C++ For loop problem

Question

I have a loop that takes two inputs, a last name and an ID, then converts it to a user id. The code looks like this:

   void User::setUserid(string ln, string id){
        string temp = "0";
        string temp2 = "0";
        for (int k = 0; k < 6; k++){
            temp += ln[k]; 
        }
        for (int i = id.length()-2; i<id.length(); i++){
            temp2 += id[i];
        }
        userid = temp+temp2;

    }

For some reason if I comment out the first for loop it will compile and build. Any ideas why the code crashes?

And if you don't comment out the first loop, what happens? What do you mean by "crashes"? Does it refuse to compile? Are there any error messages? — Hot Licks
– Hot Licks, Commented Sep 25, 2011 at 15:20
It crashes when last name = Adams, which is 5 characters. So I believe it has something to do with that — Nick
– Nick, Commented Sep 25, 2011 at 15:20
@DanielRHicks when I run it just popups with an error screen telling me "Debug assertion failed" and asks to abort. — Nick
– Nick, Commented Sep 25, 2011 at 15:22

Marcelo Cantos · Accepted Answer · 2011-09-25 15:23:32Z

3

Is ln guaranteed to have at least six characters? You may be shooting past the end of the string.

In any event, you've chosen a slow and complicated way to copy parts of strings around. This should suffice:

void User::setUserid(string ln, string id){
    userid = "0" + ln.substr(0, 6) + "0" + id.substr(id.size() - 2);
}

Note that this will produce a shorter userid if ln.size() < 6 and throw out_of_range if id.size() < 2.

edited Sep 25, 2011 at 15:23

answered Sep 25, 2011 at 15:18

Marcelo Cantos

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5 Comments

Nick Over a year ago

It crashes when last name = Adams so I believe it has something to do with that

Konrad Rudolph Over a year ago

@Nick Oh you have got to be kidding!

Nick Over a year ago

@Konrad Sorry, I'm learning how to program. This stuff isn't immediately clear to me.

Nick Over a year ago

Thank you, worked perfectly and the code is much less convoluted than mine, haha :)

Marcelo Cantos Over a year ago

@Nick: No worries. Keep at it, and don't be discouraged by negative comments.

dacwe · Accepted Answer · 2011-09-25 15:18:58Z

1

The string ln might have less characters than 6 - ln[k] will be out of bounds.

Note that the code will crash if the id string contains less then two characters (i will be negative).

answered Sep 25, 2011 at 15:18

dacwe

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