在Matlab的结构数组的每个元素更新一个字段 [英] Updating one field in every element of a Matlab struct array

问题描述

假设我有一个结构阵列改编，其中每个元素都有一堆领域，其中包括一个叫 VAL 。我想用一些恒定的量，像这样来增加每个元素的 VAL 字段：

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

这显然是工作，但我觉得应该有一个方式只有一行code要做到这一点（没有循环）。我拿出最好的是两行，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{:});

任何想法？谢谢你。

Any ideas? Thanks.

推荐答案

刚一说明，在交易是没有必要的有：

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（不福尔循环）的唯一其他方法是使用 arrayfun 来遍历数组中的每个结构：

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)

这是最后一个命令遍历在每个结构改编，并返回一个新的，其中 s.val 已定到 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.

我觉得这其实是比你的previous两班轮效率较低，在for循环，但因为它返回的复制的的改编，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.

（这是一个耻辱Matlab的不支持分层索引如 [arr.val] = num2cell（[arr.val] +3）{：} ）

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

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