In my C++ code If I print sum outside loop it gives correct answer, but not in any loop for/while, why not? -

int main() {   long long int first=0,second=1,t,n;     //here t number of cases    cin>>t;   long long int fab=first+second;   long long int sum[t];    for(long long int i=0;i<t;i++)   {     cin>>n;      while(fab<n)     {       first=second;       second=fab;       if(fab%2==0)       {         sum[i]+=fab;       }       fab=first+second;     }   }  for(int i=0;i<t;i++) {   cout<<sum[i]<<endl; }   return 0; } 

in above loop sum not providing correct answer if sum used outside loop gives appropriate answer.

this:

cin >> t; long long int sum[t]; 

is not valid c++. array in c++ must created using compile-time expression denote number of items, not variable such t.

the proper construct use standard c++ std::vector<long long>:

#include <vector> //... cin >> t; std::vector<long long> sum(t); 

the code standard c++.

the other aspect code solve issue brought in answer dietrichepp, in failed initialize vla 0. vector above have automatically initialized items 0 you.

so moral of story if used standard c++, not have had problem code.