更新 Matlab 结构数组的每个元素中的一个字段 [英] Updating one field in every element of a Matlab struct array

问题描述

假设我有一个结构数组 arr，其中每个元素都有一堆字段，包括一个名为 val 的字段.我想将每个元素的 val 字段增加一些常量，如下所示:

Suppose I have a struct array arr, where each element has a bunch of fields, including one called val. I'd like to increment each element's val field by some constant amount, like so:

for i = 1:length(arr)
    arr(i).val = arr(i).val + 3;
end

这显然有效，但我觉得应该有一种方法可以在一行代码中做到这一点(并且没有 for 循环).我想出的最好的是两行，并且需要一个临时变量:

This obviously works, but I feel there should be a way to do this in just one line of code (and no for loop). The best I've come up with is two lines and requires a temp variable:

newVals = num2cell([arr.val] + 3);
[arr.val] = deal(newVals{:});

有什么想法吗?谢谢.

推荐答案

请注意，deal 在那里不是必需的:

Just a note, the deal isn't necessary there:

[arr.val] = newVals{:}; % achieves the same as deal(newVals{:})

我知道如何做到这一点的唯一另一种方法(没有 foo 循环)是使用 arrayfun 迭代数组中的每个结构:

The only other way I know how to do this (without the foor loop) is using arrayfun to iterate over each struct in the array:

% make a struct array
arr = [ struct('val',0,'id',1), struct('val',0,'id',2), struct('val',0,'id',3) ]

% some attempts
[arr.val]=arr.val; % fine
[arr.val]=arr.val+3; % NOT fine :(

% works !
arr2 = arrayfun(@(s) setfield(s,'val',s.val+3),arr)

最后一个命令遍历 arr 中的每个结构并返回一个新的，其中 s.val 已设置为 s.val=3.

That last command loops over each struct in arr and returns a new one where s.val has been set to s.val=3.

我认为这实际上比您之前的两行代码和 for 循环效率低，因为它返回 arr 的 copy 而不是就地操作.

I think this is actually less efficient than your previous two-liner and the for loop though, because it returns a copy of arr as opposed to operating in-place.

(很遗憾 Matlab 不支持像 [arr.val]=num2cell([arr.val]+3){:} 这样的分层索引).

(It's a shame Matlab doesn't support layered indexing like [arr.val]=num2cell([arr.val]+3){:}).

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