How to input elements in an array WITHOUT inputting n? (c++)

Question

Input : 5

long long n;
cin>>n;
long long a[n];
for(long long i=0;i<n;i++){
    cin>>a[i];
}

How do I input elements of the array without inputting n?

This is what I look for :

Input : 1,2,3,4,5

cout << a[0] 

output:

1

Answer 1

The standard input filter loop in C++ is while(cin >> a) - this will read until there is no more input, or other bad things happen:

#include <vector>
#include <iterator>
#include <iostream>
int main() {
  std::vector<int> nums;
  while (std::cin >> a) {
    nums.push_back(a);
  }
  std::copy(nums.begin(), nums.end(), ostream_iterator<int>{cout, " "});
}

You could also use a one liner with input iterators - the shortest way to read elements in a vector:

#include <vector>
#include <iterator>
#include <algorithm>
#include <iostream>

int main() {
  std::vector<int> nums(std::istream_iterator<int>(std::cin), {});
  std::copy(nums.begin(), nums.end(), std::ostream_iterator<int>{std::cout, " "});
}

See Ideone example here

Assuming however that you wish to ignore all this C++ awesomeness, strongly discouraged IMHO, you can just:

#include <iostream>
int main() {
  const int MAX_SIZE = 100;
  int nums[MAX_SIZE]; 
  int a;
  int i=0;
  while (std::cin >> a) {
    nums[i++] = a;
  }

  // do your output
}

Note that you will:

1. need to guess the MAX_SIZE,
2. or manually handle reallocation once you read more elements than MAX_SIZE;

Hence: use an std::vector!!