MATLAB 中的映射函数? [英] Map function in MATLAB?

问题描述

我对 MATLAB 没有 Map 函数感到有点惊讶，所以我自己开发了一个，因为我离不开它.有没有更好的版本?是否有我缺少的某种标准的 MATLAB 函数式编程库?

I'm a little surprised that MATLAB doesn't have a Map function, so I hacked one together myself since it's something I can't live without. Is there a better version out there? Is there a somewhat-standard functional programming library for MATLAB out there that I'm missing?

function results = map(f,list)
% why doesn't MATLAB have a Map function?
results = zeros(1,length(list));
for k = 1:length(list)
    results(1,k) = f(list(k));
end

end

用法是例如

map( @(x)x^2,1:10)

推荐答案

简短的回答:内置函数 arrayfun 与您的 map 函数对数字数组所做的完全一样:

The short answer: the built-in function arrayfun does exactly what your map function does for numeric arrays:

>> y = arrayfun(@(x) x^2, 1:10)
y =

     1     4     9    16    25    36    49    64    81   100

还有两个类似的内置函数:cellfun(对元胞数组的元素进行操作)和 structfun(对结构的每个字段进行操作).

There are two other built-in functions that behave similarly: cellfun (which operates on elements of cell arrays) and structfun (which operates on each field of a structure).

但是，如果您利用矢量化，特别是使用元素化算术运算符.对于您给出的示例，矢量化解决方案是:

However, these functions are often not necessary if you take advantage of vectorization, specifically using element-wise arithmetic operators. For the example you gave, a vectorized solution would be:

>> x = 1:10;
>> y = x.^2
y =

     1     4     9    16    25    36    49    64    81   100

某些操作会自动跨元素操作(例如向向量添加标量值)，而其他操作符具有用于元素操作的特殊语法(由操作符前的 . 表示).MATLAB 中的许多内置函数旨在使用逐元素运算(通常应用于给定维度，例如 sum 和 mean 例如)，因此不需要地图功能.

Some operations will automatically operate across elements (like adding a scalar value to a vector) while others operators have a special syntax for element-wise operation (denoted by a . before the operator). Many built-in functions in MATLAB are designed to operate on vector and matrix arguments using element-wise operations (often applied to a given dimension, such as sum and mean for example), and thus don't require map functions.

总而言之，这里有一些不同的方法来对数组中的每个元素进行平方:

To summarize, here are some different ways to square each element in an array:

x = 1:10;       % Sample array
f = @(x) x.^2;  % Anonymous function that squares each element of its input

% Option #1:
y = x.^2;  % Use the element-wise power operator

% Option #2:
y = f(x);  % Pass a vector to f

% Option #3:
y = arrayfun(f, x);  % Pass each element to f separately

当然，对于如此简单的操作，选项#1 是最明智(也最有效)的选择.

Of course, for such a simple operation, option #1 is the most sensible (and efficient) choice.

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