C++ 使用 STL 将向量元素乘以标量值
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Multiply vector elements by a scalar value using STL
提问by Ismail Marmoush
Hi I want to (multiply,add,etc) vector by scalar value for example myv1 * 3, I know I can do a function with a forloop , but is there a way of doing this using STL function? Something like the {Algorithm.h :: transform function }?
嗨，我想（乘法、加法等）向量乘以标量值，例如myv1 * 3，我知道我可以用 forloop 做一个函数，但是有没有办法使用 STL 函数来做到这一点？类似于 {Algorithm.h :: 变换函数}？
回答by Oliver Charlesworth
Yes, using std::transform:
是的，使用std::transform：
std::transform(myv1.begin(), myv1.end(), myv1.begin(),
std::bind(std::multiplies<T>(), std::placeholders::_1, 3));
Before C++17 you could use std::bind1st(), which was deprecated in C++11.
在 C++17 之前，您可以使用std::bind1st()，它在 C++11 中已被弃用。
std::transform(myv1.begin(), myv1.end(), myv1.begin(),
std::bind1st(std::multiplies<T>(), 3));
For the placeholders;
对于占位符；
#include <functional>
回答by Chris Jester-Young
If you can use a valarrayinstead of a vector, it has builtin operators for doing a scalar multiplication.
如果您可以使用 avalarray而不是 a vector，则它具有用于进行标量乘法的内置运算符。
v *= 3;
If you have to use a vector, you can indeed use transformto do the job:
如果您必须使用 a vector，您确实可以使用它transform来完成这项工作：
transform(v.begin(), v.end(), v.begin(), _1 * 3);
(assuming you have something similar to Boost.Lambdathat allows you to easily create anonymous function objects like _1 * 3:-P)
（假设您有类似于Boost.Lambda 的东西，它允许您轻松创建匿名函数对象，例如_1 * 3:-P）
回答by Spandyie
Mordern C++ solution for your question.
您的问题的现代 C++ 解决方案。
std::vector<double> myarray;
double myconstant{3.3};
std::transform(myarray.begin(), myarray.end(), myarray.begin(), [&myconstant](auto& c){return c*myconstant;});
回答by slashmais
I know this not STL as you want, but it is something you can adapt as different needs arise.
我知道这不是你想要的 STL，但它是你可以随着不同需求的出现而适应的东西。
Below is a template you can use to calculate; 'func' would be the function you want to do: multiply, add, and so on; 'parm' is the second parameter to the 'func'. You can easily extend this to take different func's with more parms of varied types.
以下是您可以用来计算的模板；'func' 将是您想要执行的函数：乘法、加法等；“parm”是“func”的第二个参数。您可以轻松地扩展它以采用具有更多不同类型参数的不同 func。
template<typename _ITStart, typename _ITEnd, typename _Func , typename _Value >
_ITStart xform(_ITStart its, _ITEnd ite, _Func func, _Value parm)
{
while (its != ite) { *its = func(*its, parm); its++; }
return its;
}
...
int mul(int a, int b) { return a*b; }
vector< int > v;
xform(v.begin(), v.end(), mul, 3); /* will multiply each element of v by 3 */
Also, this is not a 'safe' function, you must do type/value-checking etc. before you use it.
此外，这不是一个“安全”功能，您必须在使用它之前进行类型/值检查等。
回答by storluffarn
I think for_eachis very apt when you want to traverse a vector and manipulate each element according to some pattern, in this case a simple lambda would suffice:
我认为for_each当您想遍历向量并根据某种模式操作每个元素时非常合适，在这种情况下，一个简单的 lambda 就足够了：
std::for_each(myv1.begin(), mtv1.end(), [](int &el){el *= 3; });
note that any variable you want to capture for the lambda function to use (say that you e.g. wanted to multiply with some predetermined scalar), goes into the bracket as a reference.
请注意，您想要为 lambda 函数捕获的任何变量（例如，您想要与某个预定标量相乘）都将作为参考放入括号中。
