Is this code right? My IT teacher says it's wrong - c++

Question

Take a string from user then change the last letter to upper and write the 2nd letter and 4th letter from the string.
Generate 10 random integers from <-10,5>. Print these numbers. To the even numbers, add 100 and print them again.
I wrote this code; is it right?
#include <iostream>
#include <string>
#include<time.h>
#include <array>
using namespace std;
int main()
{
string a;
cin >> a;
cout << a << endl;
a.back() = toupper(a.back());
cout << a<<endl;
cout << "2 letter is "<<a.at(2) << endl;
cout <<" 4 letter is "<< a.at(4) << endl;
srand(unsigned(time(0)));
int tab[10];
for (int i = 0; i < 10; i++)
{
int z = (rand() % 16) - 10;
tab[i] = z;
}
for (int i = 0; i < 10; i++)
{
cout << tab[i] << endl;
}
cout << "===============" << endl;
for (int i = 0; i < 10; i++)
{
if (tab[i] % 2 == 0)
{
tab[i] += 100;
}
}
for (int i = 0; i < 10; i++)
{
cout << tab[i] << endl;
}
}

Answer 1

This is wrong
cout << "2 letter is "<< a.at(2) << endl;
The second letter in a string is a.at(1) because in C++ array indexes start at zero, but in English we start counting at one.

Answer 2

Your code is wrong! The following two lines, in particular:
cout << "2 letter is "<<a.at(2) << endl;
cout <<" 4 letter is "<< a.at(4) << endl;
Why? Because array indexes (and string positions) begin at zero in C++, so the first letter will be a.at(0) and, thus, the second letter will be a.at(1).
So, you should have this:
cout << "2 letter is " << a.at(1) << endl;
cout <<" 4 letter is " << a.at(3) << endl;
Also, for better overall coding-style guidelines, please read this: Why is "using namespace std;" considered bad practice?

Answer 3

for (int i = 0; i < 10; i++)
{
if (tab[i] % 2 == 0)
{
tab[i] += 100;
}
}
for (int i = 0; i < 10; i++)
{
cout << tab[i] << endl;
}
I believe this is where your problem lies. Your teacher said if they are even, print them again - and by that I think he/she means to print just the even numbers again (I am taking a liberal use of language here), so change the above to:
for (int i = 0; i < 10; i++)
{
if (tab[i] % 2 == 0)
{
tab[i] += 100;
cout << tab[i] << endl;
}
}

Answer 4

There are several things I would consider sub-optimal or incorrect if I was your instructor:
Accessing at(2) and at(4) will give back 3rd and 5th characters, not 2nd and 4th. Also, when you say "2 letter" and "4 letter", do you (or your instructor) mean letter #2 and letter #4, or first 2 letters and then first 4 letters. There seems to be something lost in translation here, as was already commented.
You do not check for string length before accessing characters in it. You access a.back() without checking that the string is not empty, and at(2) and at(4) without checking it is of sufficient length. This makes it very easy to crash your program, by providing insufficient characters as input (you did not provide any input assumptions, so this may be OK with your instructor).
For the second part of the program - the one with the random numbers - you are using 4 loops, when 2 would have been perfectly sufficient: you could generate the numbers, and print them in one loop. Then, you could add 100 to the even ones, and print again, in a second loop.
Finally, when writing code, be consistent with your indentation. The first part of you code is indented by 4 characters, and the second, by 8. Only indent within blocks, and keep indentation uniform for the entire block.