C++ 检查 std::vector 中是否存在给定索引
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Check if a given index exists in std::vector
提问by leon22
I need index access to my std::vector, and therefore I must check if an index is already available to first delete them, and then set a new value.
我需要对我的 进行索引访问std::vector，因此我必须检查索引是否已经可用以首先删除它们，然后设置一个新值。
Here's my setter function:
这是我的 setter 函数：
void SetVector(int position, int value) {
std::vector<int>iterator it = testVector.begin();
// need a check here
testVector.insert(it-testVector.begin()+position, value);
}
Or is this the wrong C++ collection for my needs? (should grow dynamically, so no std:arraypossible). Could use a std::mapbut maybe it's also possible with std::vector.
或者这是我需要的错误 C++ 集合？（应该动态增长，所以std:array不可能）。可以使用 astd::map但也许也可以使用std::vector.
回答by Mike Seymour
The requirements aren't entirely clear from the question, but I'm assuming that you want to end up with testVector[position] == value, whether or not positionwas in range to begin with.
问题中的要求并不完全清楚，但我假设您希望以 结束testVector[position] == value，无论是否position在开始的范围内。
First grow the vector if it's too small. This will insert zero-values after whatever is already there.
如果向量太小，首先增加向量。这将在已经存在的任何内容之后插入零值。
if (position >= testVector.size()) {
testVector.resize(position+1);
}
Then assign the element you want to set:
然后分配要设置的元素：
testVector[position] = value;
回答by doctorlove
I don't believe the question is clear. If you want
我不相信这个问题是清楚的。如果你想
"to first delete them, and then set a new value."
“先删除它们，然后再设置一个新值。”
this might work
这可能有效
void SetVector(int position, int value) {
if (position < testVector.size()) {
testVector[position] = value;
}
else {
testVector.push_back(value);
}
}
You should really make the int positionthe testVector's size_type.
你真的应该int position把testVector's size_type。
回答by Alexis
You can use std::vector::atwho throw an exception if you don't have anything at this index.
如果您在此索引上没有任何内容，您可以使用std::vector::at抛出异常。
The function automatically checks whether n is within the bounds of valid elements in the vector, throwing an out_of_range exception if it is not (i.e., if n is greater or equal than its size). This is in contrast with member operator[], that does not check against bounds.
该函数会自动检查 n 是否在向量中有效元素的范围内，如果不在则抛出 out_of_range 异常（即，如果 n 大于或等于其大小）。这与不检查边界的成员 operator[] 形成对比。
And since you get a reference on the object at the given index, you can change/delete the value
并且由于您在给定索引处获得了对对象的引用，因此您可以更改/删除该值
void SetVector(int position, int value) {
try
{
testVector.at(position) = value;
}
catch (const std::out_of_range& oor) {
testVector.resize(position + 1);
testVector[position] = value;
}
}
回答by Sanyam Goel
first get an iterator for your vector by using
首先通过使用为您的向量获取迭代器
std::vector<int>::iterator it;
it = myvector.begin();
it = myvector.begin();
for (it=myvector.begin(); it<myvector.end(); it++)
std::cout << ' ' << *it;
Using thsi iterator you can traverse all elements and perform respective operation like remove element
使用 thsi 迭代器，您可以遍历所有元素并执行相应的操作，例如删除元素
