Printing numbers , C++

Question

Hello I have written code and it looks like this (it tooks first number from a[], then second number from power of 2 and third all even numbers):

#include <iostream>
using namespace std;

int main()
{

    int a[6] = { 1, 2, 5, 6, 7, 9 };
    int i = 0;
    int x;
    int even = 0;
    int sum;

    for (; a[i] < a[6]; i++)
    {
        for (int power = 1; power <= 64; power *= 2)
        {
            x = power % 10;
            for (int even = 0; even <= 6; even += 2)
            {
                sum = (a[i] + x + even);
                if (sum % 7 == 0)
                {
                    cout << a[i] << x << even << "  ";
                }
            }
        }
    }
}

and I want it to print out those numbers : Out: 124 142 214 248 284 518 520 588 610 626 644 680 716 786 914 948 984 But instead I get those: 124 142 160 124 142 214 284 266 520 520 610 626 644 680 662 626 644 716 786 914 984 966

I don't know where's the problem,and numbers doubles couple of times. Please could somebody help me ?

Answer 1

You've wrongly accessed a[6] , which is an Undefined Behavior.

the built-in subscript expressions work like this: If we perform a[b] on an array a , then it's equivalent to *(a+b) , while the first subobject of the array is stored in a (Here a refers to the pointer prvalue the array decaying to). Thus, to access the first element in an array, we use a[0] , ie *a

The number taking place in the declaration of an array is its size, formally speaking, the number of elements it can hold. Hence, if you do a simple math problem, the last valid subscript is size - 1 , not size .

I think here you need to change

for (; a[i] < a[6]; i++)

to

for (; a[i] < a[5]; i++)