Only read numbers out of users input - c++

Question

I'm trying to write a function which only reads four ints out of a users input like this: ewzge242jfdsiii23 So it is supposed to save only 2422.
This is my code and it just gives me some weird output, if I let it cout number.
Can you maybe see my mistakes and explain why I can't do it how I did and what I could do instead? Thanks a lot!
int readnumber ( ) {
char kar, ont_kar, ont_ont_kar;
int number;
while (kar != '\n' ){
cin.get (kar);
if (kar >= '0' && kar <= '9') {
old_kar=kar;
old_kar = old_kar*10 + (kar - '0');
old_old_kar = old_kar ;
} //if
} //while
if (old_kar < 9999) {
number=old_kar;
}//if
else {
number=old_old_kar;
}//else
}//readnumber

Answer 1

This looks too complicated, why do you need so many variables?
Also old_kar and old_old_kar are misstyped. The function does not return, that should be the main problem.
Here's a quick simple example:
unsigned readnumber(int number_of_chars) {
char ch;
unsigned number = 0;
while (number_of_chars > 0) {
std::cin.get(ch);
if ('\n' == ch)
break; // Stop on new line
if (ch < '0' or ch > '9')
continue; // Skip non-digits
--number_of_chars; // One digit processed
number = number * 10 + ch - '0'; // And added to the result
}
return number;
}
And here is a full version without break or continue:
#include <iostream> // std::cin, std::cout
#include <fstream> // std::ifstream
using namespace std;
int readnumber(int number_of_chars) {
char ch;
int number = 0;
while (number_of_chars > 0) {
std::cin.get(ch);
if ('\n' == ch)
return number;
if (ch >= '0' and ch <= '9') {
--number_of_chars;
number = number * 10 + ch - '0';
}
}
return number;
}
int main() {
int n = readnumber(4);
cout << "You entered: " << n << endl;
return 0;
}
NB: Always compile with all warnings on, this will save you a lot of time.