C++字符串迭代器
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c++ string iterator
提问by Joe
I'm trying to do an if statement inside a loop with an iterator over a string, but can't figure out how to get the current character for the if statement:
我正在尝试在循环中使用迭代器在字符串上执行 if 语句，但无法弄清楚如何获取 if 语句的当前字符：
for (std::string::iterator i=buffer.end()-1; i>=buffer.begin(); --i) {
if (!isalpha(*i) && !isdigit(*i)) {
if(i != "-") { // obviously this is wrong
buffer.erase(i);
}
}
}
Can someone help me get the current character so I can do some additional if statements?
有人可以帮我获取当前角色，以便我可以做一些额外的 if 语句吗？
回答by James McNellis
I can't figure out how to get the current character
我不知道如何获得当前角色
You do it twice here:
你在这里做两次：
if (!isalpha(*i) && !isdigit(*i))
When you dereference an iterator (*i), you get the element to which it points.
当您取消引用迭代器 ( *i) 时，您将获得它指向的元素。
"-"
This is a string literal, not a character. Character constants use single quotes, e.g., '-'.
这是一个字符串文字，而不是一个字符。字符常量使用单引号，例如，'-'。
for (std::string::iterator i=buffer.end()-1; i>=buffer.begin(); --i)
This would be much simpler with reverse iterators:
使用反向迭代器会简单得多：
for (std::string::reverse_iterator i = buffer.rbegin(); i != buffer.rend(); ++i)
回答by Luzik
if(i != "-")
should be
应该
if(*i != '-')
回答by David Rodríguez - dribeas
Other answers have solved the particular issue that you have, but you should be aware that there are different approaches to solve your actual problem: erase elements that fullfill a condition. That can be easily solved with the remove/eraseidiom:
其他答案已经解决了您遇到的特定问题，但您应该意识到有不同的方法可以解决您的实际问题：擦除满足条件的元素。这可以使用remove/erase成语轻松解决：
// C++0x enabled compiler
str.erase(
std::remove_if( str.begin(), str.end(),
[](char ch) { return !isalpha(ch) && !isdigit(ch) && ch != '-' } ),
str.end() );
While this might look cumbersome at first, once you have seen it a couple of times it will no longer be surprising, and it is an effective way of deleting elements from a vector or string.
虽然一开始这看起来很麻烦，但一旦你看过几次就不会再感到惊讶了，它是一种从向量或字符串中删除元素的有效方法。
If your compiler does not have lambda support, then you can create a functor and pass it as the third argument to remove_if:
如果您的编译器不支持 lambda，那么您可以创建一个函子并将其作为第三个参数传递给remove_if：
// at namespace level, sadly c++03 does not allow you to use local classes in templates
struct mycondition {
bool operator()( char ch ) const {
return !isalpha(ch) && !isdigit(ch) && ch != '-';
}
};
// call:
str.erase(
std::remove_if( str.begin(), str.end(), mycondition() ),
str.end() );
回答by Mark B
To get the character just say *i, but this isn't enough. Your loop isn't legal because it's not allowed to decrement before begin. You should use reverse iterators or the remove_ifalgorithm.
要获得角色只需说*i，但这还不够。您的循环不合法，因为它不允许在begin. 您应该使用反向迭代器或remove_if算法。
回答by Jerry Coffin
You have it right above in the previous ifstatement: iis an iterator, so *igives the character referred to by the iterator.
在前面的if语句中，它就在上面：i是一个迭代器，因此*i给出了迭代器所引用的字符。
Note that if you're going to iterate through a collection backwards, it's generally easier to use a reverse_iteratorwith rbeginand rend. I'd probably use a pre-packaged algorithm instead though.
请注意，如果您要向后遍历集合，通常使用reverse_iteratorwithrbegin和更容易rend。不过，我可能会使用预先打包的算法。
