在 C++ 向量的每个元素上调用函数
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Calling a function on every element of a C++ vector
提问by vikaspraj
In C++, is there a way to call a function on each element of a vector, without using a loop running over all vector elements? Something similar to a 'map' in Python.
在 C++ 中，有没有一种方法可以在向量的每个元素上调用函数，而不使用在所有向量元素上运行的循环？类似于 Python 中的“地图”。
回答by Oliver Charlesworth
Yes: std::for_each.
#include <algorithm> //std::for_each
void foo(int a) {
std::cout << a << "\n";
}
std::vector<int> v;
...
std::for_each(v.begin(), v.end(), &foo);
回答by Jerry Coffin
You've already gotten several answers mentioning std::for_each.
你已经得到了几个提到std::for_each.
While these respond to the question you've asked, I'd add that at least in my experience, std::for_eachis about the leastuseful of the standard algorithms.
虽然这些回答了您提出的问题，但至少根据我的经验，我要补充一点，这std::for_each是标准算法中最没用的。
I use (for one example) std::transform, which is basically a[i] = f(b[i]);or result[i] = f(a[i], b[i]);much more frequently than std::for_each. Many people frequently use std::for_eachto print elements of a collection; for that purpose, std::copywith an std::ostream_iteratoras the destination works much better.
我使用（为一个例子）std::transform，它基本上是a[i] = f(b[i]);或result[i] = f(a[i], b[i]);更频繁比std::for_each。许多人经常使用std::for_each打印集合的元素；为此，std::copy使用std::ostream_iteratoras 作为目的地效果会更好。
回答by Indy9000
On C++ 11: You could use a lambda. For example:
在 C++ 11 上：您可以使用 lambda。例如：
std::vector<int> nums{3, 4, 2, 9, 15, 267};
std::for_each(nums.begin(), nums.end(), [](int &n){ n++; });
回答by Alexander Poluektov
Use for_each:
使用for_each：
// for_each example
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
void myfunction (int i) {
cout << " " << i;
}
struct myclass {
void operator() (int i) {cout << " " << i;}
} myobject;
int main () {
vector<int> myvector;
myvector.push_back(10);
myvector.push_back(20);
myvector.push_back(30);
cout << "myvector contains:";
for_each (myvector.begin(), myvector.end(), myfunction);
// or:
cout << "\nmyvector contains:";
for_each (myvector.begin(), myvector.end(), myobject);
cout << endl;
return 0;
}
回答by chris
If you have C++11, there's an even shorter method: ranged-based for. Its purpose is exactly this.
如果你有 C++11，还有一个更短的方法：基于范围的 for. 它的目的正是如此。
std::vector<int> v {1,2,3,4,5};
for (int element : v)
std::cout << element; //prints 12345
You can also apply references and const to it as well, when appropriate, or use auto when the type is long.
您也可以在适当的时候对其应用引用和 const，或者在类型很长时使用 auto。
std::vector<std::vector<int>> v {{1,2,3},{4,5,6}};
for (const auto &vec : v)
{
for (int element : vec)
cout << element;
cout << '\n';
}
Output:
输出：
123
456
回答by juanchopanza
You can use std::for_eachwhich takes a pair of iterators and a function or functor.
您可以使用std::for_each，它带有一对迭代器和一个函数或函子。
回答by Mateo
The OP mentions the mapfunction in Python.
This function actually applies a function to every element of a list (or iterable)
and returns a list (or iterable) that collects all results.
In other words, it does something like this:
OP 提到了mapPython中的函数。该函数实际上将一个函数应用于列表（或可迭代）的每个元素，并返回一个收集所有结果的列表（或可迭代）。换句话说，它做这样的事情：
def f( x ) :
""" a function that computes something with x"""
# code here
return y
input = [ x1, x2, x3, ... ]
output = map( func, input )
# output is now [ f(x1), f(x2), f(x3), ...]
Hence, the closest C++ standard-library equivalent to Python's map is actually std::transform(from the ` header).
因此，与 Python 映射最接近的 C++ 标准库实际上是std::transform（来自 ` 标头）。
Example usage is as follows:
示例用法如下：
#include <vector>
#include <algorithm>
using namespace std;
double f( int x ) {
// a function that computes the square of x divided by 2.0
return x * x / 2.0 ;
}
int main( ) {
vector<int> input{ 1, 5, 10 , 20};
vector<double> output;
output.resize( input.size() ); // unfortunately this is necessary
std::transform( input.begin(), input.end(), output.begin(), f );
// output now contains { f(1), f(5), f(10), f(20) }
// = { 0.5, 12.5, 50.0, 200.0 }
return 0;
}
