c ++ find_if结构中的一个元素仅返回第一个元素

Question

I want to return the element in struct that has the a=5 . However, it cannot find it using find_if . The value the (*it).a returns is always the first element which is 0. Below is the code.

struct vect_map {
    int a;
    int b;
};
int main () {

    int iteration = 20000;
    vector< vect_map > vect;
    vect.reserve( iteration );
    for( auto i=0; i<iteration; ++i) {
        if( i==iteration-200 ) {
            vect[i].a = 5; 
            vect[i].b = 100;
        }
        else {
            vect[i].a = i; 
            vect[i].b = 1;
        }
    }
    int n=5;
    auto it = find_if( begin( vect ), end( vect ), [&n] (const vect_map &e) { return e.a==n; } );
    if( it!=vect.end() ) {
        cout << (*it).a << endl;
    }
    return 0;

Thank you.

Answer 1

vect[i] has undefined behaviour, because i is not less than vect.size() .

You need to give your vector the correct size:

vect.resize(iteration);

Answer 2

The problem is that your loop that populates the vector is incorrect: reserve(n) does not create a vector of the desired length, only allows you to push_back without reallocation up to the specified limit.

You can fix this problem either by pushing back the desired elements as you go, or by making an array of 2000 items using the appropriate constructor:

const int iteration = 20000;
vector< vect_map > vect(iteration);

Demo