adding digits in an array c++ - c++

Question

I'm new to C++ and i was trying to understand how to work with arrays. The idea I have is:
I wanted a user to input an array, the program to output the
array
Double all the values in the array and output that
And for each of the doubled values, add the digits of the doubled number
(1 digit number would remain the same), then output the new numbers as
well.
(e.g. if the array was [5, 6, 7, 8], the doubled values would be [10, 12, 14, 16] and then you would add each values digits like, [1+0, 1+2, 1+4, 1+6] to get [1, 3, 5, 7].
I put my code to show my progress, feel free to point out any errors along the way!
Any help is appreciated!
p.s. The nested loop didn't work :(
#include <iostream>
#include <string>
using namespace std;
int maxNum;
int num[20];
int main()
{
cout << "Enter an Array" << endl;
for (int i=0;i<20;i++)
{
cin >> num[i];
maxNum++;
if (num[i]==-1)
break;
}
cout <<"Your array is: " << endl;
for (int i=0;i<maxNum-1;i++)
cout << num[i];
cout << endl;
cout << "Your doubled array is:" << endl;
for (int j=0;j<maxNum-1;j++)
{
num[j]*=2;
cout << num[j];
}
cout << endl;
cout << "When the digits of each seat are added..." << endl;
for (int k=0;k<maxNum;k++)
{
for (int l=0;l<maxNum;l++)
{
int sum[20];
while (num[k]!=0)
{
sum[l]=sum[l]+num[k]%10;
num[k]=num[k]/10;
}
}
cout << sum[l];
}
cout << endl;
}

Answer 1

A few things:
maxNum and num[] are never initialized, it's dangerous.
that is not how you scan input. Ideally you woud do smth like while(cin >> tem_var){}. Or you could modify it to be if( !(cin >> num[i]) ) break;. That way you don't need to do maxNum-1 later too. (cin>>) will be True if it reads a variable succesfully and False otherwise. That way you can stop scanning by entering any non-number string, instead of running the loop for the rest of iterations, but leaving num[i] uninitialized if that happens.
you forget to output delimeters between array numbers which makes it hard to read.
cout << num[i] << "|"; or smth.
In the last part you make 3 loops: a k for loop that you never use, a l for loop to iterate num, and a k while loop to sum the digits. One of them is not necessary.
In the last part sum[] array, though filled correctly, is not outputted. You declare it inside the l loop, meaning it's deleted when you exit it. And even if you declared it outside. your cout << sum[l]; is outside the l loop, meaning it will only try to do the cout << sum[maxNum]; (the value of l the loop finishes with) while you only have [0:(maxNum-1)] elements in num and sum filled.
I'd suggest you try smth like for(k=1;k<num[l];k*=10) sum[l]+= num[l] / k % 10; instead of that while loop. It's shorter, gets the job done and leaves num[l] unchaged in case you decide to use it again afterwards.

Answer 2

You need to initialize sum array with all zeros first. You don't need nested loop.
Create a sum array to store the sum of each number and initialize it with 0's. First write a loop to traverse through the elements of the doubled array. For each element write a loop(you chose while loop) to traverse through the digits of each number and them to the corresponding sum element.
I've modified your code a little bit, go through it once.
#include <iostream>
#include <string>
using namespace std;
int maxNum;
int num[20];
int main()
{
cout << "Enter an Array" << endl;
for (int i=0;i<20;i++)
{
cin >> num[i];
maxNum++;
if (num[i]==-1)
break;
}
cout <<"Your array is: " << endl;
for (int i=0;i<maxNum-1;i++)
cout << num[i]<<' ';
cout << endl;
cout << "Your doubled array is:" << endl;
for (int j=0;j<maxNum-1;j++)
{
num[j]*=2;
cout << num[j]<<' ';
}
cout << endl;
cout << "When the digits of each seat are added..." << endl;
int sum[20];
for (int i=0;i<maxNum-1;i++)
sum[i]=0;
for (int k=0;k<maxNum-1;k++)
{
// for (int l=0;l<maxNum;l++)
// {
while (num[k]!=0)
{
sum[k]=sum[k]+num[k]%10;
num[k]=num[k]/10;
}
cout << sum[k]<<' ';
// }
}
cout << endl;
}

Answer 3

You don't need nested loop for that ,while making logic behind any program take a simple example and get the result,Don't jump directly to code. This will help you to building logics.
#include <iostream>
#include <string>
using namespace std;
int maxNum;
int num[20];
int main()
{
int sum=0;
cout << "Enter an Array" << endl;
for (int i=0;i<20;i++)
{
cin >> num[i];
maxNum++;
if (num[i]==-1)
break;
}
cout <<"Your array is: " << endl;
for (int i=0;i<maxNum;i++)
cout << num[i]<<ends;
cout << endl;
cout << "Your doubled array is:" << endl;
for (int j=0;j<maxNum;j++)
{
num[j]*=2;
cout << num[j]<<ends;
}
cout << endl;
cout << "When the digits of each seat are added..." << endl;
int r=0;
for (int k=0;k<maxNum;k++)
{
while (num[k]>0)
{
r=num[k]%10;
sum+=r;
num[k]=num[k]/10;
}
cout<<sum<<ends;
sum=0;
r=0;
}
cout << endl;
}