奇怪的std :: sort行为

Question

I am trying to sort an array of structs using an std::string ID (member of the struct). Here's the code:

struct Row {
   std::string ID;
   std::array<float, 5> scores;
   float avgScore;
};

std::array<Row, 50> records{};  

// ...
// Open a file, read some data and store them into records
// ...

// Sort the data
std::sort(records.begin(), records.end(), [](const Row& r1, const Row& r2) {
    return r1.ID > r2.ID;
});

So far everything works as expected. For example, following data:

liu 90 80 90 100 85
ols 95 95 90 93 85
kum 90 85 85 95 92

will be sorted to:

ols 95 95 90 93 85
liu 90 80 90 100 85
kum 90 85 85 95 92

However, if I simply change:

return r1.ID > r2.ID;

to:

return r1.ID < r2.ID;

for the same example, I will get:

0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0

How's that even possible?

Answer 1

std::array<Row, 50> records{}; is an array containing exactly 50 instances of Row . If your array contains 50 elements and you only specify 3 of them, there are 47 default constructed elements left in your array. It seems that you do not assign a value to all of the elements in your array when you read from your file and the remaining default constructed elements are being sorted at the front of the array.

Consider using std::vector instead if, at compile time, you aren't certain how many elements you will need.