问题描述
我不经常使用std :: transform，但是我发现它非常有用，并且我开始用这种算法替换一些for循环。
I am not using std::transform often, however I found it very useful and I am starting replacing some for loops with this algorithm.
这里有什么问题？我想保留向量vec中所有代码> 100的元素。我希望有一个带有3个元素的新std :: vector：133、144和155。但是在算法之后，大小为0。 ？
What is wrong here? I want to keep all the elements of the vector vec that have code > 100. I would expect to have a new std::vector with 3 elements: 133, 144 and 155. But after the algorithm the size is 0. What is wrong?
TEST_CASE("testing trasf1", "[tras1]") {
std::vector<Test2> vec {
{1,1},
{3,3},
{11,11},
{12,12},
{133,133},
{19,19},
{21,21},
{22,22},
{23,23},
{144,144},
{155,155}
};
std::vector<uint32_t> final_v {};
final_v.reserve(vec.size());
transform(begin(vec), end(vec), begin(final_v), [] (const Test2& elem) {
if ( elem.getCode() > 100)
return elem.getCode();
});
//REQUIRE(final.size() == 3);
cout << final_v.size() << endl;
for (const auto i : final_v) {
cout << i << endl;
}
}
推荐答案
transform 不会在输出序列中插入元素，它只是写入 * iter 并递增
transform doesn't insert elements into the output sequence, it just writes to the *iter and increments the iterator.
如果要插入到序列中，请使用 std :: back_inserter（final）作为输出迭代器。
If you want to insert to the sequence, use std::back_inserter(final) as the output iterator.
或者，先调用 final.resize（vec.size（））设置输出矢量到正确的大小。请注意，这会将向量元素初始化为零，因此对于大向量，将产生明显的时间开销。
Alternatively, call final.resize(vec.size()) first, to set the output vector to the correct size. Note that this will intialize the vector elements to zero, so for large vectors will incur a noticeable time overhead.
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