 numeric array, equivalent to [2 3 4]
    

    For d, the {} syntax extracts the contents of cells 2, 3, and 4 as a comma-separated list, then uses [] to collect these values into a numeric array. Therefore, b{2:4} is equivalent to writing b{2}, b{3}, b{4}, or 2, 3, 4.

    With respect to your call to legend, the syntax legend(a{1:2}) is equivalent to legend(a{1}, a{2}), or legend('1', '2'). Thus two arguments (two separate characters) are passed to legend. The syntax legend(b(1:2)) passes a single argument, which is a 1-by-2 string '12'.

轉載註明原文: MATLAB中的單元格和數組的連接和索引有何不同?