[英]How to copy a char array / String a variable number of times C++
in C++ : Given the std::string s = "copyme" how can I make a string b which is "copymecopymecopyme" or any generalized s + s + ... n times? 在C ++中:给定std :: string s =“ copyme”,我怎样才能使字符串b为“ copymecopymecopyme”或任何广义的s + s + ... n次?
A possible solution would be a method/macro that does a for loop, but that would be horribly inefficient. 可能的解决方案是执行for循环的方法/宏,但效率极低。
What is an efficient method of doing this string copy operation, eg as the previous author says in the style of 'string'*n in python or ruby. 什么是执行此字符串复制操作的有效方法,例如,正如前一位作者在python或ruby中以'string'* n的样式所说的那样。
a relatively efficient loop would be allocating strlen*n and then copying the chars over multiple times rather than for(...) {str+=copy_this_string;} which does multiple copies and allocates. 一个相对有效的循环是分配strlen * n,然后多次复制字符,而不是进行多次复制和分配的for(...){str + = copy_this_string;}。
another solution is using a stringbuffer. 另一个解决方案是使用字符串缓冲区。
key: without using a macro. 关键: 不使用宏。 clarification: the 'char array' in the question is because in the original question, due to a single char in the string, all answers ignored the copying of a general stream and focused on using '.'
澄清:问题中的“字符数组”是因为在原始问题中,由于字符串中只有一个字符,所有答案都忽略了通用流的复制,而专注于使用“。” since it had easily accessible methods such as the stream constructor.
因为它具有易于访问的方法,例如流构造器。
"Duplicate" : How to repeat a string a variable number of times in C++? “ Duplicate”: 如何在C ++中将字符串重复可变的次数? Except most the answers all used methods that only had a char repeated, eg the String Constructor and insert methods, which did not answer the original question and were unhelpful when I then looked for an efficient method.
除了大多数答案外,所有使用过的方法都只重复了一个char,例如String Constructor和insert方法,它们没有回答原始问题,并且在我寻找有效方法时无济于事。
If you're worried about the string '+=' operation being inefficient, one optimization you can make is to reserve memory in the target string container. 如果您担心字符串'+ ='操作效率低下,则可以进行的一种优化是在目标字符串容器中保留内存。 This may avoid inefficient re-allocations of memory.
这样可以避免无效的内存重新分配。
For example: 例如:
std::string s = "copyme";
std::string b;
target.reserve(s.length()*5);
for (int index = 0; index < 5; ++index)
{
b += s;
}
I checked the std::string implementation on my linux system and it appears this method will not cause re-allocation of the array as long as the initial reservation is large enough. 我检查了我的linux系统上的std :: string实现,并且只要初始预留足够大,此方法似乎就不会引起数组的重新分配。
template<typename _CharT, typename _Traits, typename _Alloc>
basic_string<_CharT, _Traits, _Alloc>&
basic_string<_CharT, _Traits, _Alloc>::
append(const basic_string& __str)
{
const size_type __size = __str.size();
if (__size)
{
const size_type __len = __size + this->size();
if (__len > this->capacity() || _M_rep()->_M_is_shared())
this->reserve(__len);
_M_copy(_M_data() + this->size(), __str._M_data(), __size);
_M_rep()->_M_set_length_and_sharable(__len);
}
return *this;
}
A simple loop would do. 一个简单的循环就可以了。 If you are concerned about re-allocations, you can call
std::string::reserve
on the copy string: 如果您担心重新分配,可以在复制字符串上调用
std::string::reserve
:
#include <string>
#include <iostream>
int main()
{
const int num + 5;
std::string s = "copyme";
std::string b;
b.reserve(s.size() * num); // reserve space for num* "copyme"
for (int i = 0; i < num; ++i) b += s;
std::cout << b << std::endl;
}
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