在循环中使用string.length()有效吗? [英] Is using string.length() in loop efficient?

问题描述

例如，假设string s是这样:

for(int x = 0; x < s.length(); x++)

比这更好吗?:

int length = s.length();
for(int x = 0; x < length; x++)

谢谢， 乔尔

推荐答案

通常，如果结果在迭代过程中没有改变，则应避免在循环的条件部分中调用函数.

In general, you should avoid function calls in the condition part of a loop, if the result does not change during the iteration.

因此，规范形式为:

for (std::size_t x = 0, length = s.length(); x != length; ++x);

请注意此处的三件事:

• 初始化可以初始化多个变量
• 条件用!=而不是<
表示
• 我使用前增量而不是后增量

• The initialization can initialize more than one variable
• The condition is expressed with != rather than <
• I use pre-increment rather than post-increment

(我还更改了类型，因为负长度是无意义的，并且字符串接口是用std::string::size_type定义的，在大多数实现中通常是std::size_t.)

(I also changed the type because is a negative length is non-sense and the string interface is defined in term of std::string::size_type, which is normally std::size_t on most implementations).

尽管...我承认，性能并没有提高可读性:

Though... I admit that it's not as much for performance than for readability:

• 双重初始化意味着x和length的作用域都尽可能严格
• 通过记住结果，读者不会怀疑长度在迭代过程中是否会发生变化
• 当您不需要使用旧"值创建临时目录时，使用预增量通常会更好.

• The double initialization means that both x and length scope is as tight as necessary
• By memoizing the result the reader is not left in the doubt of whether or not the length may vary during iteration
• Using pre-increment is usually better when you do not need to create a temporary with the "old" value

简而言之:使用最好的工具完成手边的工作:)

In short: use the best tool for the job at hand :)

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