I have the following code to print a vector in c++. It works fine for non-empty vectors, but for some reason it gives a segmentation fault when you try to print an empty vector.

#include <iostream>
#include <vector>

using namespace std;

void printVector(const vector<int>& v) {
    cout << "{";

    for (int i = 0; i < v.size() - 1; i++)
        cout << v[i] << ", ";

    if(v.size() > 0) cout << v[v.size() - 1];

    cout << "}" << endl;
}

int main() {
    vector<int> v = vector<int>(0);
    printVector(v);
}

For an empty vector v, v.size() = 0 so v.size() – 1 = -1 which is not > 0, therefore the loop will never be entered. The same happens with the if conditional. So the only 2 lines that should be executed are the ones that print “{” and “}n”.

Instead, this code outputs exactly (it doesn’t even print the first “{“):

Segmentation fault (core dumped)

Then I tried to add some more couts in the code, to see where it breaks. The first modification I made is

void printVector(const vector<int>& v) {
    cout << "{";
    cout << "Check 1" << endl;

    for (int i = 0; i < v.size() - 1; i++)
        cout << v[i] << ", ";

    if(v.size() > 0) cout << v[v.size() - 1];

    cout << "}" << endl;
}

Now it outputs:

{Check 1
Segmentation fault (core dumped)

Which is strange, as now the “{” does appear. My next guess was that maybe the .size() function gives an error for empty vectors, so the for loop condition is what is breaking. To test it, I made the following modification:

void printVector(const vector<int>& v) {
    cout << "{";
    int aux = v.size();
    cout << "Check 1" << " " << aux << endl;

    for (int i = 0; i < v.size() - 1; i++)
        cout << v[i] << ", ";

    if(v.size() > 0) cout << v[v.size() - 1];

    cout << "}" << endl;
}

Output:

{Check 1 0
Segmentation fault (core dumped)

I concluded that the problem can’t be with the .size() function, which was my last guess at what is going wrong. Then I asked chat gpt and it gave me this code:

void printVector(vector<int>& v) {
    cout << "{";

    for (int i = 0; i < v.size(); i++) {
        cout << v[i];
        if (i < v.size() - 1) {
            cout << ", ";
        }
    }

    cout << "}" << endl;
}

Which for some reason works! It looks like the exact same code to me. I feel I am going crazy. It is such a simple program and I can’t even get it to work / find the mistake.