Why is an incremental increase to an integer in C causing overflow?

Question

I'm working through CS50 and am currently working on a small function to determine the winner(s) of an election.

  void print_winner(void)
    {
        int max_votes = 0;
        int num_of_winners = 0;
        string winners [] = {};
    
        for (int i = 0; i < candidate_count; i++) {
        //Determine if the candidate has enough votes to be considered a winner
            if (candidates[i].votes >= max_votes) {
                //If so, increment the number of winners
                num_of_winners ++;
                //Assign the winning candidate to the winners array
                winners[i] = candidates[i].name;
                //And reset the max number of votes required to now be considered a winner
                max_votes = candidates[i].votes;
            }
           printf("number of winners  %i\n", num_of_winners);
        }
    
    for (int i = 0; i < num_of_winners; i++) {
         printf("winner  %s\n", winners[i]);
    }
        return;
    }

Whenever I run the function the counter seems to increment properly but on the last printout the number is clearly wrong:

I'm still learning C and from what I've read this sounds like either a memory allocation issue or some kind of integer overflow issue however I'm unsure what the issue is and how to fix it.

string isn't C. Your array has no size, so writing to it corrupts the stack. — Mark Tolonen
– Mark Tolonen, Commented Aug 25, 2020 at 15:43
At first sight it looks like you increase the number of winners but not the array associated with it. — Paulsor
– Paulsor, Commented Aug 25, 2020 at 15:44
you are not allocating memory for your array of winners. . You could declare winner as string winners [100] or whatever the max number of winners are. — Paul Baxter
– Paul Baxter, Commented Aug 25, 2020 at 15:46
@MarkTolonen string in the context of CS50 is a typedef to char *. Yes, it's an utter abomination that screws up a lot of C students. — Andrew Henle
– Andrew Henle, Commented Aug 25, 2020 at 15:55
Please do not add pictures of text. A simple copy&paste is sufficient. — Gerhardh
– Gerhardh, Commented Aug 25, 2020 at 15:57

Armen Tsirunyan · Accepted Answer · 2020-08-25 15:43:59Z

2

Your array winners has size 0 because you have initialized it like:

string winners [] = {};

Therefore there is no valid index i which would allow you to write winners[i]. Give a proper initial size to your array. Also, what exactly is 'string' in C?

answered Aug 25, 2020 at 15:43

Armen Tsirunyan

134k61 gold badges336 silver badges442 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Paulsor Over a year ago

Because the number of winners is not predictable it should be done dynamically. My guess is that 'string' is just a typedef of something like char*.

Ingo Leonhardt Over a year ago

candidate_count would always be big enough as initial size

mancony Over a year ago

Thanks for the answer! Makes sense, I'm coming from Javascript so used to being able to define an empty array and populate it as needed, which I appreciate is antithetical to the nature of C. As other comments have pointed out string is just a typedef that CS50 provide to make things easier at first.

John Bollinger · Accepted Answer · 2020-08-25 15:48:31Z

C does not allow zero-length arrays, so you are relying on an extension provided by your compiler for this to be accepted at all:

        string winners [] = {};

But that having been accepted, it is of no use to you, because the dimensions of C arrays are not adjustable. Thus, any attempt to access an element of your array winners overruns its bounds, producing undefined behavior. Data corruption such as you observe is one of the common manifestations of such UB.

Resolve the issue by declaring the array large enough for the largest case you want to support (and by rejecting larger cases explicitly), or else by allocating memory for it dynamically, and increasing the allocation when you find you need to do.

Collectives™ on Stack Overflow

Why is an incremental increase to an integer in C causing overflow?

2 Answers 2

3 Comments

Comments

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Related