字符串C ++中的乘法 [英] string Multiplication in C++

问题描述

这里已经有一个问题：如何在C ++中重复字符串可变次数？因为问题配制不当主要是关于字符乘法的答案。有两个正确，但昂贵的答案，所以我将在这里提高要求。

There is already a question for this here: How to repeat a string a variable number of times in C++? However because the question was poorly formulated primarily answers about character multiplication were given. There are two correct, but expensive answers, so I'll be sharpening the requirement here.

Perl提供 x 运算符： http：//perldoc.perl .org / perlop.html＃Multiplicative-Operators ，让我这样做：

Perl provides the x operator: http://perldoc.perl.org/perlop.html#Multiplicative-Operators which would let me do this:

$foo = "0, " x $bar;

我知道我可以使用辅助函数，例如另一个答案中的函数。我想知道我可以做没有我自己的帮助功能吗？我的首选项将是一个，我可以初始化一个 const字符串与，但如果我不能这样做，我很确定这可以用标准算法回答，一个lambda。

I understand that I can do this with the helper functions such as those in the other answer. I want to know can I do this without my own helper function? My preference would be something that I could initialize a const string with, but if I can't do that I'm pretty sure that this could be answered with a standard algorithm and a lambda.

推荐答案

您可以覆盖乘法运算符

#include <string>
#include <sstream>
#include <iostream>


std::string operator*(const std::string& str, size_t times)
{
    std::stringstream stream;
    for (size_t i = 0; i < times; i++) stream << str;
    return stream.str();
}

int main() {
    std::string s = "Hello World!";
    size_t times = 5;

    std::string repeated = s * times;
    std::cout << repeated << std::endl;

    return 0;
}

...或使用lambda ...

... or use a lambda ...

#include <string>
#include <sstream>
#include <iostream>

int main() {
    std::string s = "Hello World!";
    size_t times = 5;

    std::string repeated = [](const std::string& str, size_t times) {std::stringstream stream; for (size_t i = 0; i < times; i++) stream << str; return stream.str(); } (s, times);
    std::cout << repeated << std::endl;

    return 0;
}

...或者使用引用捕获的lambda ...

... or use a lambda with reference capturing ...

#include <string>
#include <sstream>
#include <iostream>

int main() {
    std::string s = "Hello World!";
    size_t times = 5;

    std::string repeated = [&s, &times]() {std::stringstream stream; for (size_t i = 0; i < times; i++) stream << str; return stream.str(); }();
    std::cout << repeated << std::endl;

    return 0;
}

---

c $ c> std :: string 结合使用 std :: string ：reserve（size_t），因为你已经知道（或可以计算）结果字符串的大小。

---

Instead of using std::stringstream you could also use std::string in combination with std::string::reserve(size_t) as you already know (or can calculate) the size of the result string.

std::string repeated; repeated.reserve(str.size() * times);
for (size_t i = 0; i < times; i++) repeated.append(str);
return repeated;

这可能更快：比较 http://goo.gl/92hH9M 与 http://goo.gl/zkgK4T

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