C++ 遍历 Map
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C++ Loop through Map
提问by NoName
I want to iterate through each element in the map<string, int>without knowing any of its string-int values or keys.
我想在map<string, int>不知道任何 string-int 值或键的情况下遍历中的每个元素。
What I have so far:
到目前为止我所拥有的：
void output(map<string, int> table)
{
map<string, int>::iterator it;
for (it = table.begin(); it != table.end(); it++)
{
//How do I access each element?
}
}
回答by P0W
You can achieve this like following :
您可以像下面这样实现：
map<string, int>::iterator it;
for ( it = symbolTable.begin(); it != symbolTable.end(); it++ )
{
std::cout << it->first // string (key)
<< ':'
<< it->second // string's value
<< std::endl ;
}
With C++11( and onwards ),
使用C++11 （及以后），
for (auto const& x : symbolTable)
{
std::cout << x.first // string (key)
<< ':'
<< x.second // string's value
<< std::endl ;
}
With C++17( and onwards ),
使用C++17 （及以后），
for( auto const& [key, val] : symbolTable )
{
std::cout << key // string (key)
<< ':'
<< val // string's value
<< std::endl ;
}
回答by Vlad from Moscow
Try the following
尝试以下
for ( const auto &p : table )
{
std::cout << p.first << '\t' << p.second << std::endl;
}
The same can be written using an ordinary for loop
可以使用普通的 for 循环编写相同的内容
for ( auto it = table.begin(); it != table.end(); ++it )
{
std::cout << it->first << '\t' << it->second << std::endl;
}
Take into account that value_type for std::mapis defined the following way
考虑到 value_type forstd::map的定义方式如下
typedef pair<const Key, T> value_type
Thus in my example p is a const reference to the value_type where Key is std::stringand T is int
因此，在我的示例中，p 是对 value_type 的常量引用，其中 Key 是std::string，T 是int
Also it would be better if the function would be declared as
如果将函数声明为
void output( const map<string, int> &table );
回答by Columbo
The value_typeof a mapis a paircontaining the key and value as it's firstand secondmember, respectively.
所述value_type的map是一种pair含有该键和值，因为它的first和second分别构件。
map<string, int>::iterator it;
for (it = symbolTable.begin(); it != symbolTable.end(); it++)
{
std::cout << it->first << ' ' << it->second << '\n';
}
Or with C++11, using range-based for:
或者在 C++11 中，使用基于范围的：
for (auto const& p : symbolTable)
{
std::cout << p.first << ' ' << p.second << '\n';
}
回答by John Mutuma
As @Vlad from Moscow says,
Take into account that value_typefor std::mapis defined the following way:
正如来自莫斯科的@Vlad 所说，考虑到value_typeforstd::map定义如下：
typedef pair<const Key, T> value_type
This then means that if you wish to replace the keyword autowith a more explicit type specifier, then you could this;
这意味着如果您希望auto用更明确的类型说明符替换关键字，那么您可以这样做；
for ( const pair<const string, int> &p : table ) {
std::cout << p.first << '\t' << p.second << std::endl;
}
Just for understanding what autowill translate to in this case.
只是为了理解auto在这种情况下会转化为什么。
