I have a problem, if you know how to solve it, please help

Question

I have a task, and I can't complete it, I don't understand what needs to be added so that the subtraction occurs in a cycle, and not once.

task:
Create a new array of the same length as the source array,
where the element in position X is equal to the difference of elements in positions
X and X-a of the source array, where a is a number entered by the user.
Elements with indexes less than a are copied into the result array.
Example: for the array {1,2,7,4,9} with the entered number 2
the result will be {1,2,6,2,2} (ie 1, 2, 7-1, 4-2, 9-7)

My attempt:

#include <iostream>

using namespace std;

int main()
{
setlocale(LC_CTYPE, "ukr");
int a[10] = { 1,2,3,4,5,6,7,8,9,10 };
int c[10] = { 1,2,3,4,5,6,7,8,9,10 };
int b;
cout << "enter a number: " << endl;
cin >> b;
cout <<"initial array: "<< endl;
for (int i = 0; i < 10; i++)
{
if (b == a [i])
{
c[i] = a[i - b];
}
cout << c[i] << endl;
}
}

Answer 1

You need to compare positions or indexes.
See 7-1 is a[2] - a[2-2] when I>=a where I=2, a=2

4-2 is a[3] - a[3-2] when I>=a where I=3, a=2...

Answer 2

you need to work on the if statement.

Now you only check if b is equal to a[i], but that's not what we want. we want to find the numbers that are equal to or greater than b. Because numbers less than b just need to be copied into c[i].

c[i] = a[i - b]; Not right, you are missing something in the last part. remember you should subtract X-a from X to get the new value, here your only using X-a.

If the second array should be created as an empty array, then we need an else statement to copy in the elements lower then b.