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[英]std::sort fails to sort elements of a std::vector<std::string>
[英]std::sort of std::vector produces illegal elements
我有一個應用程序,其中 std::sort 有時會導致核心轉儲。 我能夠將問題隔離到以下 MWE:
#include <algorithm>
#include <iostream>
#include <numeric>
#include <vector>
class DF {
public:
DF() = default;
std::vector<int> row_mapping;
std::vector<int> idx = {0, 1};
std::vector<std::vector<int>> vals = {
{0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3,
4, 4, 4, 4, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7,
8, 8, 8, 8, 9, 9, 9, 9, 10, 10, 10, 10, 11, 11, 11, 11,
12, 12, 12, 12, 13, 13, 13, 13, 13, 14, 14, 14, 14},
{2015, 2016, 2017, 2021, 2015, 2016, 2017, 2021, 2015, 2016, 2017,
2021, 2015, 2016, 2017, 2021, 2015, 2016, 2017, 2021, 2015, 2016,
2017, 2021, 2015, 2016, 2017, 2021, 2015, 2016, 2017, 2021, 2015,
2016, 2017, 2021, 2015, 2016, 2017, 2021, 2015, 2016, 2017, 2021,
2015, 2016, 2017, 2021, 2015, 2016, 2017, 2021, 2014, 2015, 2016,
2017, 2021, 2015, 2016, 2017, 2021}};
void sort(void) {
int n = vals[0].size();
std::cerr << "n = " << n << std::endl;
row_mapping.resize(n);
std::iota(begin(row_mapping), end(row_mapping), 0);
std::cerr << "row map:";
for (auto v : row_mapping) std::cerr << " " << v;
std::cerr << std::endl;
std::sort(
begin(row_mapping), end(row_mapping), [&](int i1, int i2) -> bool {
std::cerr << "cmp " << i1 << " " << i2 << std::endl;
for (auto gi : idx) {
if (vals[gi][i1] < vals[gi][i2]) return true;
}
return false;
});
}
};
int main() {
DF df;
df.sort();
}
在使用g++ -std=c++17 mwe.cpp
並運行它后,我得到:
(輸出縮寫)
n = 61
row map: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
cmp 1 30
cmp 30 60
(...)
cmp 52 257
(...)
cmp 59 55
cmp 60 56
double free or corruption (out)
Aborted (core dumped)
這很奇怪,因為我對包含值 0 到 60(包括)的向量進行排序,這些值是更復雜數據結構的索引。 但是比較 lambda 被調用,值為 257。
我想這一定是因為超出了某些數組邊界,或者因為對不再存在的數據的引用。 但是我在代碼中看不到問題!
確實這就是問題所在。
使用以下更正的 lamda,一切正常:
std::sort(
begin(row_mapping), end(row_mapping), [&](int i1, int i2) -> bool {
std::cerr << "cmp " << i1 << " " << i2 << std::endl;
for (auto gi : idx) {
if (vals[gi][i1] < vals[gi][i2]) return true;
if (vals[gi][i1] > vals[gi][i2]) return false;
}
return false;
});
您的比較操作沒有定義嚴格的弱排序。
for (int i1 = 0; i1 < vals[0].size(); ++i1)
{
for (int i2 = 1; i2 < vals[0].size(); ++i2)
{
if (!(cmp(i1, i2) ^ cmp(i2, i1)))
{
std::cerr << "incongruent comparison between " << i1 << " and " << i2 << '\n';
assert(0);
}
}
}
Output:
incongruent comparison between 0 and 52
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