反向数组 C++？ 我究竟做错了什么？

Question

int main(){
    int n;
    cin >> n;
    vector<int> arr(n);
    for(int i =0; i < n; i++){
       cin >> arr[i];
    }
    for(int i = n-1; n-1 >= 0; i--){
       cout << " "<< arr[i] << " ";
    }
    return 0;
} 

What am i doing wrong? I feel like I'm on the right track but I'm not sure whats wrong, this is the array DS problem on hacker rank. Unfortunately i don't think they reveal the answer.

Answer 1

Change the condition to i >= 0 , There's a chance that n - 1 >= 0 might lead to an infinite loop. But it doesn't matter whether it does or not, because in both cases, it'll not produce the required results.

for(int i = n-1; i >= 0; i--){
   std::cout << " "<< arr[i] << " ";
}

Or use, std::reverse .

std::reverse(arr.begin(), arr.end());
for (int i = 0; i < n; i++){
    std::cout << arr[i] << " ";
}

See the second approach live here .

Answer 2

for(int i = n-1; n-1 >= 0; i--){
   cout << " "<< arr[i] << " ";
}

You don't decrement n, which leads to an infinite loop, if n is <= 0.

Answer 3

Don't reinvent the wheel, the Standard Library will help you here.
Use std::reverse :

int main()
{
    std::vector<int> vec;
    int n = 0;
    int temp = 0;

    std::cin >> n;
    for (int i = 0; i != n; ++i) {
        std::cin >> temp;
        vec.emplace_back(temp);
    }

    std::reverse(std::begin(vec), std::end(vec));
}

Answer 4

In both the cases, you have taken the initialised the variable i=0 and incremented(in the first loop)/decremented(in the second loop) as needed. However, in the second loop you have put the condition on n, despite the fact that you never change the value of n.This will lead to an infinite loop as n-1 remains constant throughout the whole program. To sort this out,you should change the condition to i-1>0 like in the code below.

for(int i = n-1; i-1 >= 0; i--)
{
   cout << " "<< arr[i] << " ";
}

Do upvote if this helps.Cheers.

Answer 5

for(int i = n-1; n-1 >= 0; i--)

Here, you did a mistake with the condition of your for loop. You have assigned n-1 to i and in the decrements section you used i-- , So you should use a condition when the loop should be stopped or for which value of i your loop will stop. So, just change the condition as following:

for(int i = n-1; i >= 0; i--)