c++ - Why does my code only work properly one time? -

good day everyone,

i have make program suppose following:

1) asks user input string.

2) asks user input integer (lets call 'n').

3) replaces each character in string character ahead 'n' in alphabet.

4) prints new string console.

for example: if string = abc , integer = 1 result bcd.

if string = hello , integer = 4 result lipps.

if string = welcome-2-c++ , integer = 13 result jrypbzr-2-p++ (special characters $,+,/ remain unchanged).

i've written code works properly:

(code removed) 

except works 1 time. console looks like:

please insert string convert:  abc please enter modification integer:  2 resulting string is: cde please insert string convert:  abc please enter modification integer:  2 resulting string is:  please insert string convert:  

as can see, second time program runs, there's no result.

why program work first time runs?

p.s. i've done of own debugging , seems "for loop" skipped second time program runs. (?)

edit: think might have memory allocation?

you aren't initializing loop variable:

for (int i; < input_string.length(); i++) { 

should int = 0;.

---

also, convertstring can massively simplified:

string convertstring(string input_string, int mod_int) {     (char& c : input_string) {         if (std::isupper(c)) {             c = 'a' + (c - 'a' + mod_int) % 26;         }         else if (std::islower(c)) {             c = 'a' + (c - 'a' + mod_int) % 26;         }     }     return input_string; }