如何迭代向量

Question

I have such a code

#include <iostream>
#include <vector>
#include <string>

using namespace std;

void removeFirstFromVec(vector<int> & vecLink) {
    // remore first element from some vector
    vecLink.erase(vecLink.begin() + 0);
}

int main()
{
    vector<int> myVec;
    myVec.push_back(1);
    myVec.push_back(2);
    myVec.push_back(3);
    cout << "Before removal\n";
    for (auto & i : myVec) {
        cout << myVec[i-1] << endl;
    }
    removeFirstFromVec(myVec);
    cout << "After removal\n";
    for (auto & i : myVec) { // starts with 2
        cout << myVec[i-1] << endl;
    }
    return 0;
}

But on the place where I put a comment it starts with 2 instead of 0 and it causes an error. What I've done wrong or is there a way to use something brief like auto & i: myVec instead of for (int i = 0; i <...size(); i++) without such an error

Answer 1

In the range-based for loop

for (auto & i : myVec) {
    cout << myVec[i-1] << endl;
}

i is the element of the vector, not the index. It should be

for (auto & i : myVec) {
    cout << i << endl;
}

Answer 2

Iteration returns the items themselves, not indices.

vector<int> vec { 5, 3, 2 };
for (int i: vec) {
  cout << i << endl;
}

Will output this:

5
3
2

Answer 3

If you do the following:

1. Use templated-function to accept any type for the vector.

2. Use direct initialization method rather than push_back() to avoid huge number of lines.

3. Braces for single syntax is optional. Avoid it to reduce confusion during coding complex programs.

Then you can write the same code in a better manner (don't forget the comments):

#include <iostream>
#include <vector>

using namespace std;

// defining a templated-function
template<class T>
void removeFirstFromVec(vector<T>& vecLink) {
    vecLink.erase(vecLink.begin());
}

int main(void) {
    // initializing with direct-initialization method
    vector<int> myVec {1, 2, 3};

    cout << "Before removal\n";

    // no braces
    for (auto& i : myVec)
        // 'i' isn't an index here, it's an element
        cout << i << endl;

    removeFirstFromVec(myVec);

    cout << "After removal\n";

    // no braces    
    for (auto& i : myVec)
        cout << i << endl;

    return 0;
}