char + int to change the char value - c++

Question

I am new to C++, learned it for more than a month. I have a beginner-level knowledge of Python, like creating a list, modifying it, loops, etc. I don't know some codes for C++ that I know in python.
I am making a program for a school class (creative program). This is a part of my code (description at the bottom):
int number, new_one, num_letter;
char one;
cout << "You chose to encypher a message\nPlease choose an integer between 1-25:";
cin >> number;
cout << "How many letters are in your word?";
cin >> num_letter;
if (num_letter == 1)
{
cout << "Enter the first letter";
cin >> one;
new_one = one + number;
cout << "Your encrypted message is '"
<< static_cast<char>(new_one)
<< "' with the code number of "
<< number;
I am making a program where it enciphers and deciphers a message. The user chooses the number of letters of their message (maximum of 10 because I don't know how to use a for-loop in C++ yet). Then, they choose an integer. Then, they enter the letter, hit Enter, enter the letter, and hit Enter for the number of letters in their message (I don't know how to separate strings to chars in C++ yet).
When the user enters their letter and hits Enter, I cin >> that letter into the variable one, which is a char. Then, I add that one to the number the user chose, so the ASCII code of the one increases by the value of the number.
For example, when I enter 3 for number and h for the value of one, 104 (the ASCII code of h) should add up with 3, resulting in 107, which I then would static_cast to a char value.
But, when I add h and 3, instead of creating 107, it creates 155. Same for other variables. I tried cout'ing static_cast<int>(one) (in this case, the letter h) and number (which is 3). They printed 104 and 3.
But, when I add those two values, it prints 155. Why is this happening?

Answer 1

This is my solution. Hope it helps!
#include <iostream>
using namespace std;
int main()
{
int offset = 0;
int num_with_offset;
int size;
// Gets offset from user
do{
cout << "You chose to encypher a message\nPlease choose an integer between 1-25: ";
cin >> offset;
} while (offset < 1 || offset > 25);
// Gets letters in word
do{
cout << "Letters in word: ";
cin >> size;
} while(size < 0);
// Given size, init arrays
int number[size];
char one[size];
// Conversion from char to int
for(int i = 0; i < (sizeof(one)/sizeof(one[0])); i++)
{
cout << "Enter character " << (i + 1) << ": ";
cin >> one[i];
num_with_offset = one[i] + offset;
// Converts ASCII to integer and stores it into array
number[i] = static_cast<int>(num_with_offset);
}
// Prints out the new encrypted message
for(int j = 0; j < (sizeof(number)/sizeof(number[0])); j++)
{
cout << "Your encrypted message is: "
<< number[j] << " , with the code number: "
<< offset << "." << endl;
}
cout << endl << endl;
return 0;
}