c++ - How to allocate memory dynamically using pointers? -

i have following task: creat program transform array array b in followin order: http://gyazo.com/29e434ebdc1e235fe0fc97c26ae9fe9c

the size of entered user, elements. print new elements of b. "use pointers"

examples: if input 1234567, program output 8888888.

here's code:

#include <iostream>  using namespace std;  int main() {     int n;     cout << "enter size of array" << endl;     cin >> n;     int a[n], b[n];     cout << "enter elements of array" << endl;     for(int = 0; < n; i++){         cin >> a[i];     }     for(int = 0; < n; i++){         b[i] = a[i] + a[n-i-1];     }     for(int = 0; < n; i++){         cout << b[i] << " ";     }     return 0; } 

there 1 problem have, says "use pointers" don't know shall use them , must used... in advance.

int a[n], b[n]; 

this code non-standard (when n variable). compiler allows that, if try compile code in other one, won't work.

using pointers in case means, should allocate memory arrays dynamically, eg.

int * a, * b; = new int[n]; b = new int[n]; 

this code valid in terms of c++ standard.

remember though, have free allocated memory manually after don't need anymore:

delete[] a; delete[] b; 

this should work rest of code left unchanged, since [] operator works pointers well.

and finally, if writing c++ program, surely should using std::vector instead of c-style arrays...

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edit: (in response comment)

if declare variable in following way:

int * a; 

and write:

cin >> *(a + i); 

it means:

1. read data user , parse it;
2. go i "steps" starting address (it "walks" i * sizeof(*a) bytes a).
3. put data read user memory pointed place evaluated in step 2.

but when declare:

int (*a)[n]; 

the becames pointer array of n elements. since array convertible pointer first element, may think of previous line (for sake of explanation) as

int ** a; 

in such case compiler tries same described earlier. walks i pointers starting a, not *int*s. dereferences pointer getting int * , attempts put there data written user. >> operator of cin not able translate text pointer , that's why compiler complains.

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foot note

while you're learning using pointers, may think of <something> variable[] being equal <something> * variable pointing first element of array. lie (because array not a pointer, it's convertible pointer first element), helps understanding arrays have in common pointers. that's why can think of

int (* a)[]; 

as a

int ** a;