vector，vectorc++
摘自<<C++ Primer>> 5th, chapter 3
vector
a class template; a collection of objects, all of which have the same type
1 初始化
vector<T> v1
vector<T> v2(v1) vector<T> v2 = v1;
vector<T> v3(n, val) vector<T> v4(n);
vector<T> v5{a, b, c...} vector<T> v5= {a, b, c...}
例1):
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vector<int> ivec; // initally empty
vector<int> ivec2(ivec);
vector<string> svec(10, "hi!"); //10 elements, each is a string "hi!"
vector<string> articles = {"a", "an", "the"};
vector<int> v1(10, 1); // 10 elements with value 1
vector<int> v2{10, 1}; // 2 elements with values 10 and 1
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2 添加元素
it's better to create an empty vector and use push_back to add elements at run time.
例 2):
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vector<int> ivec;
for(int i = 0; i != 100; i++)
ivec.push_back(i);
sting s;
vector<sting> svec;
while(cin > s)
{
svec.push_back(s);
}
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3 操作
v.empty()
v.size() return the number of elements in v
v[n] return a reference to the element at positon n in v
v1 = v2
v1 = {a, b, c...}
v1 == v1 v1 != v2
<, <=, >, >=
例 3):
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vector<int> v{1, 2, 3, 4, 5, 6, 7, 8, 9};
for(auto &i: v) // for each element in v (i is a reference)
i * = i;
for(auto i: v)
cout << i << " ";
cout < endl;
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4 using subscripts
1) computing a vector index
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// count the number of grades by clusters of ten:
// 0~9, 10~19, ..., 90~99, 100
vector<unsigned> scores(11, 0);
unsigned grade;
while(cin >> grade)
{
if (grade <= 100)
++scores[grade/10];
}
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2) subscripting does not add elements
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vector<int> ivec;
for(decltype(ivec.size()) ix = 0; ix != 10; ++ix)
ivec[ix] = ix; // disaster: ivec has no elements
ivec.push_back(ix);
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the subscript operator on vector(and sting) fetches an existing element; it does not add an element.
