I am new to C++. In order to solve an online puzzle, I needed to extract numbers from strings and keep only the first and last number. For example 4nfhfbk56khfkvh => 456 => 46.
So I made the function given below.

int num_extr(const std::string &thisline){
 41 
 42     std::string stored_numbers = thisline;
 43     int i,k,result;
 44 
 45     k = 0;
 46 
 47     for(i=0; i<thisline.size(); i++){
 48 
 49         if(thisline[i] >= '0' && thisline[i] <= '9'){
 50             std::cout << "Number: " << thisline[i] << 'n';
 51             stored_numbers[k] = thisline[i];
 52             ++k;
 53         }
 54     }
 55     std::cout << "Stored_numbers: " << stored_numbers << 'n';
 56     std::cout << "Stored_numbers[0]: " << stored_numbers[0] << 'n';
 57     std::cout << "Stored_numbers[k-1]: " << stored_numbers[k-1] << 'n';

 58     if(k>0){
 59         result = ((int)stored_numbers[0] -48) * 10 + ((int)stored_numbers[k-1] -48);
 60     }
 61     else{
 62         result = ((int)stored_numbers[0] -48) * 10 + ((int)stored_numbers[0] -48);
 63     }
 64     return result;
 65 }

As you can see, I assign stored_numbers = thisline, in order to make the string stored_numbers
the same length as thisline (is this faster than using a function to find the length of thisline?). Then, I store the numbers I found on it, starting from [0].
This code works. This is an example of the output:

Initial string: trknlxnv43zxlrqjtwonect

Number: 4

Number: 3

Stored_numbers: 43knlxnv43zxlrqjtwonect

Stored_numbers[0]: 4

Stored_numbers[k-1]: 3

Extracted number: 43

But when I tried to not assign stored_numbers = thisline, so the code would be:

int num_extr(const std::string &thisline){
 41 
 42     std::string stored_numbers;
 43     int i,k,result;
 44 
 45     k = 0;
 46 
 47     for(i=0; i<thisline.size(); i++){
 48 
 49         if(thisline[i] >= '0' && thisline[i] <= '9'){
 50             std::cout << "Number: " << thisline[i] << 'n';
 51             stored_numbers[k] = thisline[i];
 52             ++k;
 53         }
 54     }
 55     std::cout << "Stored_numbers: " << stored_numbers << 'n';
 56     std::cout << "Stored_numbers[0]: " << stored_numbers[0] << 'n';
 57     std::cout << "Stored_numbers[k-1]: " << stored_numbers[k-1] << 'n';
 58     if(k>0){
 59         result = ((int)stored_numbers[0] -48) * 10 + ((int)stored_numbers[k-1] -48);
 60     }
 61     else{
 62         result = ((int)stored_numbers[0] -48) * 10 + ((int)stored_numbers[0] -48);
 63     }
 64     return result;
 65 }

Then the output is:

Initial string: trknlxnv43zxlrqjtwonect

Number: 4

Number: 3

Stored_numbers:

Stored_numbers[0]: 4

Stored_numbers[k-1]: 3

Extracted number: 43

So when I use stored_numbers as an array of (characters?) without the assignment, it seems like it is no longer one string, but it becomes a set of (pointers?) that cannot be presented as one string on cout…
Is this idea on the right direction? Could someone please explain this behavior in more depth?