C++将字符串传递给映射

Question

在下面for循环，我测试查询中的字母是否在 unordered_map<string, int> 中.如果不是，我打印出一些东西。

queries = [ ["a", "c"], ["b", "a"], ["a", "e"], ["a", "a"], ["x", "x"] ]

for (vector<string>& q : queries) {
    int start = index[q[0]], end = index[q[1]];
    if (index.find("z") == index.end()) {cout << "z" << '\n';
}

index仅包含 a, b, c .相应地，输出是这样的:

z
z
z
z
z

Which it should be, because one of the queries has an z in it. Printing the size of index gives 3.

However, if I run this:

for (vector<string>& q : queries) {
    int start = index[q[0]], end = index[q[1]];
    if (index.find(q[0]) == index.end()) {cout << "z" << '\n';
}

我没有得到任何输出，但我希望输出 x因为那不在 index .
为什么是这样？

Answer 1

根据运营商[]的描述cppreferance 中的 map :

Returns a reference to the value that is mapped to a key equivalent to key, performing an insertion if such key does not already exist.

index[q[0]]创建了 key "x" 的条目.